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A prism spreads white light into its various component wavelengths, or colours. (credit: © Getty Images)

That portion of the electromagnetic spectrum visible to the human eye. It ranges from the red end to the violet end of the spectrum, with wavelengths from 700 to 400 nanometres and frequencies from 4.310¹⁴ to 7.510¹⁴ Hz. Like all electromagnetic radiation, it travels through empty space at a speed of about 186,000 mi/sec (300,000 km/sec). In the mid-19th century, light was described by James Clerk Maxwell in terms of electromagnetic waves, but 20th-century physicists showed that it exhibits properties of particles as well; its carrier particle is the photon. Light is the basis for the sense of sight and for the perception of colour. optics; wave-particle duality.

For more information on light, visit Britannica.com.

Concept

Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric-powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high-intensity light, make possible a number of technologies used in everything from surgery to entertainment.

How It Works

Early Progress in Understanding of Light

The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface.

The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission—the belief that light rays originate in the eye and travel toward objects, making them visible. Some 1,500 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039), sometimes called the greatest scientist of the Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes.

The next great era of progress in studies of light began with the Renaissance (c. 1300-c. 1600.) However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1564-1642) and German astronomer Johannes Kepler (1571-1630) built the first refracting telescopes, while Dutch physicist and mathematician Willebrord Snell (1580-1626) further refined the laws of refraction.

The Spectrum

Sir Isaac Newton (1642-1727) was as intrigued with light as he was with gravity and the other concepts associated with his work. Though it was not as epochal as his contributions to mechanics, Newton's work in optics, an area of physics that studies the production and propagation of light, was certainly significant.

In Newton's time, physicists understood that a prism could be used for the diffusion of light rays—in particular, to produce an array of colors from a beam of white light. The prevailing belief was that white was a single color like the others, but Newton maintained that it was a combination of all other colors. To prove this, he directed a beam of white light through a prism, then allowed the diffused colors to enter another prism, at which point they recombined as white light.

Newton gave to the array of colors in visible light the term spectrum, (plural, "spectra") meaning the continuous distribution of properties in an ordered arrangement across an unbroken range. The term can be used for any set of characteristics for which there is a gradation, as opposed to an excluded middle. An ordinary light switch provides an example of a situation in which there is an excluded middle: there is nothing between "on" and "off." A dimmer switch, on the other hand, is a spectrum, because a very large number of gradations exist between the two extremes represented by a light switch.

Seven Colors…or Six?

The distribution of colors across the spectrum is as follows: red-orange-yellow-green-blue-violet. The reasons for this arrangement, explained below in the context of the electromagnetic spectrum, were unknown to Newton. Not only did he live in an age that had almost no understanding of electromagnetism, but he was also a product of the era called the Enlightenment, when intellectuals (scientists included) viewed the world as a highly rational, ordered mechanism. His Enlightenment viewpoint undoubtedly influenced his interpretation of the spectrum as a set of seven colors, just as there are seven notes on the musical scale.

In addition to the six basic colors listed above, Newton identified a seventh, indigo, between blue and violet. In fact, there is a noticeable band of color between blue and violet, but this is because one color fades into another. With a spectrum, there is a blurring of lines between one color and the next: for instance, orange exists at a certain point along the spectrum, as does yellow, but between them is a nearly unlimited number of orange-yellow and yellow-orange gradations.

Indigo itself is not really a distinct color—just a deep, purplish blue. But its inclusion in the listing of colors on the spectrum has given generations of students a handy mnemonic (memorization) device: the name "ROY G. BIV." These letters form an acrostic (a word constructed from the first letters of other words) for the colors of the spectrum. Incidentally, there is something arbitrary even in the idea of six colors, or for that matter seven musical notes: in both cases, there exists a very large gradation of shades, yet also in both cases, the divisions used were chosen for practical purposes.

Waves, Particles, and Other Questions Concerning Light

The Wave-Particle Controversy Begins

Newton subscribed to the corpuscular theory of light: the idea that light travels as a stream of particles. On the other hand, Dutch physicist and astronomer Christiaan Huygens (1629-1695) maintained that light travels in waves. During the century that followed, adherents of particle theory did intellectual battle with proponents of wave theory. "Battle" is not too strong a word, because the conflict was heated, and had a nationalistic element. Reflecting both the burgeoning awareness of the nation-state among Europeans, as well as Britons' sense of their own island as an entity separate from the European continent, particle theory had its strongest defenders in Newton's homeland, while continental scientists generally accepted wave theory.

According to Huygens, the appearance of the spectrum, as well as the phenomena of reflection and refraction, indicated that light was a wave. Newton responded by furnishing complex mathematical calculations which showed that particles could exhibit the behaviors of reflection and refraction as well. Furthermore, Newton challenged, if light were really a wave, it should be able to bend around corners. Yet, in 1660, an experiment by Italian physicist Francesco Grimaldi (1618-1663) proved that light could do just that. Passing a beam of light through a narrow aperture, or opening, Grimaldi observed a phenomenon called diffraction, or the bending of light.

In view of the nationalistic character that the wave-particle debate assumed, it was ironic that the physicist whose work struck a particularly forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Directing a light beam through two closely spaced pinholes onto a screen, Young reasoned that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon.

The Question of a Medium

As the nineteenth century progressed, evidence in favor of wave theory grew. Experiments in 1850 by Jean Bernard Leon Foucault (1819-1868)—famous for his pendulum—showed that light traveled faster in air than through water. Based on studies of wave motion up to that time, Foucault's work added substance to the view of light as a wave.

Foucault also measured the speed of light in a vacuum, a speed which he calculated to within 1% of its value as it is known today: 186,000 mi (299,339 km) per second. An understanding of just how fast light traveled, however, caused a nagging question dating back to the days of Newton and Huygens to resurface: how did light travel?

All types of waves known to that time traveled through some sort of medium: for instance, sound waves were propagated through air, water, or some other type of matter. If light was a wave, as Huygens said, then it, too, must have some medium. Huygens and his followers proposed a weak theory by suggesting the existence of an invisible substance called ether, which existed throughout the universe and which carried light.

Ether, of course, was really no answer at all. There was no evidence that it existed, and to many scientists, it was merely a concept invented to shore up an otherwise convincing argument. Then, in 1872, Scottish physicist James Clerk Maxwell (1831-1879) proposed a solution that must have surprised many scientists. The "medium" through which light travels, Maxwell proposed, was no medium at all; rather, the energy in light is transferred by means of radiation, which requires no medium.

Electromagnetism

Maxwell brought together a number of concepts developed by his predecessors, sorting these out and adding to them. His work led to the identification of a "new" fundamental interaction, in addition to that associated with gravity. This was the mode of particle interaction associated with electromagnetic force.

The particulars of electromagnetic force, waves, and radiation are a subject unto themselves—really, many subjects. As for the electromagnetic spectrum, it is treated at some length in an essay elsewhere in this volume, and the reader is encouraged to review that essay to gain a greater understanding of light and its place in the spectrum.

In addition, some awareness of wave motion and related phenomena would also be of great value, and, for this purpose, other essays are recommended. In the present context, a number of topics relating to these larger subjects will be handled in short order, with a minimum of explanation, to enable a more speedy transition to the subject of principal importance here: light.

Electromagnetic Waves

There is, of course, no obvious connection between light and the electromagnetic force observed in electrical and magnetic interactions. Yet, light is an example of an electromagnetic wave, and is part of the electromagnetic spectrum. The breakthrough in establishing the electromagnetic quality of light can be attributed both to Maxwell and German physicist Heinrich Rudolf Hertz (1857-1894).

In his Electricity and Magnetism (1873), Maxwell suggested that electromagnetic force might have aspects of a wave phenomenon, and his experiments indicated that electromagnetic waves should travel at exactly the same speed as light. This appeared to be more than just a coincidence, and his findings led him to theorize that the electromagnetic interaction included not only electricity and magnetism, but light as well. Some time later, Hertz proved Maxwell's hypothesis by showing that electromagnetic waves obeyed the same laws of reflection, refraction, and diffraction as light.

Hertz also discovered the photoelectric effect, the process by which certain metals acquire an electrical potential when exposed to light. He could not explain this behavior, and, indeed, there was nothing in wave theory that could account for it. Strangely, after more than a century in which acceptance of wave theory had grown, he had encountered something that apparently supported what Newton had said long before: that light traveled in particles rather than waves.

The Wave-Particle Debate Revisited

One of the modern physicists whose name is most closely associated with the subject of light is Albert Einstein (1879-1955). In the course of proving that matter is convertible to energy, as he did with the theory of relativity, Einstein predicted that this could be illustrated by accelerating to speeds close to that of light. (Conversely, he also showed that it is impossible for matter to reach the speed of light, because to do so would—as he proved mathematically—result in the matter acquiring an infinite amount of mass, which, of course, is impossible.)

Much of Einstein's work was influenced by that of German physicist Max Planck (1858-1947), father of quantum theory. Quantum theory and quantum mechanics are, of course, far too complicated to explain in any depth here. It is enough to say that they called into question everything physicists thought they knew, based on Newton's theories of classical mechanics. In particular, quantum mechanics showed that, at the subatomic level, particles behave in ways not just different from, but opposite to, the behavior of larger physical objects in the observable world. When a quantity is "quantized," its values or properties at the atomic or subatomic level are separate from one another—meaning that something can both be one thing and its opposite, depending on how it is viewed.

Interpreting Planck's observations, Einstein in a 1905 paper on the photoelectric effect maintained that light is quantized—that it appears in "bundles" of energy that have characteristics both of waves and of particles. Though light travels in waves, as Einstein showed, these waves sometimes behave as particles, which is the case with the photoelectric effect. Nearly two decades later, American physicist Arthur Holly Compton (1892-1962) confirmed Einstein's findings and gave a name to the "particles" of light: photons.

Light's Place in the Electromagnetic Spectrum

The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. As discussed earlier, concerning the visible color spectrum, each of these occupies a definite place on the spectrum, but the divisions between them are not firm: in keeping with the nature of a spectrum, one band simply "blurs" into another.

Of principal concern here is an area near the middle of the electromagnetic spectrum. Actually, the very middle of the spectrum lies within the broad area of infrared light, which has frequencies ranging from 10¹² to just over 10¹⁴ Hz, with wavelengths of approximately 10⁻¹ to 10⁻³ centimeters. Even at this point, the light waves are oscillating at a rate between 1 and 100 trillion times a second, and the wavelengths are from 1 millimeter to 0.01 millimeters. Yet, over the breadth of the electromagnetic spectrum, wavelengths get much shorter, and frequencies much greater.

Infrared lies just below visible light in frequency, which is easy to remember because of the name: red is the lowest in frequency of all the colors, as discussed below. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Neither infrared nor ultraviolet can be seen, yet we experience them as heat. In the case of ultraviolet (UV) light, the rays are so powerful that exposure to even the minuscule levels of UV radiation that enter Earth's atmosphere can cause skin cancer.

Ultraviolet light occupies a much narrower band than infrared, in the area of about 10¹⁵ to 10¹⁶ Hz—in other words, oscillations between 1 and 10 quadrillion times a second. Wavelengths in this region are from just above 10⁻⁶ to about 10⁻⁷ centimeters. These are often measured in terms of a nanometer (nm)—equal to one-millionth of a millimeter—meaning that the wavelength range is from above 100 down to about 10 nm.

Between infrared and ultraviolet light is the region of visible light: the six colors that make up much of the world we know. Each has a specific range and frequency, and together they occupy an extremely narrow band of the electromagnetic spectrum: from 4.3 · 10¹⁴ to 7.5 · 10¹⁴ Hz in frequency, and from 700 down to 400 nm in wavelength. To compare its frequency range to that of the entire spectrum, for instance, is the same as comparing 3.2 to 100 billion.

Real-Life Applications

Colors

Unlike many of the topics addressed by physics, color is far from abstract. Numerous expressions in daily life describe the relationship between energy and color: "red hot," for instance, or "blue with cold." In fact, however, red—with a smaller frequency and a longer wavelength than blue—actually has less energy; therefore, blue objects should be hotter.

The phenomenon of the red shift, discovered in 1923 by American astronomer Edwin Hubble (1889-1953), provides a clue to this apparent contradiction. As Hubble observed, the light waves from distant galaxies are shifted to the red end, and he reasoned that this must mean those galaxies are moving away from the Milky Way, the galaxy in which Earth is located.

To generalize from what Hubble observed, when something shows red, it is moving away from the observer. The laws of thermodynamics state that where heat is involved, the movement is always away from an area of high temperature and toward an area of low temperature. Heated molecules that reflect red light are, thus, to use a colloquialism, "showing their tail end" as they move toward an area of low temperature. By contrast, molecules of low temperature reflect bluish or purple light because the tendency of heat is to move toward them.

There are other reasons, aside from heat, that some objects tend to be red and others blue—or another color. Chemical factors may be involved: atoms of neon, for example, can be made to vibrate at a particular wavelength, producing a specific color. In any case, the color that an object reflects is precisely the color that it does not absorb: thus, if something is red, that means it has absorbed every color of the spectrum but red.

Why Is the Sky Blue?

The placement of colors on the electromagnetic spectrum provides an answer to that age-old question posed by generations of children to their parents: "Why is the sky blue?" Electromagnetic radiation is scattered as it enters the atmosphere, but all forms of radiation are not scattered equally. Those having shorter wavelengths—that is, toward the blue end of the spectrum—tend to scatter more than those with longer wavelengths, on the red and orange end.

Yet the longer-wavelength light becomes visible at sunset, when the Sun's light enters the atmosphere at an angle. In addition, the dim quality of evening light means that it is easiest to see light of longer wavelengths. This effect is known as Rayleigh scattering, after English physicist John William Strutt, Lord Rayleigh (1842-1919), who discovered it in 1871. Thanks to Rayleigh's discovery, there is an explanation not only for the question of why the sky is blue, but why sunsets are red, orange, and gold.

Rainbows

On the subject of color as children perceive it, many a child has been fascinated by a rainbow, seeing in them something magical. It is easy to understand why children perceive these beautiful phenomena this way, and why people have invented stories such as that of the pot of gold at the end of a rainbow. In fact, a rainbow, like many other "magical" aspects of daily life, can be explained in terms of physics.

A rainbow, in fact, is simply an illustration of the visible light spectrum. Rain drops perform the role of tiny prisms, dispersing white sunlight, much as scientists before Newton had learned to do. But if there is a pot of gold at the end of the rainbow, it would be impossible to find. In order for a rainbow to be seen, it must be viewed from a specific perspective: the observer must be in a position between the sunlight and the raindrops.

Sunlight strikes raindrops in such a way that they are refracted, then reflected back at an angle so that they represent the entire visible light spectrum. Though they are beautiful to see, rainbows are neither magical nor impossible to reproduce artificially. Such rainbows can be produced, for instance, in the spiral of small water droplets emerging from a water hose, viewed when one's back is to the Sun.

Perception of Light and Color

People literally live and die for colors: the colors of a flag, for instance, present a rallying point for soldiers, and different colors are assigned specific political meanings. Blue, both in the American and French flags, typically stands for liberty. Red can symbolize the blood shed by patriots, or it can mean some version of fraternity or brotherhood. Such is the case with the red of the French tricolor (red-white-blue); likewise, the red in the flag of the former Soviet Union and other Communist countries stood for the alleged international brotherhood of all working peoples. In Islamic countries, by contrast, green stands for the unity of all Muslims.

These are just a few examples, drawn from a specific realm—politics—illustrating the meanings that people ascribe to colors. Similarly, people find meanings in images presented to them by light itself. In his Republic, the ancient Greek philosopher Plato (c. 427-347 B.C.) offered a complex parable, intended to illustrate the difference between reality and illusion, concerning a group of slaves who do not recognize the difference between sunlight and the light of a torch in a cave. Modern writers have noted the similarities between Plato's cave and a phenomenon which the ancient philosopher could hardly have imagined: a movie theatre, in which an artificial light projects images—images that people sometimes perceive as being all too real—onto a screen.

People refer to "tricks of the light," as, for instance, when one seems to see an image in a fire. One particularly well-known "trick of the light," a mirage, is discussed below, but there are also manmade illusions created by light, shapes, and images. An optical illusion is something that produces a false impression in the brain, causing one to believe that something is as it appears, when, in fact, it is not. When two lines of equal length are placed side by side, but one has arrows pointed outward at either end while the other line has arrows pointing inward, it appears that the line with the inward-pointing arrows are shorter.

This is an example of the ways in which human perception plays a role in what people see. That topic, of course, goes far beyond physics and into the realms of psychology and the social sciences. Nonetheless, it is worthwhile to consider, from a physical standpoint, how humans see what they see—and sometimes see things that are not there.

A Mirage

Because they can be demonstrated in light waves as well as in sound waves, diffraction and interference are discussed in separate essays. As for refraction, or the bending of light waves, this phenomenon can be seen in the familiar example of a mirage. While driving down a road on a hot day, one may observe that there are pools of water up ahead, but by the time one approaches them, they disappear.

Of course, the pools were never there; light itself has created an optical illusion of sorts. As light moves from one material to another, it bends with a different angle of refraction, and, though, in this instance, it is traveling entirely through air, it is moving through regions of differing temperature. Light waves travel faster through warm air than through cool air, and, thus, when the light enters the area over the heated surface of the asphalt, it experiences refraction. The waves are thus bent, creating the impression of a reflection, which suggests to the observer that there is water up ahead.

How the Eye Sees Color

White, as noted earlier, is the combination of all colors; black is the absence of color. Where ink, dye, or other forms of artificial pigmentation are concerned, of course, black is a "real" color, but in terms of light, it is not. In the same way, the experience of coldness is real, yet "cold" does not exist as a physical phenomenon: it is simply the absence of heat.

The mixture of pigmentation is an entirely different matter from the mixture of light. In artificial pigmentation, the primary colors—the three colors which, when mixed, yield the remainder of the shades on the rainbow—are red, blue, and yellow. Red mixed with blue creates purple, blue mixed with yellow makes green, and red mixed with yellow yields orange. Black and white are usually created by using natural substances of that color—chalk for white, for instance, or various oxides for black. For light, on the other hand, blue and red are primary colors, but the third primary color is green, not yellow. From these three primary colors, all other shades of the visible spectrum can be made.

The mechanism of the human eye responds to the three primary colors of the visible light spectrum: thus, the eye's retina is equipped with tiny cones that respond to red, blue, and green light. The cones respond to bright light; other structures called rods respond to dim light, and the pupil regulates the amount of light that enters the eye.

The eye responds with maximum sensitivity to light at the middle of the visible color spectrum—specifically, green light with a wavelength of about 555 nm. The optimal wavelength for maximum sensitivity in dim light is around 510 nm, on the blue end. It is difficult for the eye to recognize red light, at the far end of the spectrum, against a dark background. However, this can be an advantage in situations of relative darkness, which is why red light is often used to maintain vision for sailors, amateur astronomers, and the military on night maneuvers. Because there is not much difference between the darkness and the red light, the eye adjusts and is able to see beyond the red light into the darkness. A bright yellow or white light in such situations, on the other hand, would minimize visibility in areas beyond the light.

Artificial Light

Prehistoric Lighting Technology

Prehistoric humans did not know it, but they were making use of electromagnetic radiation when they lit and warmed their caves with light from a fire. Though it would seem that warmth was more essential to human survival than artificial light, in fact, it is likely that both functions emerged at about the same time: once humans began using fire for warmth, it would have been a relatively short time before they comprehended the power of fire to drive out both darkness and the fierce creatures (for instance, bears) that came with it.

These distant forebears advanced to the fashioning of portable lighting technology in the form of torches or rudimentary lamps. Torches were probably made by binding together resinous material from trees, while lamps were made either from stones with natural depressions, or from soft rocks—for example, soap-stone or steatite—into which depressions were carved by using harder material. Most of the many hundreds of lamps found by archaeologists at sites in southwestern France are made of either limestone or sandstone. Limestone was a particularly good choice, since it conducts heat poorly; lamps made of sandstone, a good conductor of heat, usually had carved handles to protect the hands of the user.

Artificial Light in Pre-Modern Times

The history of lighting is generally divided into four periods, each of which overlap, and which together illustrate the slow pace of change in illumination technology. First was the primitive, a period encompassing the torches and lamps of prehistoric human beings—though, in fact, French peasants used the same lighting methods depicted in nearby cave paintings until World War I.

Next came the classical stage, the world of Greece and Rome. Earlier civilizations, such as that of Egypt, belong to the primitive era in lighting—before the relatively widespread adoption of the candle and of vegetable oil as fuel. Third was the medieval stage, which saw the development of metal lamps. Last came the modern or invention stage, which began with the creation of the glass lantern chimney by Leonardo da Vinci (1452-1519) in 1490, culminated with Thomas Edison's (1847-1931) first practical incandescent bulb in 1879, and continues today.

At various times, ancient peoples used the fat of seals, horses, cattle, and fish as fuel for lamps. (Whale oil, by contrast, entered widespread use only during the nineteenth century.) Primitive humans sometimes used entire animals—for example, the storm petrel, a bird heavy in fat—to provide light. Even without such cruel excesses, however, animal fat made for a smoky, dangerous, foul-smelling fire.

The use of vegetable oils, a much more efficient medium for lighting, did not take hold until Greek, and especially, Roman times. Animal oils remained in use, however, among the poor, whose homes often reeked with the odor of castor oil or fish oil. Because virtually all fuels came from edible sources, times of famine usually meant times of darkness as well.

The candle, as well as the use of vegetable oils, dates back to earliest antiquity, but the use of candles only became common among the richest citizens of Rome. Because it used animal fat, the candle was apparently a return to an earlier stage, but its hardened tallow actually represented a much safer, more stable fuel than lamp oil.

Incandescent Light

Lighting technology in the period from about 1500 to the late nineteenth century involved a number of improvements, but in one respect, little had progressed since prehistoric times: people were still burning fuel to provide illumination. This all changed with the invention of the incandescent bulb, which, though it is credited to Edison, was the product of experimentation that took place throughout the nineteenth century. As early as 1802, British scientist Sir Humphry Davy (1778-1829) showed that electricity running through thin strips of metal could heat them enough to cause them to give off light—that is, electromagnetic radiation.

Edison, in fact, was just one of several inventors in the 1870s attempting to develop a practical incandescent lamp. His innovation lay in his understanding of the parameters necessary for developing such a lamp—in particular, decreasing the electrical resistance in the lamp filament (the part that is heated) so that less energy would be required to light it. On October 19, 1879, using low-resistance filaments of carbon or platinum, combined with a high-resistance carbon filament in a vacuum-sealed glass container, Edison produced the first practical lightbulb.

Much has changed in the design of light-bulbs during the decades following Edison's ingenious invention, of course, but his design provided the foundation. There is just one problem with incandescent light, however—a problem inherent in the definition and derivation of the word incandescent, which comes from a Latin root meaning "to become hot." The efficiency of a light is determined by the ratio of light, or usable energy, to heat—which, except in the case of a campfire, is typically not a desirable form of energy where lighting is concerned.

Amazingly, only about 10% of the energy output from a typical incandescent light bulb is in the form of visible light; the rest comes through the infrared region of the spectrum, producing heat rather than light that people can use. The visible light tends to be in the red and yellow end of the spectrum—closer to infrared—but a blue-tinted bulb helps to absorb some of the red and yellow, providing a color balance. This, however, only further diminishes the total light output, and, hence, in many applications today, fluorescent light takes the place of incandescent light.

Lasers

A laser is an extremely focused, extremely narrow, and extremely powerful beam of light. Actually, the term laser is an acronym, standing for L ight A mplification by S imulated E mission of R adiation. Simulated emission involves bringing a large number of atoms into what is called an "excited state." Generally, most atoms are in a ground state, and are less active in their movements, but the energy source that activates a laser brings about population inversion, a reversal of the ratios, such that the majority of atoms within the active medium are in an excited rather than a ground state. To visualize this, picture a popcorn popper, with the excited atoms being the popping kernels, and the ground-state atoms the ones remaining unpopped. As the atoms become excited, and the excited atoms outnumber the ground ones, they start to cause a multiplication of the resident photons. This is simulated emission.

A laser consists of three components: an optical cavity, an energy source, and an active medium. To continue the popcorn analogy, the "popper" itself—the chamber which holds the laser—is the optical cavity, which, in the case of a laser, involves two mirrors facing one another. One of these mirrors fully reflects light, whereas the other is a partly reflecting mirror. The light not reflected by the second mirror escapes as a highly focused beam. As with the popcorn popper, the power source involves electricity, and the active medium is analogous to the oil in a conventional popper.

Types of Lasers

There are four types of lasers: solid-state, semiconductor, gas, and dye. Solid-state lasers are generally very large and extremely powerful. Having a crystal or glass housing, they have been implemented in nuclear energy research, and in various areas of industry. Whereas solid-state lasers can be as long as a city block, semiconductor lasers can be smaller than the head of a pin. Semiconductor lasers (involving materials such as arsenic that conduct electricity, but do not do so as efficiently as the metals typically used as conductors) are applied for the intricate work of making compact discs and computer microchips.

Gas lasers contain carbon dioxide or other gases, activated by electricity in much the same way the gas in a neon sign is activated. Among their applications are eye surgery, printing, and scanning. Finally, dye lasers, as their name suggests, use different colored dyes. (Laser light itself, unlike ordinary light, is monochromatic.) Dye lasers can be used for medical research, or for fun—as in the case of laser light shows held at parks in the summertime.

Laser Applications

Laser beams have a number of other useful functions, for instance, the production of compact discs (CDs). Lasers etch information onto a surface, and because of the light beam's qualities, can record far more information in much less space than the old-fashioned ways of producing phonograph records.

Lasers used in the production of CD-ROM (Read-Only Memory) disks are able to condense huge amounts of information—a set of encyclopedias or the New York metropolitan phone book—onto a disk one can hold in the palm of one's hand. Laser etching is also used to create digital videodiscs (DVDs) and holograms. Another way that lasers affect everyday life is in the field of fiber optics, which uses pulses of laser light to send information on glass strands.

Before the advent of fiber-optic communications, telephone calls were relayed on thick bundles of copper wire; with the appearance of this new technology, a glass wire no thicker than a human hair now carries thousands of conversations. Lasers are also used in scanners, such as the price-code checkers at supermarkets and various kinds of tags that prevent thefts of books from libraries or clothing items from stores. In an industrial setting, heating lasers can drill through solid metal, or in an operating room, lasers can remove gallstones or cataracts. Lasers are also used for guiding missiles, and to help building contractors ensure that walls and floors and ceilings are in proper alignment.

Where to Learn More

Burton, Jane and Kim Taylor. The Nature and Science of Colors. Milwaukee, WI: Gareth Stevens Publishing, 1998.

Glover, David. Color and Light. New York: Dorling Kindersley Publishing, 2001.

Kalman, Bobbie and April Fast. Cosmic Light Shows. New York: Crabtree Publishing, 1999.

Kurtus, Ron. "Visible Light" (Web site). <http://www.school-for-champions.com/science/light.html> (May 2, 2001).

"Light Waves and Color." The Physics Classroom (Web site). <http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/lighttoc.html> (May 2, 2001).

Miller-Schroeder, Patricia. The Science and Light of Color. Milwaukee, WI: Gareth Stevens Publishing, 2000.

Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997.

Riley, Peter D. Light and Color. New York: F. Watts, 1999.

Taylor, Helen Suzanne. A Rainbow Is a Circle: And Other Facts About Color. Brookfield, CT: Copper Beech Books, 1999.

"Visible Light Waves" NASA: National Aeronautics and Space Administration (Web site). <http://imagers.gsfc.nasa.gov/ems/visible.html> (May 2, 2001).

The term light, as commonly used, refers to the kind of radiant electromagnetic energy that is associated with vision. In a broader sense, light includes the entire range of radiation known as the electromagnetic spectrum. The branch of science dealing with light, its origin and propagation, its effects, and other phenomena associated with it is called optics. Spectroscopy is the branch of optics that pertains to the production and investigation of spectra. See also Optics; Spectroscopy.

Principal effects

The electromagnetic spectrum is a broad band of radiant energy that extends over a range of wavelengths running from trillionths of inches to hundreds of miles; wavelengths of visible light are measured in hundreds of thousandths of an inch. Arranged in order of increasing wavelength, the radiation making up the electromagnetic spectrum is termed gamma rays, x-rays, ultraviolet rays, visible light, infrared waves, microwaves, radio waves, and very long electromagnetic waves. See also Electromagnetic radiation.

The fact that light travels at a finite speed or velocity is well established. In round numbers, the speed of light in vacuum or air may be said to be 186,000 mi/s or 300,000 km/s. Measurements of the speed of light, c, which had attracted physicists for 308 years, came to an end in 1983 when the new definition of the meter fixed the value of the speed of light. Highly precise values of c were obtained by extending absolute frequency measurements into a region of the electromagnetic spectrum where wavelengths can be most accurately measured. These advances were facilitated by the use of stabilized lasers and high-speed tungsten-nickel diodes which were used to measure the lasers' frequencies. The measurements of the speed of light and of the frequency of lasers yielded a value of the speed of light limited only by the standard of length which was then in use. This permitted a redefinition of the meter in which the value of the speed of light assumed an exact value, 299,792,458 m/s. The meter is defined as the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. See also Laser.

One of the most easily observed facts about light is its tendency to travel in straight lines. Careful observation shows, however, that a light ray spreads slightly when passing the edges of an obstacle. This phenomenon is called diffraction. The reflection of light is also well known. Reflection of light from smooth optical surfaces occurs so that the angle of reflection equals the angle of incidence, a fact that is most readily observed with a plane mirror. When light is reflected irregularly and diffusely, the phenomenon is termed scattering. The scattering of light by gas particles in the atmosphere causes the blue color of the sky. See also Diffraction; Reflection of electromagnetic radiation.

The type of bending of light rays called refraction is caused by the fact that light travels at different speeds in different media—faster, for example, in air than in either glass or water. Refraction occurs when light passes from one medium to another in which it moves at a different speed. Familiar examples include the change in direction of light rays in going through a prism, and the bent appearance of a slick partially immersed in water. See also Refraction of waves.

In the phenomenon called interference, rays of light emerging from two parallel slits combine on a screen to produce alternating light and dark bands. This effect can be obtained quite easily in the laboratory, and is observed in the colors produced by a thin film of oil on the surface of a pool of water. Polarization of light is usually shown with Polaroid disks. Such disks are quite transparent individually. When two of them are placed together, however, the degree of transparency of the combination depends upon the relative orientation of the disks. It can be varied from ready transmission of light to almost total opacity, simply by rotating one disk with respect to the other. See also Interference of waves; Polarized light.

When light is absorbed by certain substances, chemical changes take place. This fact forms the basis for the science of photochemistry.

Theory

Phenomena involving light may be classed into three groups: electromagnetic wave phenomena, corpuscular or quantum phenomena, and relativistic effects. The relativistic effects appear to influence similarly the observation of both corpuscular and wave phenomena. See also Relativity.

Wave phenomena

Interference and diffraction are the most striking manifestations of the wave character of light. Their fundamental similarity can be demonstrated in a number of experiments. The wave aspect of the entire spectrum of electromagnetic radiation is most convincingly shown by the similarity of diffraction pictures produced on a photographic plate, placed at some distance behind a diffraction grating, by radiations of different frequencies, such as x-rays and visible light. The interference phenomena of light are, moreover, very similar to interference of electronically produced microwaves and radio waves.

Polarization demonstrates the transverse character of light waves. Further proof of the electromagnetic character of light is found in the possibility of inducing, in a transparent body that is being traversed by a beam of plane-polarized light, the property of rotating the plane of polarization of the beam when the body is placed in a magnetic field. See also Faraday effect.

The fact that the velocity of light had been calculated from electric and magnetic parameters (permittivity and permeability) was at the root of Maxwell's conclusion in 1865 that “light, including heat and other radiations if any, is a disturbance in the form of waves propagated…according to electromagnetic laws.” Finally, the observation that electrons and neutrons can give rise to diffraction patterns quite similar to those produced by visible light has made it necessary to ascribe a wave character to particles. See also Electron diffraction; Neutron diffraction.

Corpuscular phenomena

In its interactions with matter, light exchanges energy only in discrete amounts, called quanta. This fact is difficult to reconcile with the idea that light energy is spread out in a wave, but is easily visualized in terms of corpuscles, or photons, of light.

The radiation from theoretically perfect heat radiators, called blackbodies, involves the exchange of energy between radiation and matter in an enclosed cavity. The observed frequency distribution of the radiation emitted by the enclosure at a given temperature of the cavity can be correctly described by theory only if one assumes that light of frequency ν is absorbed in integral multiples of a quantum of energy equal to hν, where h is a fundamental physical constant called Planck's constant.

When a monochromatic beam of electromagnetic radiation illuminates the surface of a solid (or less commonly, a liquid), electrons are ejected from the surface in the phenomenon known as photoemission or the external photoelectric effect. It is found that the emission of these photoelectrons, as they are called, is immediate, and independent of the intensity of the light beam, even at very low light intensities. This fact excludes the possibility of accumulation of energy from the light beam until an amount corresponding to the kinetic energy of the ejected electron has been reached.

The scattering of x-rays of frequency ν₀ by the lighter elements is caused by the collision of x-ray photons with electrons. Under such circumstances, both a scattered x-ray photon and a scattered electron are observed, and the scattered x-ray has a lower frequency than the impinging x-ray. The kinetic energy of the impinging x-ray, the scattered x-ray, and the scattered electron, as well as their relative directions, are in agreement with calculations involving the conservation of energy and momentum. See also Compton effect; Heat radiation; Photon.

Quantum theories

The need for reconciling Maxwell's theory of the electromagnetic field, which describes the electromagnetic wave character of light, with the particle nature of photons, which demonstrates the equally important corpuscular character of light, has resulted in the formulation of several theories which go a long way toward giving a satisfactory unified treatment of the wave and the corpuscular picture. These theories incorporate, on one hand, the theory of quantum electrodynamics, first set forth by P. A. M. Dirac, P. Jordan, W. Heisenberg, and W. Pauli, and on the other, the earlier quantum mechanics of L. de Broglie, Heisenberg, and E. Schrödinger. Unresolved theoretical difficulties persist, however, in the higher-than-first approximations of the interactions between light and elementary particles.

Dirac's synthesis of the wave and corpuscular theories of light is based on rewriting Maxwell's equations in a Hamiltonian form resembling the Hamiltonian equations of classical mechanics. Using the same formalism involved in the transformation of classical into wave-mechanical equations by the introduction of the quantum of action hν, Dirac obtained a new equation of the electromagnetic field. The solutions of this equation require quantized waves, corresponding to photons. The superposition of these solutions represents the electromagnetic field. The quantized waves are subject to Heisenberg's uncertainty principle. The quantized description of radiation cannot be taken literally in terms of either photons or waves, but rather is a description of the probability of occurrence in a given region of a given interaction or observation. See also Hamilton's equations of motion; Quantum electrodynamics; Quantum field theory; Quantum mechanics; Relativistic quantum theory; Uncertainty principle.

noun

1. Electromagnetic radiation that makes vision possible: illumination. See light/darkness.
2. The act of physically illuminating or the condition of being filled with light: illumination, lighting. See light/darkness.
3. The particular angle from which something is considered: angle², aspect, facet, frame of reference, hand, phase, regard, respect, side. See perspective.
4. The faculty of seeing: eye, eyesight, seeing, sight, vision. See see/not see.

verb

1. To cause to burn or undergo combustion: enkindle, fire, ignite, kindle. Slang torch. Idioms: setafireon fire, set fire to. See hot/cold/lukewarm, start/end.
2. To provide, cover, or fill with light: illume, illuminate, illumine, lighten¹. See light/darkness.
3. To make lively or animated: animate, brighten, enliven. See happy/unhappy.

adjective

Of a light color or complexion: alabaster, fair, ivory, pale. See colors/colorless.

Definition: blond, fair
Antonyms: brunette, dark, dusky

adj

Definition: funny, cheery
Antonyms: gloomy, grave, serious, solemn

adj

Definition: illuminated
Antonyms: black, dark, darkened, dim, gloomy, obscure

adj

Definition: not heavy
Antonyms: dense, heavy, weighted

adj

Definition: sexually carefree
Antonyms: chaste, dark

adj

Definition: simple, easy
Antonyms: complicated, difficult, heavy, laborious, serious

n

Definition: context, point of view; understanding
Antonyms: misconception, misunderstanding

n

Definition: luminescence from sun or other source
Antonyms: dark, darkness, night, obscurity

n

Definition: understanding
Antonyms: ignorance

v

Definition: illuminate
Antonyms: darken, dull, obscure

v

Definition: start on fire
Antonyms: drench, extinguish, put out, quench

v

Definition: step down; land
Antonyms: mount

1. An aperture through which daylight is admitted to the interior of a building.
2. A pane of glass, a window, or a compartment of a window.
3. An artificial source of illumination. Also see ceiling light, dead light, divided light, dome light, elliptical fanlight, fanlight, lantern light, leaded light, pavement light, quarter-round light, semi-circular light, semielliptical light, sidelight, skylight, sodium light, sunburst light, transom light.
4. A spirelight.

Although in physics light is well understood as electromagnetic energy of a very specific wavelength (between 390 and 740 nanometers: a nanometer is 10^-9 metre), the fact that this is just the energy that illuminates the world for us is evidently contingent upon the nature of our senses, and upon the adaptations they have made to the environment. Light can thus come to inherit the problems of secondary qualities (see primary/secondary qualities), and just as the fact that trees are green can seem due to our design as much as to non-human nature, so the fact that it is light or dark may seem disturbingly anthropocentric or subjective. Descartes put the problem by asking ‘could nature not also have established some sign, which would make us have the sensation of light, even if the sign contained nothing in itself which is similar to this sensation?’ (Le Monde). Light has been associated with the original primal creative principle in many religious and philosophical traditions, including Zoroastrianism and Neoplatonism.

Spiritualists believed that light had a destructive effect upon the physical phenomena of Spiritualism, which psychic research attempted to document. Quite apart from the fact that darkness hid much fraud, Spiritualists developed arguments to suggest that light had an inherent inhibiting effect on psychic phenomena. For example, it is known that light waves have very rapid vibrations (the visible light waves are from 3900 angstroms to 7700 angstroms; that is, the wave lengths range from 0.00000077 to 0.00000039 meters). Broadcasting practice demonstrates that the fast vibrations tend to nullify the slower vibrations on which radio is based. When the days are long and the sunlight intense, radio reception drops down. With the on-coming of night it improves again. With short waves which vibrate faster, reception is better.

It is claimed that psychic vibrations are in the same position. The slowest light vibration is red, and its destructive effect is correspondingly less. Filtering of daylight by glasses of various colors makes little difference. Cold light, devoid of actinic rays, is the least injurious. "I have had many opportunities," wrote Sir William Crookes, "of testing the action of light of different sources and colours, such as sunlight, diffused daylight, moonlight, gas, lamp and candle light, electric light from a vacuum tube, homogeneous yellow light, etc. The interfering rays appear to be those at the extreme end of the spectrum." He found moonlight ideal.

Sulphide of zinc or calcium screens have also been tried. They have the disadvantage that their illumination is poor unless they are extremely large, and the intensity of their phosphorescence rapidly diminishes. Gustav Geley experimented with biological light. It did not appear to affect the phenomena. However, the cultures of photogenic microbes are very unstable. In Brazil, luminous insects were tried with some apparent success.

Meanwhile, some of the more notable mediums worked primarily in lighted rooms and were able to produce extraordinary phenomena. D. D. Home seldom sat in darkness. Eusapia Palladino once levitated a table in blazing sunshine. French psychic researcher Dr. Joseph Maxwell was probably right in stating that the action of light is not such as to constitute an insurmountable obstacle to the production of telekinetic movements.

The supposed problem of light was highlighted in an incident reported in the issue of Psychic Research (January 1930). According to a communication by Irving Gaertner of St. Louis, Missouri, in a sitting with Eveling Burnside and Myrtle Larsen in Camp Chesterfield, Indiana, a ray of light, owing to the turning of a switch outside, penetrated through a crack between the lower edge of the door and the floor into the séance room.

"Agonized groans were heard (presumably from the entranced medium, Mrs. Larsen) and one of the two trumpets which had been levitated for the voice immediately fell at the feet of Mr. Nelson. At the same moment, Mrs. Nelson received an electric shock which formed a blister on one of her fingers, resembling one which would be produced by a burning of the skin. All the sitters testified to having felt the electric shock both in the region of the solar plexus, the back and the forehead."

Larsen was reportedly discovered prostrate on the floor, minus any heartbeat and her body rigid. It took considerable effort to restore her to consciousness. Burnside, the other medium, suffered from the shock for several days after the sitting. Frederick Bligh Bond, editor of Psychic Research, speculated about the nature of the electric shock: "Is it the light, qua light, which in this case causes the violent disturbance of conditions, or is it light as an avenue of conductivity, linking the psychic circuit to the current on the wires of the lamp in the hall?"

The dangers of the shock from unexpected light were considered an interesting matter in J. Hewat McKenzie 's report on the mediumship of Ada Besinnet in the April 1922 issue of Psychic Science. The smallest red spark burning was sufficient to prevent the medium from going into trance.

"Upon another occasion, when drawing the electric plug from the wall socket, behind a piece of furniture, and about 8 feet from the medium, the small spark, about 1/16 inch long, which usually accompanies the withdrawal of a plug of this kind when the power is on, was sufficient to create such a psychic shock that the medium immediately fell forward on the table in a cataleptic state."

That psychic structures may objectively exist beyond the range of our optical capacity was demonstrated by quartz lens photography. The quartz lens transmits ultra-violet rays to make visible on the photographic plate things not visible to the eyes. Mrs. J. H. McKenzie and Major Mowbray experimented in this field with the mediums J. Lynn and Lewis. The quartz lens not only disclosed fluorescing lights; vibrating, spinning substances; and psychic rods, but also the dematerialization of the medium's hand when added force had to be borrowed.

Similar results were achieved by Daniel Frost Comstock in séances with "Margery the Medium" (Mina Stinson Crandon) in Boston. Several of his exposed plates showed curious, indefinable white patches, one of which was fairly recognizable as a human face, although it could not be identified. The most important advance in this field of research was registered at the Institut Métapsychique International in Paris with the mediumship of Rudi Schneider in 1931.

Over the first half of the twentieth century, critics claimed that the alleged destructive effect of light on psychic phenomena and the health of the medium were a subterfuge to cover fraud in the darkness of the séance room. In no case was any true physical harm done to mediums by the shining of light, and over the long run, physical mediumship of the type popular in the early twentieth century disappeared under the scrutiny of psychic researchers and the continued improvement of observational techniques.

Electromagnetic radiation in a band of frequencies that can be received by the human eye.

1) Refers to a player being short of a complete bet.
2) Refers to a player who has not anted.
3) Refers to sitting down at a table to play.

SoundPoker Says: 1) For example, if a player is “light by $20” it means they are $20 short of a full bet.
2) For example, when a player asks “Who’s light?”, they are asking who has not anted into the pot yet.
3) Part of the phrase “Light and fight.”

See Also: Ante, Full Bet Rule, Half Bet Rule

Quotes:

"The pursuit of perfection, then, is the pursuit of sweetness and light." - Matthew Arnold

"The light shines in the darkness and the darkness comprehended it not." - Bible

"It's not necessary to blow out your neighbor's light to let your own shine." - M. R. Dehaan

"Light is the first of painters. There is no object so foul that intense light will not make it beautiful." - Ralph Waldo Emerson

"Give light, and the darkness will disappear of itself." - Desiderius Erasmus

"You can't have a light without a dark to stick it in." - Arlo Guthrie

See more famous quotes about Light

The type of electromagnetic wave that is visible to the human eye. Visible light runs along a spectrum from the short wavelengths of violet to the longer wavelengths of red. (See photon.)

1. electromagnetic radiation capable of producing a visual sensation in the eye. It comprises wavelengths in the range 380 — 780 nm (frequencies 385 — 789 THz), although the term is sometimes loosely extended to include some ultraviolet and infrared radiation in adjacent parts of the spectrum.
2. (in chemistry) describing any (especially metallic) element whose density is relatively small (usually <5000 kg m⁻³).
3. (in physics) of less than the usual mass. Compare heavy.

• Beautiful, attractive, or well-formed
• Optics - light: electromagnetic radiation, ranging in wavelength from 4 x 10^-7 to 7 x 10^-7 meter, that causes sensation of vision
• Lamps and Mirrors - light: device for illumination by electricity
• Newspaper Names
• Directions - light: arrive at and come to rest
• Affectionate Reference
• Active, Lively, Awake, or Limber
• Special, Bright, Skillful, or Unusual
• New Age - light: positive energy; the source

"Visible light" redirects here. For light that cannot be seen with human eye, see Electromagnetic radiation. For other uses, see Light (disambiguation) and Visible light (disambiguation).

The Sun is Earth's primary source of light. About 44% of the sun's electromagnetic radiation that reaches the ground is in the visible light range.

Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.^[1] Visible light has a wavelength in the range of about 380 nanometres to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.

Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR) is experimentally found to move at exactly this same speed in vacuum.

In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.^[2][3] This article focuses on visible light. See the electromagnetic radiation article for the general term.

Contents 1 Speed of visible light 2 Electromagnetic spectrum and visible light 3 Optics 3.1 Refraction 4 Light sources 5 Units and measures 6 Light pressure 7 Historical theories about light, in chronological order 7.1 Classical Greece and Hellenism 7.2 Classical India 7.3 Descartes 7.4 Particle theory 7.5 Wave theory 7.6 Quantum theory 7.7 Electromagnetic theory as explanation for all types of visible light and all EM radiation 8 See also 9 Notes 10 References

Speed of visible light

Main article: Speed of light

The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation are believed to move at exactly this same speed in vacuum.

Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.^[4] Unfortunately, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.

Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.

The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. For example the speed of light in water is about 3/4 of that in vacuum. However, the slowing process in matter is thought to result not from actual slowing of particles of light, but rather from their absorption and re-emission from charged particles in matter.

As an extreme example of the nature of light-slowing in matter, two independent teams of physicists were able to bring light to a "complete standstill" by passing it through a Bose-Einstein Condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.^[5] However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.

Electromagnetic spectrum and visible light

Main article: Electromagnetic spectrum

Electromagnetic spectrum with light highlighted

Generally, EM radiation, or EMR (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region that we perceive as light, ultraviolet, X-rays and gamma rays.

The behaviour of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.

ERM in the visible light region consists of quanta (called photons) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which lead to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humansinfrared because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina. This change triggers the sensation of vision.

There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, and this is how living animals detect it.

Above the range of visible light, ultraviolet light becomes invisible to humans mostly because it is absorbed by the tissues of the eye and in particular the lens. Humans with natural eye lenses removed, as well as many animals with eyes that do not require lenses (such as insects and shrimp) are able to directly detect ultraviolet visually, by quantum photon-absorption mechanisms, in much the same chemical way that normal humans detect visible light.

Optics

Main article: Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light.

Refraction

Main article: Refraction

An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.

A cloud illuminated by sunlight

Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by Snell's Law:

where is the angle between the ray and the surface normal in the first medium, is the angle between the ray and the surface normal in the second medium, and n₁ and n₂ are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.

When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.

The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.

Light sources

See also: List of light sources

There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight, the radiation emitted by the chromosphere of the Sun at around 6,000 Kelvin peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units ^[6] and roughly 44% of sunlight energy that reaches the ground is visible.^[7] Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum. The peak of the blackbody spectrum is in the deep infrared, at about 10 micrometer wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is heated to "red hot" or "white hot". Blue-white thermal emission is not often seen, except in stars (the commonly seen pure-blue colour in a gas flame or a welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm, and is not seen in stars or pure thermal radiation)).

Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.

Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.

Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.

A city illuminated by artificial lighting

Certain other mechanisms can produce light:

• Bioluminescence
• Cherenkov radiation
• Electroluminescence
• Scintillation
• Sonoluminescence
• triboluminescence

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

• Particle–antiparticle annihilation
• Radioactive decay

Units and measures

Main articles: Photometry (optics) and Radiometry

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.

Table 1. SI radiometry units

• v
• t
• e

Quantity	Symbol[nb 1]	SI unit	Symbol	Dimension	Notes
Radiant energy	Qe[nb 2]	joule	J	M⋅L2⋅T−2	energy
Radiant flux	Φe[nb 2]	watt	W	M⋅L2⋅T−3	radiant energy per unit time, also called radiant power.
Spectral power	Φeλ[nb 2][nb 3]	watt per metre	W⋅m−1	M⋅L⋅T−3	radiant power per wavelength.
Radiant intensity	Ie	watt per steradian	W⋅sr−1	M⋅L2⋅T−3	power per unit solid angle.
Spectral intensity	Ieλ[nb 3]	watt per steradian per metre	W⋅sr−1⋅m−1	M⋅L⋅T−3	radiant intensity per wavelength.
Radiance	Le	watt per steradian per square metre	W⋅sr−1⋅m−2	M⋅T−3	power per unit solid angle per unit projected source area. confusingly called "intensity" in some other fields of study.
Spectral radiance	Leλ[nb 3] or Leν[nb 4]	watt per steradian per metre3 or watt per steradian per square metre per hertz	W⋅sr−1⋅m−3 or W⋅sr−1⋅m−2⋅Hz−1	M⋅L−1⋅T−3 or M⋅T−2	commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.
Irradiance	Ee[nb 2]	watt per square metre	W⋅m−2	M⋅T−3	power incident on a surface, also called radiant flux density. sometimes confusingly called "intensity" as well.
Spectral irradiance	Eeλ[nb 3] or Eeν[nb 4]	watt per metre3 or watt per square metre per hertz	W⋅m−3 or W⋅m−2⋅Hz−1	M⋅L−1⋅T−3 or M⋅T−2	commonly measured in W⋅m−2⋅nm−1 or 10−22W⋅m−2⋅Hz−1, known as solar flux unit.[nb 5]
Radiant exitance / Radiant emittance	Me[nb 2]	watt per square metre	W⋅m−2	M⋅T−3	power emitted from a surface.
Spectral radiant exitance / Spectral radiant emittance	Meλ[nb 3] or Meν[nb 4]	watt per metre3 or watt per square metre per hertz	W⋅m−3 or W⋅m−2⋅Hz−1	M⋅L−1⋅T−3 or M⋅T−2	power emitted from a surface per wavelength or frequency.
Radiosity	Je or Jeλ[nb 3]	watt per square metre	W⋅m−2	M⋅T−3	emitted plus reflected power leaving a surface.
Radiant exposure	He	joule per square metre	J⋅m−2	M⋅T−2
Radiant energy density	ωe	joule per metre3	J⋅m−3	M⋅L−1⋅T−2
See also: SI · Radiometry · Photometry · (Compare)

Table 2. SI photometry units

• v
• t
• e

Quantity	Symbol[nb 6]	SI unit	Symbol	Dimension	Notes
Luminous energy	Qv [nb 7]	lumen second	lm⋅s	T⋅J [nb 8]	units are sometimes called talbots
Luminous flux	Φv [nb 7]	lumen (= cd⋅sr)	lm	J	also called luminous power
Luminous intensity	Iv	candela (= lm/sr)	cd	J	an SI base unit, luminous flux per unit solid angle
Luminance	Lv	candela per square metre	cd/m2	L−2⋅J	units are sometimes called nits
Illuminance	Ev	lux (= lm/m2)	lx	L−2⋅J	used for light incident on a surface
Luminous emittance	Mv	lux (= lm/m2)	lx	L−2⋅J	used for light emitted from a surface
Luminous exposure	Hv	lux second	lx⋅s	L−2⋅T⋅J
Luminous energy density	ωv	lumen second per metre3	lm⋅s⋅m−3	L−3⋅T⋅J
Luminous efficacy	η [nb 7]	lumen per watt	lm/W	M−1⋅L−2⋅T3⋅J	ratio of luminous flux to radiant flux
Luminous efficiency	V			1	also called luminous coefficient
See also: SI · Photometry · Radiometry · (Compare)

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The cone cells in the human eye are of three types which respond differently across the visible spectrum, and the cumulative response peaks at a wavelength of around 555 nm. Therefore, two sources of light which produce the same intensity (W/m²) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw power by a quantity called luminous efficacy, and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and charge-coupled devices (CCD) tend to respond to some infrared, ultraviolet or both.

Light pressure

Main article: Radiation pressure

Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by c, the speed of light.  Due to the magnitude of c, the effect of light pressure is negligible for everyday objects.  For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U. S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.^[8]  However, in nanometer-scale applications such as NEMS, the effect of light pressure is more significant, and exploiting light pressure to drive NEMS mechanisms and to flip nanometer-scale physical switches in integrated circuits is an active area of research.^[9]

At larger scales, light pressure can cause asteroids to spin faster,^[10] acting on their irregular shapes as on the vanes of a windmill.  The possibility to make solar sails that would accelerate spaceships in space is also under investigation.^[11][12]

Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.^[13] This should not be confused with the Nichols radiometer, in which the (slight) motion caused by torque (though not enough for full rotation against friction) is directly caused by light pressure.^[14]

Historical theories about light, in chronological order

Classical Greece and Hellenism

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In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.

In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:

"The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." – On the nature of the Universe

Despite being similar to later particle theories, Lucretius's views were not generally accepted.

Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics.^[15]

Classical India

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In ancient India, the Hindu schools of Samkhya and Vaisheshika, from around the early centuries CE developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.

On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivi), water (pani), fire (agni), and air (vayu) Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms.^{[citation needed]} The Vishnu Purana refers to sunlight as "the seven rays of the sun".^{[citation needed]}

The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy.^{[citation needed]}

Descartes

René Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Witelo as well as the "species" of Bacon, Grosseteste, and Kepler.^[16] In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves.^{[citation needed]} Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media.

Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as the start of modern physical optics.^[17]

Particle theory

Main article: Corpuscular theory of light

Pierre Gassendi.

Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.

Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words it would become what is now called a black hole. Laplace withdrew his suggestion later, after a wave theory of light became firmly established as the model for light (as has been explained, neither a particle or wave theory is fully correct). A translation of Newton's essay on light appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

Thomas Young's sketch of the two-slit experiment showing the diffraction of light. Young's experiments supported the theory that light consists of waves.

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarised, if it were a transverse wave. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye.

Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.

Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. By the year 1821, Fresnel was able to show via mathematical methods that polarisation could be explained only by the wave theory of light and only if light was entirely transverse, with no longitudinal vibration whatsoever.

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850.^[18] His result supported the wave theory, and the classical particle theory was finally abandoned, only to partly re-emerge in the 20th century.

Quantum theory

In 1900 Max Planck, attempting to explain black body radiation suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "quanta" (from a Latin word for "how much." In 1905, Albert Einstein used the idea of light quanta to explain the photoelectric effect, and suggested that these light quanta had a "real" existence. These light particles were named photons in In 1923 Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called Compton scattering) could be explained by a particle-theory of X-rays but not a wave theory. In 1926 Gilbert N. Lewis named these liqht quanta photons.

Eventually the modern theory of quantum quantum mechanics came to picture light as (in some sense) both a particle and a wave, and (in another sense), as a phenomenon which is neither a particle or a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, modern physics sees light as something that can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles), and sometimes another macroscopic metaphor (water waves), but is actually something that cannot be fully imagined. As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both.

Electromagnetic theory as explanation for all types of visible light and all EM radiation

Main article: Electromagnetic radiation

A linearly polarised light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion (a transverse wave).

In 1845, Michael Faraday discovered that the plane of polarisation of linearly polarised light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation.^[19] This was the first evidence that light was related to electromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.^[20] Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.

Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.

In the quantum theory, photons are seen as wave packets of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as spectral lines).

See also

Wikimedia Commons has media related to: Light

Look up light in Wiktionary, the free dictionary.

Wikiquote has a collection of quotations related to: Light

• Automotive lighting
• Ballistic photon
• Color temperature
• Electromagnetic spectrum
• Fermat's principle
• Huygens' principle
• International Commission on Illumination
• Journal of Luminescence
• Light beam – in particular about light beams visible from the side
• Light Fantastic (TV series)
• Light mill
• Light pollution
• Light therapy
• Lighting
• Luminescence: The Journal of Biological and Chemical Luminescence
• Photic sneeze reflex
• Photometry
• Photon
• Rights of Light
• Risks and benefits of sun exposure
• Spectroscopy
• Visible spectrum
• Wave–particle duality

Notes

1. ^ Standards organizations recommend that radiometric quantities should be denoted with a suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
2. ^ ^{a b c d e} Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant emittance.
3. ^ ^{a b c d e f} Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek) to indicate a spectral concentration. Spectral functions of wavelength are indicated by "(λ)" in parentheses instead, for example in spectral transmittance, reflectance and responsivity.
4. ^ ^{a b c} Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek)—not to be confused with the suffix "v" (for "visual") indicating a photometric quantity.
5. ^ NOAA / Space Weather Prediction Center includes a definition of the solar flux unit (SFU).
6. ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
7. ^ ^{a b c} Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
8. ^ "J" is the recommended symbol for the dimension of luminous intensity in the International System of Units.

References

1. ^ CIE (1987). International Lighting Vocabulary. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.
   By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.”
2. ^ Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 978-0-8194-6093-6, http://books.google.com/?id=6mb0C0cFCEYC&pg=PA4 
3. ^ Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN 978-81-7008-592-8, http://books.google.com/?id=IryMtwHHngIC&pg=PA1416#v=onepage&q= 
4. ^ Scientific Method, Statistical Method and the Speed of Light. Statistical Science 2000, Vol. 15, No. 3, 254–278
5. ^ Harvard News Office (2001-01-24). "Harvard Gazette: Researchers now able to stop, restart light". News.harvard.edu. http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html. Retrieved 2011-11-08. 
6. ^ http://thulescientific.com/LYNCH%20&%20Soffer%20OPN%201999.pdf
7. ^ "Reference Solar Spectral Irradiance: Air Mass 1.5". http://rredc.nrel.gov/solar/spectra/am1.5/. Retrieved 2009-11-12. 
8. ^ Tang, Hong X. (October 2009), "May the Force of Light Be with You", IEEE Spectrum: pp. 41 – 45, http://www.spectrum.ieee.org/semiconductors/devices/photonics-breakthrough-for-silicon-chips, retrieved 7 September 2010 .
9. ^ See, for example, nano-opto-mechanical systems research at Yale University.
10. ^ Kathy A. (2004-02-05). "Asteroids Get Spun By the Sun". Discover Magazine. http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/. 
11. ^ "Solar Sails Could Send Spacecraft 'Sailing' Through Space". NASA. 2004-08-31. http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html. 
12. ^ "NASA team successfully deploys two solar sail systems". NASA. 2004-08-09. http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html. 
13. ^ P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).
14. ^ Nichols, E.F & Hull, G.F. (1903) The Pressure due to Radiation, The Astrophysical Journal,Vol.17 No.5, p.315–351.
15. ^ Ptolemy and A. Mark Smith (1996), Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary, Diane Publishing, p. 23, ISBN 0-87169-862-5 
16. ^ Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0-521-28436-8, ISBN 978-0-521-28436-3
17. ^ 'Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0-521-28436-8, ISBN 978-0-521-28436-3
18. ^ David Cassidy, Gerald Holton, James Rutherford (2002), Understanding Physics, Birkhäuser, ISBN 0-387-98756-8, http://books.google.com/?id=rpQo7f9F1xUC&pg=PA382 
19. ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87.
20. ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87

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Dansk (Danish)
1.
n. - lys, skær, dag, dagslys, lysåbning, rude, fyr, fyrtårn, ild, tændstik
v. tr. - tænde, oplyse, lede, føre
v. intr. - tændes
adj. - lys, klar

idioms:

• bring to light    bringe frem for dagens lys, trække frem i lyset
• come to light    komme for dagen, komme for en dag
• hide one's light under a bushel    sætte sit lys under en skæppe
• in a bad light    i et dårligt lys
• in the light of    i lyset af, på baggrund af, i betragtning af, i relation til
• light bulb    elektrisk pære
• light dawns on someone    der går et lys op for nogen
• light of one's life    sit livs lys
• light pen    lyspen
• light stylus    lyspen
• light up    lyse op, oplyse
• light woman    letsindig kvinde, tomhjernet kvinde
• light year    lysår
• out like a light    gå ud som et lys
• pedestrian-crossing lights    lys i fodgængerovergang
• set light to    sætte ild til
• the light at the end of the tunnel    der er lys for enden af tunnellen, der er lys forude

2.
adj. - let, mild, ubetydelig, ringe, letsindig, tankeløs, overfladisk, tomhjernet, letbevæbnet
adv. - let, mildt, ringe, letsindigt, tankeløst

idioms:

• light aircraft    lille fly
• light in the head    ør i hovedet, svimmel, tankeløs, overgiven
• light industry    let industri
• light into    angribe
• light on    træffe på, støde på
• light opera    operette
• light out    bryde op, stikke af
• light reading    let læsning
• light sleeper    person der sover let
• light upon    falde over
• make light of    ikke tage alvorligt, bagatellisere
• make light work of it    gøre det let for sig selv

3.
v. intr. - lande, dale ned, sætte sig

Nederlands (Dutch)
opsteken, verlichten, bijlichten, aansteken, ontsteken, uitstappen, aan wal gaan, licht, verlichting, schijnsel, vuurtje (voor sigaret), lamp, (mv) koplichten, (mv) verkeerslichten, venster/bovenlicht, inzicht, ster, knappe kop, zaklantaarn, licht-, weinig calorieën bevattend, lichtgewicht, lichtbewapend, gemakkelijk, luchtig, lichtzinnig, met weinig bagage, helder

Français (French)
1.
n. - (Phys) radiation électromagnétique, lumière, clarté, lueur (dans les yeux), réverbère, feu (sur un bateau), (Aut) phare, feu arrière, veilleuse, flamme, (fig) jour, angle, aspect, lumière de (d'un témoignage), vue (arch), (fig) découverte (d'une vérité), (Transp) feux (npl), illuminations (npl)
v. tr. - allumer, éclairer, illuminer
v. intr. - s'allumer, s'éclairer, s'illuminer
adj. - léger, faible, meuble (une terre)

idioms:

• bring to light    mettre en lumière, révéler
• come to light    être découvert, être dévoilé
• hide one's light under a bushel    cacher son talent
• in a bad light    sous un mauvais jour
• in light of    compte-tenu de, à cause de, en prenant considération de
• in the light of    à la lumière de, selon
• light at the end of the tunnel    la lumière au bout du tunnel
• light bulb    ampoule
• light dawns on someone    comprendre la vérité
• light of one's life    son rayon de soleil
• light pen    (Comput) crayon optique
• light stylus    (Comput) crayon optique
• light up    allumer, s'allumer, (fig) s'éclairer, briller de joie (les yeux)
• light woman    femme légère
• light year    (Astron) année-lumière
• out like a light    (s'endormir) tout de suite
• pedestrian-crossing lights    feu pour piétons
• set light to    mettre feu à

2.
adj. - clair, blond, léger, modéré (un buveur), peu actif (une affaire), léger (une sentence), délicat (un mouvement), peu fatigant (un travail), facile (à lire), pas sérieux (une liaison), enjoué (une humeur), (Culin) allégé, light
adv. - légèrement, sans chargement (une locomotive)

idioms:

• be light on    ne pas être sévère sur (une sanction)
• go light on    y aller mollo
• light aircraft    avion léger
• light in the head    léger dans sa tête
• light industry    industrie légère
• light into    tomber dessus, rentrer dedans (qn)
• light on    rencontrer (qn), trouver (qn) par hasard
• light opera    opérette
• light out    (US) décamper
• light reading    lecture facile
• light sleeper    (avoir) le sommeil léger
• light upon    tomber sur (qn) par hasard
• make light of    traiter (qch) à la légère, attacher peu d'importance à
• make light work of    faire qch sans peine

3.
v. intr. - descendre de (d'un cheval, d'une voiture), se poser sur (un regard), se poser sur (une branche), tomber sur, s'abattre sur, s'enfuir rapidement (arch)

Deutsch (German)
1.
n. - Licht, Lichtschein, Ampel, Leuchtfeuer, Feuer, Fenster, Erleuchtung
v. - beleuchten, erhellen, anzünden, brennen, (auf)leuchten
adj. - hell, licht

idioms:

• bring to light    aufdecken
• come to light    ans Licht kommen
• hide one's light under a bushel    sein Licht unter den Scheffel stellen
• in a bad light    in einem schlechten Licht
• in light of    angesichts
• in the light of    im Licht
• light at the end of the tunnel    Licht am Ende des Tunnels
• light bulb    Glühbirne
• light dawns on someone    jmdm. geht ein Licht auf
• light of one's life    Sonne des Lebens
• light pen    Lichtstift
• light stylus    Lichtstift
• light up    erhellen, aufleuchten, sich eine Zigarette anzünden
• light woman    leichtes Mädchen
• light year    Lichtjahr
• out like a light    sofort weg
• pedestrian-crossing lights    Fußgängerampel
• set light to    etwas anzünden

2.
adj. - leicht, mild, gering, leichtfertig, behende
adv. - leicht, nicht schwer

idioms:

• be light on    knapp
• go light on    [auf etw. (+Akk)] kommen od. stoßen
• light aircraft    leichtes Flugzeug
• light in the head    leicht benommen
• light industry    Leichtindustrie
• light into    jmdn. angreifen
• light on    auf etwas stoßen
• light opera    Operette
• light out    fortgehen
• light reading    leichte Lektüre
• light sleeper    jmd. der einen leichten Schlaf hat
• light upon    auf etwas stoßen
• make light of    auf die leichte Schulter nehmen
• make light work of    leicht damit fertigwerden

3.
v. - stoßen auf, absteigen, sich niederlassen

Ελληνική (Greek)
n. - φως, λάμψη, φωτιά, λάμπα, κερί, φωτιά για άναμμα τσιγάρου, φωτισμός, σκοπιά, άποψη, όψη, φεγγίτης, παράθυρο
v. - ανάβω, φωτίζω/-ομαι, αστράφτω, λάμπω, διαφωτίζω, ελαφρώνω, αστράφτω, αποβιβάζομαι, αφιππεύω
adj. - ελαφρός, ανάλαφρος, ανοιχτόχρωμος, απαλός, ευκολονόητος, εύθυμος
adv. - με ελάχιστες αποσκευές

idioms:

• bring to light    φέρνω στο φως
• come to light    έρχομαι στο φως
• hide one's light under a bushel    είμαι υπερβολικά μετριόφρων
• in a bad light    δυσμενώς
• in the light of    υπό το φως
• light aircraft    ελαφρά αεροσκάφη
• light bulb    λάμπα, λαμπτήρας
• light dawns on someone    (κάποτε επιτέλους) καταλαβαίνω ότι
• light in the head    ζαλισμένος
• light industry    ελαφρά βιομηχανία
• light into    επιτίθεμαι σε, καταφέρομαι εναντίον
• light of one's life    φως της ζωής μου
• light on    κάθομαι σε
• light opera    οπερέτα, ελαφρά όπερα
• light out    φεύγω βιαστικά
• light pen    (Η/Υ) φωτεινή πένα
• light reading    ελαφρό ανάγνωσμα
• light sleeper    αυτός που κοιμάται ελαφρά
• light stylus    (Η/Υ) φωτεινή πένα/γραφίδα
• light up    ανάβω, φωτίζω/-ομαι, λάμπω, ακτινοβολώ, αστράφτω, διαφωτίζω
• light upon    ανακαλύπτω, "πέφτω πάνω σε", βρίσκω τυχαία
• light woman    ελαφριά (γυναίκα)
• light year    έτος φωτός
• make light of    παίρνω αψήφιστα
• make light work of it    ξεμπερδεύω στα γρήγορα
• out like a light    (αποκοιμιέμαι) μονομιάς
• pedestrian-crossing lights    φανάρια για πεζούς
• set light to    βάζω φωτιά σε
• the light at the end of the tunnel    φως στην άκρη του τούνελ

Italiano (Italian)
illuminare, accendere, schiarirsi, luce, lume, fanale, illuminazione, leggero, facile, chiaro

idioms:

• bring to light    svelare
• come to light    viene rivelato
• in a bad light    in cattiva luce
• in the light of    alla luce di
• light aircraft    aereo leggero
• light bulb    lampadina
• light dawns on someone    qualcuno comincia a capire
• light in the head    idea luminosa
• light industry    industria leggera
• light into    attaccare
• light of one's life    luce degli occhi
• light on/upon    imbattersi in
• light opera    operetta
• light out    partire
• light reading    letture leggere
• light sleeper    dal sonno leggero
• light up    ubriacarsi
• light upon    imbattersi
• light woman    donna leggera
• light year    anno luce
• make light of    dare poca importanza a
• out like a light    addormentato (o svenuto) di colpo
• pedestrian-crossing lights    semaforo pedonale
• phased traffic lights    semafori sincronizzati
• rear light    fanale posteriore
• set light/fire to    dar fuoco a
• shed/throw/cast light on    fare luce su
• street light    lampione
• the light at the end of the tunnel    un lumicino nel buio
• unit of light    unità di luce

Português (Portuguese)
n. - luz (f), iluminação (f), fogo (m) (para acender cigarro), clarabóia (f), compreensão (f), esclarecimento (m)
v. - iluminar, acender, animar
adj. - leve, suave, vivo, superficial
adv. - facilmente

idioms:

• bring to light    elucidar
• come to light    desvendar-se
• in a bad light    tornar-se uma pessoa ruim
• in the light of    à luz de
• light aircraft    avião leve
• light bulb    lâmpada incandescente
• light dawns on someone    alguém que finalmente entende alguma coisa
• light industry    indústria da energia elétrica
• light into    ofender alguém
• light of one's life    luz da vida (de alguém)
• light on/upon    topar com, encontrar-se por acaso
• light opera    ópera de natureza alegre ou cômica
• light out    (gír.) escapar, sumir
• light reading    leitura leve
• light sleeper    um sono leve
• light up    acender (cigarro), iluminar, alegrar
• light upon    uma luz sobre
• light woman    mulher tranqüila
• light year    ano-luz
• make light of    não levar alguém à sério
• out like a light    estar inconsciente
• pedestrian-crossing lights    farol de pedestres
• set light to    atear fogo em alguma coisa
• the light at the end of the tunnel    a luz no fim do túnel

Русский (Russian)
свет, огонь, информация, аспект, знаменитость, зажигать, светить, освещать, светлый, легкий, нетрудный, легко

idioms:

• bring to light    обнаружить, вывести на чистую воду
• come to light    обнаруживаться, выявляться
• in a bad light    в невыгодном свете
• in the light of    в свете чего-л.
• light aircraft    легкий самолет
• light bulb    электролампочка
• light dawns on someone    внезапно понять что-л.
• light in the head    ветер в голове
• light industry    легкая промышленность
• light into    нападать, набрасываться
• light of one's life    светочь жизни
• light on/upon    неожиданно натолкнуться на что-л.
• light opera    оперетта
• light out    убегать
• light reading    развлекательная литература
• light sleeper    чутко спящий человек
• light up    освещать, светиться
• light upon    обнаруживать
• light woman    женщина легкого поведения
• light year    световой год
• make light of    недооценивать что-л.
• out like a light    немедленно заснул
• pedestrian-crossing lights    знак пешеходного перехода
• phased traffic lights    система светофоров
• set light/fire to    зажечь, поджечь
• shed/throw/cast light on    пролить свет на что-л.
• the light at the end of the tunnel    свет в конце туннеля

Español (Spanish)
1.
n. - luz, lumbre, resplandor, lámpara, faro, iluminación, energía lumínica, energía radiante, encendedor, (fig) líder destacado, comprensión
v. tr. - alumbrar, iluminar, encender, clarear, dar brillo
v. intr. - encenderse, iluminarse, brillar
adj. - alumbrado, claro, de luz, luminoso

idioms:

• bring to light    sacar a luz, descubrir
• come to light    descubrirse, salir a luz
• hide one's light under a bushel    ser muy modesto
• in a bad light    aspecto negativo
• in light of    a la luz de, en vista de que
• in the light of    a la luz de
• light at the end of the tunnel    la luz al final del túnel, vislumbrar el fin de sus problemas, solución
• light bulb    bombilla, foco
• light dawns on someone    caer en la cuenta, comprender
• light of one's life    la luz de la vida de uno
• light pen    (comp) lápiz óptico
• light stylus    (comp) lápiz óptico
• light up    iluminar, iluminarse, prender, encender (cigarro, etc.)
• light woman    mujer ligera de cascos
• light year    año luz
• out like a light    dormirse en seguida, caerse redondo
• pedestrian-crossing lights    semáforo para peatones
• set light to    pegar fuego a, encender

2.
adj. - de poco peso, leve, liviano, ligero, superficial, suave
adv. - a la ligera, con poca carga

idioms:

• be light on    dar con, tropezar con, suceder por casualidad
• go light on    emborracharse
• light aircraft    avión ligero, avioneta
• light in the head    ligero de cascos
• light industry    industria ligera
• light into    embestir, atacar
• light on    dar con, tropezar con
• light opera    opereta, zarzuela
• light out    largarse, poner pies en polvorosa
• light reading    lecturas de entretenimiento
• light sleeper    de sueño ligero
• light upon    posarse, dar con, tropezar con
• make light of    no dar importancia a, no tomar en serio
• make light work of    despachar algo rápidamente, hacer algo sin mucho esfuerzo

3.
v. intr. - posarse, descender, caer sobre, arremeter contra, increpar duramente, salir de pronto, suceder inesperadamente

Svenska (Swedish)
n. - ljus, belysning, dagsljus, dager, lampa, låga, tändsticka, klarhet, (sjö.) fyr, lanterna, ljusöppning, fönster, glasruta (i drivhus), (konst.) ljusparti (på tavla), (sl.) krita, kredit
v. - tända, lysa upp, belysa, förse med belysning, lysa ngn (på väg), tändas, ta eld, (börj
adj. - lätt, för lätt, inte fullviktig, underviktig, lätt lastad, som är avsedd för lätt last, (sjö.) utan last, tom, (mil.) lätt, lös, sandig, oviktig, obetydlig, ringa, lindrig, tanklös, lättsinnig, ytlig, flyktig, lättledd, lätt (färdig), (typ.) mager
adv. - lätt

中文（简体）(Chinese (Simplified))
1. 光, 光亮, 光线, 光源, 点, 照亮, 点燃, 使容光焕发, 点着, 变亮, 明亮的, 浅色的

idioms:

• bring to light    发现, 揭发
• come to light    被发现, 众所周知
• hide one's light under a bushel    不露锋芒
• in a bad light    在暗处, 看...坏的方面
• in the light of    根据, 依照
• light aircraft    轻型飞机, 小型飞机
• light bulb    灯泡
• light dawns on someone    某人茅塞顿开
• light in the head    头晕的, 愚蠢的
• light industry    轻工业
• light into    痛打, 大吃
• light of one's life    心爱的人, 亲爱者, 生命之光
• light on    停落于
• light opera    光动电驿, 轻歌剧
• light out    匆匆离去
• light pen    光笔
• light reading    轻松读物
• light sleeper    睡眠中易被惊醒的人
• light stylus    光笔
• light up    点燃, 照亮
• light upon    停落于, 想到
• light woman    轻浮女人, 荡妇
• light year    光年
• make light of    轻视
• make light work of it    轻松解决某事
• out like a light    很快入睡, 立刻昏厥
• pedestrian-crossing lights    人行横道灯
• set light to    点燃
• the light at the end of the tunnel    成功之望

2. 明亮的, 浅色的, 轻地, 轻装地, 点着, 变亮

中文（繁體）(Chinese (Traditional))
1.
n. - 光, 光亮, 光線, 光源
v. tr. - 點, 照亮, 點燃, 使容光煥發
v. intr. - 點著, 變亮
adj. - 明亮的, 淺色的

idioms:

• bring to light    發現, 揭發
• come to light    被發現, 眾所周知
• hide one's light under a bushel    不露鋒芒
• in a bad light    在暗處, 看...壞的方面
• in the light of    根據, 依照
• light aircraft    輕型飛機, 小型飛機
• light bulb    燈泡
• light dawns on someone    某人茅塞頓開
• light in the head    頭暈的, 愚蠢的
• light industry    輕工業
• light into    痛打, 大吃
• light of one's life    心愛的人, 親愛者, 生命之光
• light on    停落於
• light opera    光動電驛, 輕歌劇
• light out    匆匆離去
• light pen    光筆
• light reading    輕鬆讀物
• light sleeper    睡眠中易被驚醒的人
• light stylus    光筆
• light up    點燃, 照亮
• light upon    停落於, 想到
• light woman    輕浮女人, 蕩婦
• light year    光年
• make light of    輕視
• make light work of it    輕鬆解決某事
• out like a light    很快入睡, 立刻昏厥
• pedestrian-crossing lights    人行橫道燈
• set light to    點燃
• the light at the end of the tunnel    成功之望

2.
adj. - 明亮的, 淺色的
adv. - 輕地, 輕裝地
v. intr. - 點著, 變亮

한국어 (Korean)
1.
n. - 빛, 밝기, 가시광선, 주간, 불꽃, 발견 , 대가
v. tr. - 점화하다, 조명하다, 밝게 하다, 등불을 밝히고 안내하다
v. intr. - 불이 붙다, 밝아지다, 빛나다
adj. - 밝은, 연한, 흰 빛을 띤

idioms:

• bring to light    드러내다, 폭로하다
• come to light    밝은 데로 나오다, 나타내다
• in a bad light    잘 보이지 않는 곳에, 불리한 위치에
• in the light of    관점, ~을 고려하여
• light up    밝게 하다, 비추다
• set light to    불을 일으키다
• the light at the end of the tunnel    어려운 일의 종말의 기미

2.
adj. - 가벼운, 적은 , 불명확한, 너그러운, 경쾌한, 경솔한
adv. - 가볍게 , 경쾌하게, 짐을 가지지 않고

idioms:

• light into    욕하다, 꾸짖다
• light on    우연히 발견하다, ~과 맞닥뜨리다, 떨어지다
• light out    총총히 떠나다, 급히 떠나다
• light upon    우연히 발견하다
• make light of    ~을 경시하다, 얕보다
• make light work of it    빠르고 쉽게 성취하다

3.
v. intr. - 내리다, 머물다

日本語 (Japanese)
n. - 光, 明かり, 明るさ, 日中, 日の明かり, 火, 灯火, 信号灯, 輝き, 見方, 光明, 日光, 知識, 脚光, マッチ
adj. - 明るい, 薄い, 軽い, 量目不足の, 容易な, 肩の凝らない, 少しの, もたれない, 弱い, 軽快な, 浮ついた
adv. - 軽く, 身軽に, 軽装で
v. - 火をつける, 火がつく, 明るくする, 明るくなる, 降りる

idioms:

• come to light    明るみに出る
• in the light of    照らして, …として
• light aircraft    軽飛行機
• light bulb    電球
• light dawns on someone    人を叱る
• light in the head    めまいがする, ばかな
• light industry    軽工業
• light into    激しく攻撃する, 叱る
• light of one's life    人生の光明
• light on/upon    十分でなく
• light opera    軽喜歌劇
• light out    全速で走る, 急いで立ち去る, 逃げ出す
• light reading    軽い読み物
• light sleeper    眠りの浅い人
• light the fuse    導火線に火をつける
• light up    火入
• light upon    出会う, 見付ける
• light year    光年
• make light of    軽んじる, 軽視する
• out like a light    すぐ寝てしまう
• pedestrian-crossing lights    歩行者横断用信号
• reversing light    後退灯
• shed/throw/cast light on    光を当てる, 明らかにする
• the light at the end of the tunnel    トンネルを抜けると見える光

العربيه (Arabic)
‏(الاسم) ضياء, نور (فعل) أضاء, نور, أشعل, أوقد (صفه) يكشف, يعلن, خفيف الوزن, مضىء, غير معتم, رقيق, تافه, لطيف (ظرف) يجعله معلوما أو معروفا‏

עברית (Hebrew)
n. - ‮אור, מקור אור, אש, חלון, צוהר, אספקט, אור יום, איש מופת, מואר, שטוף-אור, בהיר, נאורות, תקווה, אושר, אמצעי להדלקת אש, שמשה, בעיקר בחממה‬
v. tr. - ‮האיר, הדליק, האיר דרך, (שמחה) האירה את פניו‬
v. intr. - ‮נדלק, הואר, הדליק סיגריה, זרח משמחה‬
adj. - ‮מואר, שטוף-אור, בהיר‬
adj. - ‮קל, קליל, קל-דעת, עליז‬
adv. - ‮בקלות, קלילות, ללא מיטען‬
v. intr. - ‮ירד מכלי-רכב, נחת, הגיע (במקרה) ל-, נפל על‬

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