electromagnetic spectrum

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electromagnetic spectrum

electromagnetic spectrum frequencies in hertz A. gamma rays B. x-rays C. ultraviolet rays D. visible light E. infrared rays F. microwaves G. radio waves (Precision Graphics)

The entire range of radiation extending in frequency from approximately 10²³ hertz to 0 hertz or, in corresponding wavelengths, from 10 ^-13 centimeter to infinity and including, in order of decreasing frequency, cosmic-ray photons, gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

The range of wavelengths over which electromagnetic radiation extends. The longest waves (10⁵–10⁻³ metres) are radio waves, the next longest (10⁻³–10⁻⁶ m) are infrared waves, then comes the narrow band (4–7 × 10⁻⁷ m) of visible radiation, followed by ultraviolet waves (10⁻⁷–10⁻⁹ m) and X-rays and gamma radiation (10⁻⁹–10⁻¹⁴ m).

Concept

One of the most amazing aspects of physics is the electromagnetic spectrum—radio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma rays—as well as the relationship between the spectrum and electromagnetic force. The applications of the electromagnetic spectrum in daily life begin the moment a person wakes up in the morning and "sees the light." Yet visible light, the only familiar part of the spectrum prior to the eighteenth and nineteenth centuries, is also its narrowest region. Since the beginning of the twentieth century, uses for other bands in the electromagnetic spectrum have proliferated. At the low-frequency end are radio, short-wave radio, and television signals, as well as the microwaves used in cooking. Higher-frequency waves, all of which can be generally described as light, provide the means for looking deep into the universe—and deep into the human body.

How It Works

Electromagnetism

The ancient Romans observed that a brushed comb would attract particles, a phenomenon now known as static electricity and studied within the realm of electrostatics in physics. Yet, the Roman understanding of electricity did not extend any further, and as progress was made in the science of physics—after a period of more than a thousand years, during which scientific learning in Europe progressed very slowly—it developed in areas that had nothing to do with the strange force observed by the Romans.

The fathers of physics as a serious science, Galileo Galilei (1564-1642) and Sir Isaac Newton (1642-1727), were concerned with gravitation, which Newton identified as a fundamental force in the universe. For nearly two centuries, physicists continued to believe that there was only one type of force. Yet, as scientists became increasingly aware of molecules and atoms, anomalies began to arise—in particular, the fact that gravitation alone could not account for the strong forces holding atoms and molecules together to form matter.

Foundations of Electromagnetic Theory

At the same time, a number of thinkers conducted experiments concerning the nature of electricity and magnetism, and the relationship between them. Among these were several giants in physics and other disciplines—including one of America's greatest founding fathers. In addition to his famous (and highly dangerous) experiment with lightning, Benjamin Franklin (1706-1790) also contributed the names "positive" and "negative" to the differing electrical charges discovered earlier by French physicist Charles Du Fay (1698-1739).

In 1785, French physicist and inventor Charles Coulomb (1736-1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the inverse of the square of the distance between objects. That attraction itself, however, resulted not from gravity, but from electrical charge, according to Coulomb.

A few years later, German mathematician Johann Karl Friedrich Gauss (1777-1855) developed a mathematical theory for finding the magnetic potential of any point on Earth, and his contemporary, Danish physicist Hans Christian Oersted (1777-1851), became the first scientist to establish the existence of a clear relationship between electricity and magnetism. This led to the foundation of electromagnetism, the branch of physics devoted to the study of electrical and magnetic phenomena.

French mathematician and physicist André Marie Ampère (1775-1836) concluded that magnetism is the result of electricity in motion, and, in 1831, British physicist and chemist Michael Faraday (1791-1867) published his theory of electromagnetic induction. This theory shows how an electrical current in one coil can set up a current in another through the development of a magnetic field. This enabled Faraday to develop the first generator, and for the first time in history, humans were able to convert mechanical energy systematically into electrical energy.

Maxwell and Electromagnetic Force

A number of other figures contributed along the way; but, as yet, no one had developed a "unified theory" explaining the relationship between electricity and magnetism. Then, in 1865, Scottish physicist James Clerk Maxwell (1831-1879) published a groundbreaking paper, "On Faraday's Lines of Force," in which he outlined a theory of electromagnetic force—the total force on an electrically charged particle, which is a combination of forces due to electrical and/or magnetic fields around the particle.

Maxwell had thus discovered a type of force in addition to gravity, and this reflected a "new" type of fundamental interaction, or a basic mode by which particles interact in nature. Newton had identified the first, gravitational interaction, and in the twentieth century, two other forms of fundamental interaction—strong nuclear and weak nuclear—were identified as well.

In his work, Maxwell drew on the studies conducted by his predecessors, but added a new statement: that electrical charge is conserved. This statement, which did not contradict any of the experimental work done by the other physicists, was based on Maxwell's predictions regarding what should happen in situations of electromagnetism; subsequent studies have supported his predictions.

Electromagnetic Radiation

So far, what we have seen is the foundation for modern understanding of electricity and magnetism. This understanding grew enormously in the late nineteenth and early twentieth centuries, thanks both to the theoretical work of physicists, and the practical labors of inventors such as Thomas Alva Edison (1847-1931) and Serbian-American electrical engineer Nikola Tesla (1856-1943). But our concern in the present context is with electromagnetic radiation, of which the waves on the electromagnetic spectrum are a particularly significant example.

Energy can travel by conduction or convection, two principal means of heat transfer. But the energy Earth receives from the Sun—the energy conveyed through the electromagnetic spectrum—is transferred by another method, radiation. Whereas conduction of convection can only take place where there is matter, which provides a medium for the energy transfer, radiation requires no medium. Thus, electromagnetic energy passes from the Sun to Earth through the vacuum of empty space.

Electromagnetic Waves

The connection between electromagnetic radiation and electromagnetic force is far from obvious. Even today, few people not scientifically trained understand that there is a clear relationship between electricity and magnetism—let alone a connection between these and visible light. The breakthrough in establishing that connection can be attributed both to Maxwell and to German physicist Heinrich Rudolf Hertz (1857-1894).

Maxwell had suggested that electromagnetic force carried with it a certain wave phenomenon, and predicted that these waves traveled at a certain speed. In his Treatise on Electricity and Magnetism (1873), he predicted that the speed of these waves was the same as that of light—186,000 mi (299,339 km) per second—and theorized that the electromagnetic interaction included not only electricity and magnetism, but light as well. A few years later, while studying the behavior of electrical currents, Hertz confirmed Maxwell's proposition regarding the wave phenomenon by showing that an electrical current generated some sort of electromagnetic radiation.

In addition, Hertz found that the flow of electrical charges could be affected by light under certain conditions. Ultraviolet light had already been identified, and Hertz shone an ultraviolet beam on the negatively charged side of a gap in a current loop. This made it easier for an electrical spark to jump the gap. Hertz could not explain this phenomenon, which came to be known as the photoelectric effect. Indeed, no one else could explain it until quantum theory was developed in the early twentieth century. In the meantime, however, Hertz's discovery of electromagnetic waves radiating from a current loop led to the invention of radio by Italian physicist and engineer Guglielmo Marconi (1874-1937) and others.

Light: Waves or Particles?

At this point, it is necessary to jump backward in history, to explain the progression of scientists' understanding of light. Advancement in this areatook place over a long period of time: at the endof the first millennium A.D., the Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039) showedthat light comes from the Sun and other self-illuminated bodies—not, as had been believed up tothat time—from the eye itself. Thus, studies inoptics, or the study of light and vision, were—compared to understanding of electromagnetismitself—relatively advanced by 1666, when Newton discovered the spectrum of colors in light. AsNewton showed, colors are arranged in a sequence, and white light is a combination of allcolors.

Newton put forth the corpuscular theory of light—that is, the idea that light is made up of particles—but his contemporary Christiaan Huygens (1629-1695), a Dutch physicist and astronomer, maintained that light appears in the form of a wave. For the next century, adherents of Newton's corpuscular theory and of Huygens's wave theory continued to disagree. Physicists on the European continent began increasingly to accept wave theory, but corpuscular theory remained strong in Newton's homeland.

Thus, it was ironic that the physicist whose work struck the most forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Young directed a beam of light through two closely spaced pinholes onto a screen, reasoning 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.

By the time of Hertz, wave theory had become dominant; but the photoelectric effect also exhibited aspects of particle behavior. Thus, for the first time in more than a century, particle theory gained support again. Yet, it was clear that light had certain wave characteristics, and this raised the question—which is it, a wave or a set of particles streaming through space?

The work of German physicist Max Planck (1858-1947), father of quantum theory, and of Albert Einstein (1879-1955), helped resolve this apparent contradiction. Using Planck's quantum principles, Einstein, in 1905, showed that light appears in "bundles" of energy, which travel as waves but behave as particles in certain situations. Eighteen years later, American physicist Arthur Holly Compton (1892-1962) showed that, depending on the way it is tested, light appears as either a particle or a wave. These particles he called photons.

Wave Motion and Electromagnetic Waves

The particle behavior of electromagnetic energy is beyond the scope of the present discussion, though aspects of it are discussed elsewhere. For the present purposes, it is necessary only to view the electromagnetic spectrum as a series of waves, and in the paragraphs that follow, the rudiments of wave motion will be presented in short form.

A type of harmonic motion that carries energy from one place to another without actually moving any matter, wave motion is related to oscillation, harmonic—and typically periodic—motion in one or more dimensions. Oscillation involves no net movement, but only movement in place; yet individual waves themselves are oscillating, even as the overall wave pattern moves.

The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves: waves in which a uniform series of crests and troughs follow each other in regular succession. Periodic waves are divided into longitudinal and transverse waves, the latter (of which light waves are an example) being waves in which the vibration or motion is perpendicular to the direction in which the wave is moving. Unlike longitudinal waves, such as those that carry sound energy, transverse waves are fairly easy to visualize, and assume the shape that most people imagine when they think of waves: a regular up-and-down pattern, called "sinusoidal" in mathematical terms.

Parameters of Wave Motion

A period (represented by the symbol T) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength.

Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after Hertz himself: a single Hertz (the term is both singular and plural) is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 10³ or 1,000 cycles per second); megahertz (MHz; 10⁶ or 1 million cycles per second); and gigahertz (GHz; 10⁹ or 1 billion cycles per second.)

Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength; and, thus, it is possible to describe waves in terms of either. According to quantum theory, however, electromagnetic waves can also be described in terms of photon energy level, or the amount of energy in each photon. Thus, the electromagnetic spectrum, as we shall see, varies from relatively long-wave length, low-frequency, low-energy radio waves on the one end to extremely short-wave-length, high-frequency, high-energy gamma rays on the other.

The other significant parameter for describing a wave—one mathematically independent from those so far discussed—is amplitude. Defined as the maximum displacement of a vibrating material, amplitude is the "size" of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity. The amplitude of a light wave, for instance, determines the intensity of the light.

A Right-Hand Rule

Physics textbooks use a number of "right-hand rules": devices for remembering certain complex physical interactions by comparing the lines of movement or force to parts of the right hand. In the present context, a right-hand rule makes it easier to visualize the mutually perpendicular directions of electromagnetic waves, electric field, and magnetic field.

A field is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electrical field and magnetic field are simply regions in which electrical and magnetic components, respectively, of electromagnetic force are exerted.

Hold out your right hand, palm perpendicular to the floor and thumb upright. Your fingers indicate the direction that an electromagnetic wave is moving. Your thumb points in the direction of the electrical field, as does the heel of your hand: the electrical field forms a plane perpendicular to the direction of wave propagation. Similarly, both your palm and the back of your hand indicate the direction of the magnetic field, which is perpendicular both to the electrical field and the direction of wave propagation.

The Electromagnetic Spectrum

As stated earlier, an electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with mutually perpendicular electrical and magnetic fields that emanate from it.

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. Though each occupies a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply "blurs" into another.

Frequency Range of the Electromagnetic Spectrum

The range of frequencies for waves in the electromagnetic spectrum is from approximately 10² Hz to more than 10²⁵ Hz. These numbers are an example of scientific notation, which makes it possible to write large numbers without having to include a string of zeroes. Without scientific notation, the large numbers used for discussing properties of the electromagnetic spectrum can become bewildering.

The first number given, for extremely low-frequency radio waves, is simple enough—100—but the second would be written as 1 followed by 25 zeroes. (A good rule of thumb for scientific notation is this: for any power n of 10, simply attach that number of zeroes to 1. Thus 10⁶ is 1 followed by 6 zeroes, and so on.) In any case, 10²⁵ is a much simpler figure than 10,000,000,000,000,000,000,000,000—or 10 trillion trillion. As noted earlier, gigahertz, or units of 1 billion Hertz, are often used in describing extremely high frequencies, in which case the number is written as 10¹⁶ GHz. For simplicity's sake, however, in the present context, the simple unit of Hertz (rather than kilo-, mega-, or gigahertz) is used wherever it is convenient to do so.

Wavelengths on the Electromagnetic Spectrum

The range of wavelengths found in the electromagnetic spectrum is from about 10⁸ centimeters to less than 10⁻¹⁵ centimeters. The first number, equal to 1 million meters (about 621 mi), obviously expresses a great length. This figure is for radio waves of extremely low frequency; ordinary radio waves of the kind used for actual radio broadcasts are closer to 10⁵ centimeters (about 328 ft).

For such large wavelengths, the use of centimeters might seem a bit cumbersome; but, as with the use of Hertz for frequencies, centimeters provide a simple unit that can be used to measure all wavelengths. Some charts of the electromagnetic spectrum nonetheless give figures in meters, but for parts of the spectrum beyond microwaves, this, too, can become challenging. The ultra-short wavelengths of gamma rays, after all, are equal to one-trillionth of a centimeter. By comparison, the angstrom—a unit so small it is used to measure the diameter of an atom—is 10 million times as large.

Energy Levels on the Electromagnetic Spectrum

Finally, in terms of photon energy, the unit of measurement is the electron volt (eV), which is used for quantifying the energy in atomic particles. The range of photon energy in the electromagnetic spectrum is from about 10⁻¹³ to more than 10¹⁰ electron volts. Expressed in terms of joules, an electron volt is equal to 1.6 · 10⁻¹⁹ J.

To equate these figures to ordinary language would require a lengthy digression; suffice it to say that even the highest ranges of the electromagnetic spectrum possess a small amount of energy in terms of joules. Remember, however, that the energy level identified is for a photon—a light particle. Again, without going into a great deal of detail, one can just imagine how many of these particles, which are much smaller than atoms, would fit into even the smallest of spaces. Given the fact that electromagnetic waves are traveling at a speed equal to that of light, the amount of photon energy transmitted in a single second is impressive, even for the lower ranges of the spectrum. Where gamma rays are concerned, the energy levels are positively staggering.

Real-Life Applications

The Radio Sub-Spectrum

Among the most familiar parts of the electromagnetic spectrum, in modern life at least, is radio. In most schematic representations of the spectrum, radio waves are shown either at the left end or the bottom, as an indication of the fact that these are the electromagnetic waves with the lowest frequencies, the longest wavelengths, and the smallest levels of photon energy. Included in this broad sub-spectrum, with frequencies up to about 10⁷ Hertz, are long-wave radio, short-wave radio, and microwaves. The areas of communication affected are many: broadcast radio, television, mobile phones, radar—and even highly specific forms of technology such as baby monitors.

Though the work of Maxwell and Hertz was foundational to the harnessing of radio waves for human use, the practical use of radio had its beginnings with Marconi. During the 1890s, he made the first radio transmissions, and, by the end of the century, he had succeeded in transmitting telegraph messages across the Atlantic Ocean—a feat which earned him the Nobel Prize for physics in 1909.

Marconi's spark transmitters could send only coded messages, and due to the broad, long-wave length signals used, only a few stations could broadcast at the same time. The development of the electron tube in the early years of the twentieth century, however, made it possible to transmit narrower signals on stable frequencies. This, in turn, enabled the development of technology for sending speech and music over the airwaves.

Broadcast Radio

The Development of Am and Fm

A radio signal is simply a carrier: the process of adding information—that is, complex sounds such as those of speech or music—is called modulation. The first type of modulation developed was AM, or amplitude modulation, which Canadian-American physicist Reginald Aubrey Fessenden (1866-1932) demonstrated with the first United States radio broadcast in 1906. Amplitude modulation varies the instantaneous amplitude of the radio wave, a function of the radio station's power, as a means of transmitting information.

By the end of World War I, radio had emerged as a popular mode of communication: for the first time in history, entire nations could hear the same sounds at the same time. During the 1930s, radio became increasingly important, both for entertainment and information. Families in the era of the Great Depression would gather around large "cathedral radios"—so named for their size and shape—to hear comedy programs, soap operas, news programs, and speeches by important public figures such as President Franklin D. Roosevelt.

Throughout this era—indeed, for more than a half-century from the end of the first World War to the height of the Vietnam Conflict in the mid-1960s—AM held a dominant position in radio. This remained the case despite a number of limitations inherent in amplitude modulation: AM broadcasts flickered with popping noises from lightning, for instance, and cars with AM radios tended to lose their signal when going under a bridge. Yet, another mode of radio transmission was developed in the 1930s, thanks to American inventor and electrical engineer Edwin H. Armstrong (1890-1954). This was FM, or frequency modulation, which varied the radio signal's frequency rather than its amplitude.

Not only did FM offer a different type of modulation; it was on an entirely different frequency range. Whereas AM is an example of a long-wave radio transmission, FM is on the microwave sector of the electromagnetic spectrum, along with television and radar. Due to its high frequency and form of modulation, FM offered a "clean" sound as compared with AM. The addition of FM stereo broadcasts in the 1950s offered still further improvements; yet despite the advantages of FM, audiences were slow to change, and FM did not become popular until the mid-to late 1960s.

Signal Propagation

AM signals have much longer wavelengths, and smaller frequencies, than do FM signals, and this, in turn, affects the means by which AM signals are propagated. There are, of course, much longer radio wavelengths; hence, AM signals are described as intermediate in wavelength. These intermediate-wavelength signals reflect off highly charged layers in the ionosphere between 25 and 200 mi (40-332 km) above Earth's surface. Short-wave-length signals, such as those of FM, on the other hand, follow a straight-line path. As a result, AM broadcasts extend much farther than FM, particularly at night.

At a low level in the ionosphere is the D layer, created by the Sun when it is high in the sky. The D layer absorbs medium-wavelength signals during the day, and for this reason, AM signals do not travel far during daytime hours. After the Sun goes down, however, the D layer soon fades, and this makes it possible for AM signals to reflect off a much higher layer of the ionosphere known as the F layer. (This is also sometimes known as the Heaviside layer, or the Kennelly-Heaviside layer, after English physicist Oliver Heaviside and British-American electrical engineer Arthur Edwin Kennelly, who independently discovered the ionosphere in 1902.) AM signals "bounce" off the F layer as though it were a mirror, making it possible for a listener at night to pick up a signal from halfway across the country.

The Sun has other effects on long-wave and intermediate-wave radio transmissions. Sunspots, or dark areas that appear on the Sun in cycles of about 11 years, can result in a heavier buildup of the ionosphere than normal, thus impeding radio-signal propagation. In addition, occasional bombardment of Earth by charged particles from the Sun can also disrupt transmissions.

Due to the high frequencies of FM signals, these do not reflect off the ionosphere; instead, they are received as direct waves. For this reason, an FM station has a fairly short broadcast range, and this varies little with regard to day or night. The limited range of FM stations as compared to AM means that there is much less interference on the FM dial than for AM.

Distribution of Radio Frequencies

In the United States and most other countries, one cannot simply broadcast at will; the airwaves are regulated, and, in America, the governing authority is the Federal Communications Commission (FCC). The FCC, established in 1934, was an outgrowth of the Federal Radio Commission, founded by Congress seven years earlier. The FCC actually "sells air," charging companies a fee to gain rights to a certain frequency. Those companies may in turn sell that air to others for a profit.

At the time of the FCC's establishment, AM was widely used, and the federal government assigned AM stations the frequency range of 535 kHz to 1.7 MHz. Thus, if an AM station today is called, for instance, "AM 640," this means that it operates at 640 kHz on the dial. The FCC assigned the range of 5.9 to 26.1 MHz to short-wave radio, and later the area of 26.96 to 27.41 MHz to citizens' band (CB) radio. Above these are microwave regions assigned to television stations, as well as FM, which occupies the range from 88 to 108 MHz.

The organization of the electromagnetic spectrum's radio frequencies—which, of course, is an entirely arbitrary, humanmade process—is fascinating. It includes assigned frequencies for everything from garage-door openers to deep-space radio communications. The FCC recognizes seven divisions of radio carriers, using a system that is not so much based on rational rules as it is on the way that the communications industries happened to develop over time.

The Seven Fcc Divisions

Most of what has so far been described falls under the heading of "Public Fixed Radio Services": AM and FM radio, other types of radio such as shortwave, television, various other forms of microwave broadcasting, satellite systems, and communication systems for federal departments and agencies. "Public Mobile Services" include pagers, air-to-ground service (for example, aircraft-to-tower communications), offshore service for sailing vessels, and rural radio-telephone service. "Commercial Mobile Radio Services" is the realm of cellular phones, and "Personal Communications Service" that of the newer wireless technology that began to challenge cellular for market dominance in the late 1990s.

"Private Land Mobile Radio Service" (PMR) and "Private Operational-Fixed Microwave Services" (OFS) are rather difficult to distinguish, the principal difference being that the former is used exclusively by profit-making businesses, and the latter mostly by nonprofit institutions. An example of PMR technology is the dispatching radios used by taxis, but this is only one of the more well-known forms of internal electronic communications for industry. For instance, when a film production company is shooting a picture and the director needs to speak to someone at the producer's trailer a mile away, she may use PMR radio technology. OFS was initially designated purely for nonprofit use, and is used often by schools; but banks and other profit-making institutions often use OFS because of its low cost.

Finally, there is the realm of "Personal Radio Services," created by the FCC in 1992. This branch, still in its infancy, will probably one day include video-on-demand, interactive polling, online shopping and banking, and other activities classified under the heading of Interactive Video and Data Services, or IVDS. Unlike other types of video technology, these will all be wireless, and, therefore, represent a telecommunications revolution all their own.

Microwaves

Microwave Communication

Though microwaves are treated separately from radio waves, in fact, they are just radio signals of a very short wavelength. As noted earlier, FM signals are actually carried on microwaves, and, as with FM in particular, microwave signals in general are very clear and very strong, but do not extend over a great geographical area. Nor does microwave include only high-frequency radio and television; in fact, any type of information that can be transmitted via telephone wires or coaxial cables can also be sent via a microwave circuit.

Microwaves have a very narrow, focused beam: thus, the signal is amplified considerably when an antenna receives it. This phenomenon, known as "high antenna gain," means that microwave transmitters need not be highly powerful to produce a strong signal. To further the reach of microwave broadcasts, transmitters are often placed atop mountain peaks, hilltops, or tall buildings. In the past, a microwave-transmitting network such as NBC (National Broadcasting Company) or CBS (Columbia Broadcasting System) required a network of ground-based relay stations to move its signal across the continent. The advent of satellite broadcasting in the 1960s, however, changed much about the way signals are beamed: today, networks typically replace, or at least augment, ground-based relays with satellite relays.

The first worldwide satellite TV broadcast, in the summer of 1967, featured the Beatles singing their latest song "All You Need Is Love." Due to the international character of the broadcast, with an estimated 200 million viewers, John Lennon and Paul McCartney wrote a song with simple, universal lyrics, and the result was just another example of electronic communication uniting large populations. Indeed, the phenomenon of rock music, and of superstardom as people know it today, would be impossible without many of the forms of technology discussed here. Long before the TV broadcast, the Beatles had come to fame through the playing of their music on the radio waves—and, thus, they owed much to Maxwell, Hertz, and Marconi.

Microwave Ovens

The same microwaves that transmit FM and television signals—to name only the most obviously applications of microwave for communication—can also be harnessed to cook food. The microwave oven, introduced commercially in 1955, was an outgrowth of military technology developed a decade before.

During World War II, the Raytheon Manufacturing Company had experimented with a magnetron, a device for generating extremely short-wavelength radio signals as a means of improving the efficiency of military radar. While working with a magnetron, a technician named Percy Spencer was surprised to discover that a candy bar in his pocket had melted, even though he had not felt any heat. This led him to considering the possibilities of applying the magnetron to peacetime uses, and a decade later, Raytheon's "radar range" hit the market.

Those early microwave ovens had none of varied power settings to which modern users of the microwave—found today in two-thirds of all American homes—are accustomed. In the first microwaves, the only settings were "on" and "off," because there were only two possible adjustments: either the magnetron would produce, or not produce, microwaves. Today, it is possible to use a microwave for almost anything that involves the heating of food that contains water—from defrosting a steak to popping popcorn.

As noted much earlier, in the general discussion of electromagnetic radiation, there are three basic types of heat transfer: conduction, convection, and radiation. Without going into too much detail here, conduction generally involves heat transfer between molecules in a solid; convection takes place in a fluid (a gas such as air or a liquid such as water); and radiation, of course, requires no medium.

A conventional oven cooks through convection, though conduction also carries heat from the outer layers of a solid (for example, a turkey) to the interior. A microwave, on the other hand, uses radiation to heat the outer layers of the food; then conduction, as with a conventional oven, does the rest. The difference is that the microwave heats only the food—or, more specifically, the water, which then transfers heat throughout the item being heated—and not the dish or plate. Thus, many materials, as long as they do not contain water, can be placed in a microwave oven without being melted or burned. Metal, though it contains no water, is unsafe because the microwaves bounce off the metal surfaces, creating a microwave buildup that can produce sparks and damage the oven.

In a microwave oven, microwaves emitted by a small antenna are directed into the cooking compartment, and as they enter, they pass a set of turning metal fan blades. This is the stirrer, which disperses the microwaves uniformly over the surface of the food to be heated. As a microwave strikes a water molecule, resonance causes the molecule to align with the direction of the wave. An oscillating magnetron causes the microwaves to oscillate as well, and this, in turn, compels the water molecules to do the same. Thus, the water molecules are shifting in position several million times a second, and this vibration generates energy that heats the water.

Radio Waves for Measurement and Ranging

Radar

Radio waves can be used to send communication signals, or even to cook food; they can also be used to find and measure things. One of the most obvious applications in this regard is radar, an acronym for RA dio D etection A nd R anging.

Radio makes it possible for pilots to "see" through clouds, rain, fog, and all manner of natural phenomena—not least of which is darkness. It can also identify objects, both natural and manmade, thus enabling a peacetime pilot to avoid hitting another craft or the side of a mountain. On the other hand, radar may help a pilot in wartime to detect the presence of an enemy. Nor is radar used only in the skies, or for military purposes, such as guiding missiles: on the ground, it is used to detect the speeds of objects such as automobiles on an interstate highway, as well as to track storms.

In the simplest model of radar operation, the unit sends out microwaves toward the target, and the waves bounce back off the target to the unit. Though the speed of light is reduced somewhat, due to the fact that waves are traveling through air rather than through a vacuum, it is, nonetheless, possible to account for this difference. Hence, the distance to the target can be calculated using the simple formula d = vt, where d is distance, v is velocity, and t is time.

Typically, a radar system includes the following: a frequency generator and a unit for controlling the timing of signals; a transmitter and, as with broadcast radio, a modulator; a duplexer, which switches back and forth between transmission and reception mode; an antenna; a receiver, which detects and amplifies the signals bounced back to the antenna; signal and data processing units; and data display units. In a monostatic unit—one in which the transmitter and receiver are in the same location—the unit has to be continually switched between sending and receiving modes. Clearly, a bistatic unit—one in which the transmitter and receiver antennas are at different locations—is generally preferable; but on an airplane, for instance, there is no choice but to use a monostatic unit.

In order to determine the range to a target—whether that target be a mountain, an enemy aircraft, or a storm—the target itself must first be detected. This can be challenging, because only a small portion of the transmitted pulse comes back to the receiving antenna. At the same time, the antenna receives reflections from a number of other objects, and it can be difficult to determine which signal comes from the target. For an aircraft in a wartime situation, these problems are compounded by the use of enemy countermeasures such as radar "jamming." Still another difficulty facing a military flyer is the fact that the use of radar itself—that is, the transmission of microwaves—makes the aircraft detectable to opposing forces.

Telemetry

Telemetry is the process of making measurements from a remote location and transmitting those measurements to receiving equipment. The earliest telemetry systems, developed in the United States during the 1880s, monitored the distribution and use of electricity in a given region, and relayed this information back to power companies using telephone lines. By the end of World War I, electric companies used the power lines themselves as information relays, and though such electrical telemetry systems remain in use in some sectors, most modern telemetry systems apply radio signals.

An example of a modern telemetry application is the use of an input device called a transducer to measure information concerning an astronaut's vital signs (heartbeat, blood pressure, body temperature, and so on) during a manned space flight. The transducer takes this information and converts it into an electrical impulse, which is then beamed to the space monitoring station on Earth. Because this signal carries information, it must be modulated, but there is little danger of interference with broadcast transmissions on Earth. Typically, signals from spacecraft are sent in a range above 10¹⁰ Hz, far above the frequencies of most microwave transmissions for commercial purposes.

Light: Invisible, Visible, and Invisible Again

Between about 10¹³ and 10¹⁷ Hz on the electromagnetic spectrum is the range of light: infrared, visible, and ultraviolet. Light actually constitutes a small portion of the spectrum, and the area of visible light is very small indeed, extending from about 4.3 · 10¹⁴ to 7.5 · 10¹⁴ Hz. The latter, incidentally, is another example of scientific notation: not only is it easier not to use a string of zeroes, but where a coefficient or factor (for example, 4.3 or 7.5) is other than a multiple of 10, it is preferable to use what are called significant figures—usually a single digit followed by a decimal point and up to 3 decimal places.

Infrared light lies just below visible light in frequency, and this is easy to remember because of the name: red is the lowest in frequency of all the colors. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Visible light itself, by far the most familiar part of the spectrum—especially prior to the age of radio communications—is discussed in detail elsewhere.

Infrared Light

Though we cannot see infrared light, we feel it as heat. German-English astronomer William Herschel (1738-1822), first scientist to detect infrared radiation from the Sun, demonstrated its existence in 1800 by using a thermometer. Holding a prism, a three-dimensional glass shape used for diffusing beams of light, he directed a beam of sunlight toward the thermometer, which registered the heat of the infrared rays.

Eighty years later, English scientist Sir William Abney (1843-1920) developed infrared photography, a method of capturing infrared radiation, rather than visible light, on film. By the mid-twentieth century, infrared photography had come into use for a variety of purposes. Military forces, for instance, may use infrared to detect the presence of enemy troops. Medicine makes use of infrared photography for detecting tumors, and astronomers use infrared to detect stars too dim to be seen using ordinary visible light.

The uses of infrared imaging in astronomy, as a matter of fact, are many. The development in the 1980s of infrared arrays, two-dimensional grids which produce reliable images of infrared phenomena, revolutionized infrared astronomy. Because infrared penetrates dust much more easily than does visible light, infrared astronomy makes it easier to see regions of the universe where stars—formed from collapsing clouds of gas and dust—are in the process of developing. Because hydrogen molecules emit infrared radiation, infrared astronomy helps provide clues regarding the distribution of this highly significant chemical element throughout the universe.

Ultraviolet Light

Very little of the Sun's ultraviolet light penetrates Earth's atmosphere—a fortunate thing, since ultraviolet (UV) radiation can be very harmful to human skin. A suntan, as a matter of fact, is actually the skin's defense against these harmful UV rays. Due to the fact that Earth is largely opaque, or resistant, to ultraviolet light, the most significant technological applications of UV radiation are found in outer space.

In 1978 the United States, in cooperation with several European space agencies, launched the International Ultraviolet Explorer (IUE), which measured the UV radiation from tens of thousands of stars, nebulae, and galaxies. Despite the progress made with IUE, awareness of its limitations—including a mirror of only 17 in (45 cm) on the telescope itself—led to the development of a replacement in 1992.

This was the Extreme Ultraviolet Explorer (EUVE), which could observe UV phenomena over a much higher range of wavelengths than those observed by IUE. In addition, the Hubble Space Telescope, launched by the United States in 1990, includes a UV instrument called the Goddard High Resolution Spectrograph. With a mirror measuring 8.5 ft (2.6 m), it is capable of observing objects much more faint than those detected earlier by IUE.

Ultraviolet astronomy is used to study the winds created by hot stars, as well as stars still in the process of forming, and even stars that are dying. It is also useful for analyzing the densely packed, highly active sectors near the centers of galaxies, where both energy and temperatures are extremely high.

X Rays

Though they are much higher in frequency than visible light—with wavelengths about 1,000 times shorter than for ordinary light rays—x rays are a familiar part of modern life due to their uses in medicine. German scientist Wilhelm Röntgen (1845-1923) developed the first x-ray device in 1895, and, thus, the science of using xray machines is called roentgenology.

The new invention became a curiosity, with carnivals offering patrons an opportunity to look at the insides of their hands. And just as many people today fear the opportunities for invasion of privacy offered by computer technology, many at the time worried that x rays would allow robbers and peeping toms to look into people's houses. Soon, however, it became clear that the most important application of x rays lay in medicine.

How a Medical X-Ray Machine Works

Due to their very short wavelengths, x rays can pass through substances of low density—for example, fat and other forms of soft tissue—without their movement being interrupted. But in materials of higher density, such as bone, atoms are packed closely together, and this provides x rays with less space through which to travel. As a result, x-ray images show dark areas where the rays traveled completely through the target, and light images of dense materials that blocked the movement of the rays.

Medical x-ray machines are typically referred to either as "hard" or "soft." Soft x rays are the ones with which most people are more familiar. Operating at a relatively low frequency, these are used to photograph bones and internal organs, and provided the patient does not receive prolonged exposure to the rays, they cause little damage. Hard x rays, on the other hand, are designed precisely to cause damage—not to the patient, but to cancer cells. Because they use high voltage and high-frequency rays, hard x rays can be quite dangerous to the patient as well.

Other Applications

X-ray crystallography, developed in the early twentieth century, is devoted to the study of the interference patterns produced by x rays passing through materials that are crystalline in Structure. Each of these discoveries, in turn, transformed daily life: insulin, by offering hope to diabetics, penicillin, by providing a treatment for a number of previously fatal illnesses, and DNA, by enabling scientists to make complex assessments of genetic information.

In addition to the medical applications, the scanning capabilities of x-ray machines make them useful for security. A healthy person receives an x ray at a doctor's office only once in a while; but everyone who carries items past a certain point in a major airport must submit to x-ray security scanning. If one is carrying a purse or briefcase, for instance, this is placed on a moving belt and subjected to scanning by a low-power device that can reveal the contents.

Gamma Rays

At the furthest known reaches of the electromagnetic spectrum are gamma rays, ultra high-frequency, high-energy, and short-wavelength forms of radiation. Human understanding of gamma rays, including the awesome powers they contain, is still in its infancy.

In 1979, a wave of enormous energy passed over the Solar System. Though its effects on Earth were negligible, instruments aboard several satellites provided data concerning an enormous quantity of radiation caused by gamma rays. As to the source of the rays themselves, believed to be a product of nuclear fusion on some other body in the universe, scientists knew nothing.

The Compton Gamma Ray Observatory Satellite, launched by NASA (National Aeronautics and Space Administration) in 1991, detected a number of gamma-ray bursts over the next two years. The energy in these bursts was staggering: just one of these, scientists calculated, contained more than a thousand times as much energy as the Sun will generate in its entire lifetime of 10 billion years.

Some astronomers speculate that the source of these gamma-ray bursts may ultimately be a distant supernova, or exploding star. If this is the case, scientists may have found the supernova; but do not expect to see it in the night sky. It is not known just how long ago it exploded, but its light appeared on Earth some 340,000 years ago, and during that time it was visible in daylight for more than two years. So great was its power that the effects of this stellar phenomenon are still being experienced.

Where to Learn More

Branley, Franklyn Mansfield. The Electromagnetic Spectrum: Key to the Universe. Illustrated by Leonard D. Dank. New York: Crowell, 1979.

Electromagnetic Spectrum (Web site). <http://www.jsc.mil/images/speccht.jpg> (April 25, 2001).

The Electromagnetic Spectrum (Web site). <http://library.thinkquest.org/11119/index.htm> (April 30, 2001).

The Electromagnetic Spectrum (Web site). <http://www.ospi.wednet.edu:8001/curric/space/lfs/emspectr.html> (April 30, 2001).

Electromagnetic Spectrum/NASA: National Aeronautics and Space Administration (Web site). <http://imagine.gsfc.nasa.gov/docs/science/know_11/emspectrum.html> (April 30, 2001).

"How the Radio Spectrum Works." How Stuff Works (Web site). <http://www.howstuffworks.com/radio~spectrum.htm> (April 25, 2001).

Internet Resources for Sound and Light (Web site). <http://electro.sau.edu/SLResources.html> (April 25, 2001).

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

"Radio Electronics Pages" ePanorama.net (Web site). <http://www.epanorama.net/radio.html> (April 25, 2001).

Skurzynski, Gloria. Waves: The Electromagnetic Universe. Washington, D.C.: National Geographic Society, 1996.

A family of radiant energies that travel in wave form, have neither mass nor charge, and travel at the speed of light. Radiations within the spectrum vary only in wavelength. X-ray photons and light rays are examples of electromagnetic radiation.

physics Electromagnetic waves (light, radio waves, electricity, x-rays, etc.) cover a continuum of frequencies, increasingly exploited technologically by mankind and detected in nature. The main types, with indications of usage, are shown broadly in Table 13, grouped by decades. Naturally occurring waves of many frequencies reach Earth's surface, from outer space, in the solar wind, through the interaction of such waves with Earth's magnetosphere, and from atmospheric storms; natural waves below 1 kHz have been recorded. See UHF, VHF, HF, MF, LF, and VLF; see also visible light.

(click to enlarge)
The spectrum of electromagnetic waves ranges from low-frequency radio waves to high-frequency gamma … (credit: © Merriam-Webster Inc.)

Total range of frequencies or wavelengths of electromagnetic radiation. The spectrum ranges from waves of long wavelength (low frequency) to those of short wavelength (high frequency); it comprises, in order of increasing frequency (or decreasing wavelength): very-low-frequency to ultrahigh-frequency radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. In a vacuum, all waves of the electromagnetic spectrum travel at the same speed: 299,792,458 m/sec (186,282 mi/sec).

For more information on electromagnetic spectrum, visit Britannica.com.

James Clerk Maxwell's model for the electromagnetic nature of light did not at first consider extension to other forms of energy propagation; but before long Heinrich Hertz had discovered radio waves and showed that they obeyed similar laws: soon afterwards Wilhelm Röntgen discovered X-rays. The electromagnetic spectrum had expanded from less than one octave to a range of frequencies extending from a few kilohertz to some 100 exahertz (10¹⁸ Hz), the shortest wavelength gamma radiation.

In Maxwell's model all the radiation is characterized by a very high-frequency alternating electric field linked to a magnetic field at right angles to it (Fig. 1). Maxwell derived a figure for the speed of light using only electric and magnetic constants, and subsequent measurements by Fizeau and others showed his calculations to be correct. Later work by de Broglie and Planck showed that the photon (the fundamental particle of light energy) was not alone in being associated with a wavelength: all fundamental particles, such as electrons and protons were, also had corresponding wavelengths.

For the photographer, the most important part of the electromagnetic spectrum is the visible region, and to a lesser extent its near neighbours the optical infrared and ultraviolet. The visible spectrum can be broadly separated into three regions defined by wavelength: blue (400-500 nanometres), green (500-600 nm), and red (600-700 nm). Human vision reaches a little beyond these limits, but only by a small amount. The optical infrared region extends roughly from 750 to 1, 500 nm; special lens materials can produce optical images up to about 1, 500 nm, but above this scanning devices are required. The optical ultraviolet region is roughly 250-360 nm, using lenses of special material. Radiation of less than 100 nm wavelength will still affect silver halides, but passes straight through lenses without refraction, and can only be used for shadowgraphs: it is, of course, useful in diagnostic radiography. Fig. 2 summarizes the various regions of the electromagnetic spectrum.

Fig.1

Fig.2

— Graham Saxby

The electromagnetic spectrum is the range of wavelengths. It ranges from gamma rays with very short wavelengths (10^-5 nanometers) and high energy through radio waves with longer wavelengths (10³ meters) and less energy. Visible light, seen as color, occurs between 380 and 750 nanometers; ultraviolet light has a shorter wavelength, while infrared light has a greater wavelength.

The electromagnetic spectrum consists of all the frequencies at which electromagnetic waves can occur, ordered from zero to infinity. Radio waves, visible light, and x rays are examples of electromagnetic waves at different frequencies. Every part of the electromagnetic spectrum is exploited for some form of military, security, or espionage activity; the entire spectrum is also key to science and industry.

Basic Physics

Electromagnetic waves have been known since the midnineteenth century, when their behavior was first described by the equations of Scottish physicist James Clerk Maxwell (1831–1879). Electromagnetic waves, according to Maxwell's equations, are generated whenever an electrical charge (e.g., an electron) is accelerated, that is, changes its direction of motion, its speed, or both. An electromagnetic wave is so named because it consists of an electric and a magnetic field propagating together through space. As the electric field varies with time, it renews the magnetic field; as the magnetic field varies, it renews the electric field. The two components of the wave, which always point at right angles both to each other and to their direction of motion, are thus mutually sustaining, and form a wave which moves forward through empty space indefinitely.

The rate at which energy is periodically exchanged between the electric and magnetic components of a given electromagnetic wave is the frequency, [.nu], of that wave and has units of cycles per second or Hertz (Hz); the linear distance between the wave's peaks is termed its wavelength, [.lambda], and has units of length (e.g., meters). The speed at which a wave travels is the product of its wavelength and its frequency, V = [.nu][.lambda]; in the case of electromagnetic waves, Maxwell's equations require that this velocity equal the speed of light, c (>186,000 miles per second [300,000 km/sec]). Since the velocity of all electromagnetic waves is fixed, the wavelength [.lambda] of an electromagnetic wave always determines its frequency [.nu], or vice versa, by the relationship c = [.nu][.lambda] The higher the frequency (i.e., the shorter the wavelength) of an electromagnetic wave, the higher in the spectrum it is said to be. Since a wave cannot have a frequency less than zero, the spectrum is bound by zero at its lower end. In theory, it has no upper limit.

Electromagnetic waves and matter. All atoms and molecules at temperatures above absolute zero radiate electromagnetic waves at specific frequencies that are determined by the details of their internal structure. In quantum physics, this radiation must often be described as consisting of particles called photons rather than as waves; however, this article will restrict itself to the classical (continuous-wave) treatment of electromagnetic radiation, which is adequate for most technological purposes.

Not only do atoms and molecules radiate electromagnetic waves at certain frequencies, they can absorb them at the same frequencies. All material objects, therefore, are continuously absorbing and radiating electromagnetic waves having various frequencies, thus exchanging energy with other objects, near and far. This makes it possible to observe objects at a distance by detecting the electromagnetic waves that they radiate or reflect, or to affect them in various ways by beaming electromagnetic waves at them. These facts make the manipulation of electromagnetic waves at various frequencies (i.e., from various parts of the electromagnetic spectrum) fundamental to many fields of technology and science, including radio communication, radar, infrared sensing, visible-light imaging, lasers, x rays, astronomy, and more.

The Spectrum

The spectrum has been divided by physicists into a number of frequency ranges or bands denoted by convenient names. The points at which these bands begin and end do not correspond to shifts in the physics of electromagnetic radiation; rather, they reflect the importance of different frequency ranges for human purposes. Below, the various parts of the spectrum are named in order, lowestfrequency to highest-frequency, and their properties described.

Radio. Radio waves are typically produced by time-varying electrical currents in relatively large objects (i.e., at least centimeters across). This category of electromagnetic waves extends from the lowest-frequency, longest-wave-length electromagnetic waves up into the gigahertz (GHz; billions of cycles per second) range. The U.S. government officially allocates sub-bands of the radio frequency spectrum to various military and commercial purposes from 9 × 10³ Hz to 3 × 10¹¹ Hz, dividing this part of the spectrum up into over 450 non-overlapping frequency bands. These bands are exploited by different users and technologies: for example, broadcast FM is transmitted using frequencies on the order of 10⁶ Hz, while television signals are transmitted using frequencies on the order of 10⁸ Hz (about a hundred times higher). In general, higher-frequency signals can be used to transmit lower-frequency information, but not the reverse; thus a voice signal with a maximum frequency content of 20 kHz (kilohertz, thousands of Hertz) can, if desired, be transmitted on a signal centered in the Ghz range, but it is impossible to transmit a television signal over a broadcast FM station. From 10⁹ to 3 × 10¹¹ Hz, radio waves are termed microwaves; these are used for high-speed communications links, heating food, radar, and electromagnetic weapons, that is, devices designed to irritate or injure people or to disable enemy devices. The microwave frequencies used for communications and radar are subdivided still further into frequency bands with special designations, such as "X BAND" and "Y band." Microwave radiation from the Big Bang, the cosmic explosion in which the Universe originated, pervades all of space.

Infrared. Electromagnetic waves from approximately 10¹² to 5 µlt 10¹⁴ Hz are termed infrared radiation. The word infrared means "below red," and is assigned to these waves because their frequencies are just below those of red light, the lowest-frequency light visible to human beings. Infrared radiation is typically produced by molecular vibrations and rotations (i.e., heat) and causes or accelerates such motions in the molecules of objects that absorb it; it is, therefore, perceived by the body through the increased warmth of skin exposed to it. Since all objects above absolute zero emit infrared radiation, electronic devices sensitive to infrared can form images even in the absence of visible light. Because of their ability to "see" at night, imaging devices that electronically create visible images from infrared light are important in security systems, on the battlefield, and in observations of the Earth from space for both scientific and military purposes.

Visible. Visible light consists of elecromagnetic waves with frequencies in the 4.3 × 10¹⁴ to 7.5 × 10¹⁴ Hz range. Waves in this narrow band are typically produced by rearrangements (orbital shifts) in the outer electrons of atoms. Most of the energy in the sunlight that reaches the Earth's surface consists of electromagnetic waves in this narrow frequency range; our eyes have therefore evolved to be sensitive to this band of the electromagnetic spectrum. Photovoltaic cells—electronic devices which turn incident electromagnetic radiation into electricity—are also designed to work primarily in this band, and for the same reason. Because half the Earth is liberally illuminated by visible light at all times, this band of the spectrum, though narrow (less than an octave), is essential to thousands of applications, including all forms of natural and many forms of mechanical vision.

Ultraviolet. Ultraviolet light consists of electromagnetic waves with frequencies in the 7.5 × 10¹⁴ to 10¹⁶ Hz range. It is typically produced by rearrangements in the outer and intermediate electrons of atoms. Ultraviolet light is invisible, but can cause chemical changes in many substances: for living things, consequences of these chemical changes can include skin burns, blindness, or cancer. Ultraviolet light can also cause some substances to give off visible light (flouresce), a property useful for mineral detection, art-forgery detection, and other applications. Various industrial processes employ ultraviolet light, including photolithography, in which patterned chemical changes are produced rapidly over an entire film or surface by projecting patterned ultraviolet light onto it. Most ultraviolet light from the sun is absorbed by a thin layer of ozone (O₃ in the stratosphere, making the Earth's surface much more hospitable to life than it would be otherwise; some chemicals produced by human industry (e.g., chlorfluorocarbons) destroy ozone, threatening this protective layer.

X rays. Electromagnetic waves with frequences from about 10¹⁶ to 10¹⁹ Hz are termed x rays. x rays are typically produced by rearrangements of electrons in the innermost orbitals of atoms. When absorbed, they are capable of ejecting electrons entirely from atoms and thus ionizing them (i.e., causing them to have a net positive electric charge). Ionization is destructive to living tissues because ions may abandon their original molecular bonds and form new ones, altering the structure of a DNA molecule or some other aspect of cell chemistry. However, x rays are useful in medical diagnosis and in security systems (e.g., airline luggage scanners) because they can pass entirely through many solid objects; both traditional contrast images of internal structure (often termed "x RAYS" for short) and modern computerized axial tomography images, which give much more information, depend on the penetrating power of x rays. x rays are produced in large quantities by nuclear explosions (as are electromagnetic waves at all other frequencies above the radio band), and have been proposed for use in a space-based ballistic-missile defense system as follows: X-rays emitted by an orbital nuclear explosion would stimulate coherent, highly-directional x-ray emission (x-ray lasing) in special fibers placed next to the warhead that had been pre-aimed at ballistic warheads arcing through space. The resulting xray laser bursts would disable the warheads or knock them off course. There are, however, many technical and political problems with such a scheme, and its feasibility has never been demonstrated.

Gamma rays. All electromagnetic waves above about 3 × 10¹⁹ Hz are termed gamma rays ([.gamma] rays). Gamma rays are typically produced by rearrangements of particles in atomic nuclei. A nuclear explosion produces large quantities of gamma radiation, which is both directly and indirectly destructive of life. By interacting with the Earth's magnetic field, gamma rays from a high-altitude nuclear explosion can cause an intense pulse of radio waves termed an electromagnetic pulse (EMP). EMP may be powerful enough to burn out unprotected electronics on the ground over a wide area; most military hardware is therefore "hardened" against EMP to some degree, although hardening standards vary from one sector of the military to another.

Radio-frequency spectrum allocation. Radio waves present a unique regulatory problem, for only one broadcaster at a particular frequency can function in a given area. (Signals from overlapping same-frequency broadcasts would be received simultaneously by antennas, interfering with each other.) Throughout the world, therefore, governments regulate the radio portion of the electromagnetic spectrum, a process termed spectrum allocation. In the U.S., since the passage of the Communications Act of 1934, the radio spectrum has been deemed a public resource. Individual private broadcasters are given licenses allowing them to use specific portions of this resource, that is, specific sub-bands of the radio spectrum. The United States Commerce Department's National Telecommunications and Information Administration (NTIA) and FCC (Federal Communications Commission) oversee the spectrum allocation process, which is subject to intense lobbying by various telecommunications stakeholders.

Military and security significance of the electromagnetic spectrum. Virtually all forms of military, espionage, and security activity exploit some portion of the electromagnetic spectrum. The transmission, reception, and interception of radio messages are perhaps the most obvious examples, second to the use of light in the visible spectrum for ordinary vision and most technical imaging. More exotic direct applications of electromagnetic radiation are also under development, including the direct use of electromagnetic waves (e.g., laser light) as a destructive weapon, and for various other methods of electronic warfare, defined by the U.S. Joint Chiefs of Staff as "any military action involving the use of electromagnetic and directed energy to control the electromagnetic spectrum or to attack the enemy." Jamming of enemy transmissions and protection of friendly forces against enemy jamming attempts are typical forms of electronic warfare.

In summary, it can be said that the manipulation of every level of the electromagnetic spectrum is of urgent technological interest, but most work is being done in the radio through the visible portions of the spectrum (below 7.5 × 10¹⁴ Hz), where communications, radar, and imaging can be accomplished.

Further Reading

Electronic

"Electromagnetic Spectrum Use in Joint Military Operations." Chairman of the Joint Chiefs of Staff Instruction. May 1, 2000. <http://www.dtic.mil/doctrine/jel/cjcsd/cjcsi/3320_01.pdf> (Jan. 30, 2003).

Schroeder, Norbert. "Radio Frequency Spectrum Allocations in the United States." National Telecommunications and Information Administration. July 1, 2000. <http://www.ntia.doc.gov/osmhome/chart_00.htm> (Jan. 30, 2003).

The family of electromagnetic waves. The electromagnetic spectrum, starting from the waves with the longest wavelengths (and least energy), consists of radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma radiation. Members of the family differ from one another only in their wavelength, or frequency. For example, the wavelength of blue light is roughly half that of red light, but the waves corresponding to the two colors are otherwise identical.

List or diagram showing the range of electromagnetic radiation.

(DOD) The range of frequencies of electromagnetic radiation from zero to infinity. It is divided into 26 alphabetically designated bands. See also electronic warfare.

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.^[1] The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from below frequencies used for modern radio to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The long wavelength limit is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.

Although some radiations are marked as N for no in the diagram, some waves do in fact penetrate the atmosphere, although extremely minimally compared to the other radiations

Legend^[2][3][4]

γ= Gamma rays	MIR= Mid infrared	HF= High freq.
HX= Hard X-Rays	FIR= Far infrared	MF= Medium freq.
SX= Soft X-Rays	Radio waves	LF= Low freq.
EUV= Extreme ultraviolet	EHF= Extremely high freq.	VLF= Very low freq.
NUV= Near ultraviolet	SHF= Super high freq.	VF/ULF= Voice freq.
Visible light	UHF= Ultra high freq.	SLF= Super low freq.
NIR= Near Infrared	VHF= Very high freq.	ELF= Extremely low freq.
		Freq=Frequency

Contents 1 Range of the spectrum 2 Rationale 3 Types of radiation 3.1 Radio frequency 3.2 Microwaves 3.2.1 Terahertz radiation 3.3 Infrared radiation 3.4 Visible radiation (light) 3.5 Ultraviolet light 3.6 X-rays 3.7 Gamma rays 4 See also 5 References 6 External links

Range of the spectrum

EM waves are typically described by any of the following three physical properties: the frequency f, wavelength λ, or photon energy E. Frequencies range from 2.4x10²³ Hz (1 GeV gamma rays) down to tiny fractions of Hertz (nanohertz for astronomical scale waves). Wavelength is inversely proportional to the wave frequency, so gamma rays have very short wavelengths that are fractions of the size of atoms, whereas wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma rays have the highest energy (around a billion electron volts) and radio waves have very low energy (around femto electron volts). These relations are illustrated by the following equations:

or

or

Where:

c = 299,792,458 m/s (speed of light in vacuum) and

h = 6.62606896(33)×10⁻³⁴ J·s (Planck's constant).

Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased. Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated.

Generally, EM radiation is classified by wavelength into radio wave, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum (photon) it carries.

Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon which has a wavelength of 21.12 cm. Also, frequencies of 30 Hz and below can be produced by and are important in the study of certain stellar nebulae^[5] and frequencies as high as 2.9×10²⁷ Hz have been detected from astrophysical sources.^[6]

Rationale

Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can be so different that it seems to be justified to refer to different types of radiation. At the same time, there is a continuum containing all these "different kinds" of electromagnetic radiation. Thus we refer to a spectrum, but divide it up based on the different interactions with matter.

Region of the spectrum	Main interactions with matter
Radio	Collective oscillation of charge carriers in bulk material (plasma oscillation). An example would be the oscillation of the electrons in an antenna.
Microwave through far infrared	Plasma oscillation, molecular rotation
Near infrared	Molecular vibration, plasma oscillation (in metals only)
Visible	Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only)
Ultraviolet	Excitation of molecular and atomic valence electrons, including ejection of the electrons (photoelectric effect)
X-rays	Excitation and ejection of core atomic electrons
Gamma rays	Energetic ejection of core electrons in heavy elements, excitation of atomic nuclei, including dissociation of nuclei
High energy gamma rays	Creation of particle-antiparticle pairs. At very high energies a single photon can create a shower of high energy particles and antiparticles upon interaction with matter.

Types of radiation

The electromagnetic spectrum

While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power.

The distinction between X and gamma rays is based on sources: gamma rays are the photons generated from nuclear decay or other nuclear and subnuclear/particle process, whereas X-rays are generated by electronic transitions involving highly energetic inner atomic electrons. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays, even though their energy may exceed 6 megaelectronvolts (0.96 pJ),^[7] whereas there are Many (77 Known to be less than 10 keV (1.6 fJ)) low-energy nuclear transitions (e.g. the 7.6 eV (1.22 aJ) nuclear transition of Th-229), and despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin.^[8]

Also, the region of the spectrum of the particular electromagnetic radiation is reference-frame dependent (on account of the Doppler shift for light) so EM radiation which one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the cosmic microwave background. It was produced, when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos. However, for particles moving near the speed of light, this radiation will be blue-shifted in their rest frame. The highest energy cosmic ray protons are moving such that, in their rest frame, this radiation is blueshifted to high energy gamma rays which interact with the proton to produce bound quark-antiquark pairs (pions). This is the source of the GZK limit.

Radio frequency

Main article: Radio frequency

Radio waves generally are utilized by antennas of appropriate size (according to the principle of resonance), with wavelengths ranging from hundreds of meters to about one millimeter. They are used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use radio waves. The use of the radio spectrum is regulated by many governments through frequency allocation.

Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up, causing thermal effects and sometimes burns. This is exploited in microwave ovens.

Microwaves

Main article: Microwaves

Plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.

The super high frequency (SHF) and extremely high frequency (EHF) of microwaves come next up the frequency scale. Microwaves are waves which are typically short enough to employ tubular metal waveguides of reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi, although this is at intensity levels unable to cause thermal heating.

Volumetric heating, as used by microwaves, transfer energy through the material electro-magnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.

When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.

Terahertz radiation

Main article: Terahertz radiation

Terahertz radiation is a region of the spectrum between far infrared and microwaves. Until recently, the range was rarely studied and few sources existed for microwave energy at the high end of the band (sub-millimetre waves or so-called terahertz waves), but applications such as imaging and communications are now appearing. Scientists are also looking to apply terahertz technology in the armed forces, where high frequency waves might be directed at enemy troops to incapacitate their electronic equipment.

Infrared radiation

Main article: Infrared radiation

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:

• Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimetre" in astronomy, reserving far infrared for wavelengths below 200 μm.
• Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.
• Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical processes that are relevant for this range are similar to those for visible light.

Visible radiation (light)

Main article: Visible spectrum

Above infrared in frequency comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. The light we see with our eyes is really a very small portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if you could see it) would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end.

Electromagnetic radiation with a wavelength between 380 nm and 760 nm (790–400 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant.

If radiation having a frequency in the visible region of the EM spectrum reflects off of an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.

At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image. The coding used in such data is similar to that used with radio waves.

Ultraviolet light

Main article: Ultraviolet

The amount of penetration of UV relative to altitude in Earth's ozone

Next in frequency comes ultraviolet (UV). This is radiation whose wavelength is shorter than the violet end of the visible spectrum, and longer than that of an x-ray.

Being very energetic, UV can break chemical bonds, making molecules unusually reactive or ionizing them (see photoelectric effect), in general changing their mutual behavior. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer, if the radiation irreparably damages the complex DNA molecules in the cells (UV radiation is a proven mutagen). The Sun emits a large amount of UV radiation, which could quickly turn Earth into a barren desert. However, most of it is absorbed by the atmosphere's ozone layer before reaching the surface.

X-rays

Main article: X-rays

After UV come X-rays, which are also ionizing, but due to their higher energies they can also interact with matter by means of the Compton effect. Hard X-rays have shorter wavelengths than soft X-rays. As they can pass through most substances, X-rays can be used to 'see through' objects, most notably diagnostic x-ray images in medicine (a process known as radiography), as well as for high-energy physics and astronomy. Neutron stars and accretion disks around black holes emit X-rays, which enable us to study them. X-rays are given off by stars, and strongly by some types of nebulae.

Gamma rays

Main article: Gamma rays

After hard X-rays come gamma rays, which were discovered by Paul Villard in 1900. These are the most energetic photons, having no defined lower limit to their wavelength. They are useful to astronomers in the study of high energy objects or regions, and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. Gamma rays are also used for the irradiation of food and seed for sterilization, and in medicine they are used in radiation cancer therapy and some kinds of diagnostic imaging such as PET scans. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.

Note that there are no precisely defined boundaries between the bands of the electromagnetic spectrum. Radiation of some types have a mixture of the properties of those in two regions of the spectrum. For example, red light resembles infrared radiation in that it can resonate some chemical bonds.

See also

Electronics portal

• Atmospheric window
• Bandplan
• Cosmic rays
• Electromagnetic spectroscopy
• Ionizing radiation
• Ozone layer
• Radiant energy
• Radiation
• Spectroscopy
• V band
• W band
• Electroencephalography

References

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (November 2007)

1. ^ "Imagine the Universe! Dictionary". http://imagine.gsfc.nasa.gov/docs/dict_ei.html#em_spectrum. 
2. ^ What is Light? – UC Davis lecture slides
3. ^ The Electromagnetic Spectrum, The Physics Hypertextbook
4. ^ Definition of frequency bands on vlf.it
5. ^ J. J. Condon and S. M. Ransom. "Essential Radio Astronomy: Pulsar Properties". National Radio Astronomy Observatory. http://www.cv.nrao.edu/course/astr534/Pulsars.html. Retrieved 2008-01-05. 
6. ^ A. A. Abdo; B. Allen; D. Berley; E. Blaufuss; S. Casanova; C. Chen; D. G. Coyne; R. S. Delay; B. L. Dingus; R. W. Ellsworth; L. Fleysher; R. Fleysher; I. Gebauer; M. M. Gonzalez; J. A. Goodman; E. Hays; C. M. Hoffman; B. E. Kolterman; L. A. Kelley; C. P. Lansdell; J. T. Linnemann; J. E. Mc Enery; A. I. Mincer; I. V. Moskalenko; P. Nemethy; D. Noyes; J. M. Ryan&#x A;;&#x A; F. W. Samuelson&#x A;;&#x A; P. M. Saz Parkinson; M. Schneider; A. Shoup&#x A;;&#x A; G. Sinnis&#x A;;&#x A; A. J. Smith; A. W. Strong; G. W. Sullivan; V. Vasileiou; G. P. Walker; D. A. Williams; X. W. Xu; G. B. Yodh (2007-03-20). "Discovery of TeV Gamma‐Ray Emission from the Cygnus Region of the Galaxy". The Astrophysical Journal Letters 658: L33. doi:10.1086/513696. 
7. ^ CORRECTIONS TO MUONIC X-RAYS AND A POSSIBLE PROTON HALO slac-pub-0335 (1967)
8. ^ Hyperphysics (see Gamma-Rays

External links

Wikimedia Commons has media related to: Electromagnetic spectrum

• Australian Radiofrequency Spectrum Allocations Chart (from Australian Communications and Media Authority)
• Canadian Table of Frequency Allocations (from Industry Canada)
• U.S. Frequency Allocation Chart — Covering the range 3 kHz to 300 GHz (from Department of Commerce)
• UK frequency allocation table (from Ofcom, which inherited the Radiocommunications Agency's duties, pdf format)
• Flash EM Spectrum Presentation / Tool - Very complete and customizable.
• How to render the color spectrum / Code - Only approximately right.

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