Speed of Light

Denoted with the symbol "c," the speed of light is 299,792,458 metres per second and is often rounded as 300,000 kilometres per second or 186,000 miles per second.

4,236 Questions
Speed of Light
Visible Light Spectrum

Does x ray travel faster than visible light?

X-rays travel at the same speed as visible light. All frequencies of electro-magnetic radiation (of which light is a very narrow band) travel at the speed of light (C) in a vacuum: 299,792,458 meters per second.

The speed of light varies in different medium. Given an angle of incidence other than 90 or 0, the wave will shift direction an amount determined by its frequency. The higher the frequency, the greater the shift. The transition between two medium of different refractive indices causes a variation in the phase velocity of the wave, but does not affect its frequency.

Planet Earth
Speed of Light
The Solar System

How much time would taken by sun rays to get to the sun from the earth?

8.3 seconds

Domestic Dogs
Units of Measure
Speed of Light

How many lightyears is in one year in human years?

A lightyear is actually a unit of distance not time. In which case the answer would be 5.88 trillion miles. That's the distance that light will travel in one year in space

If a human would try to travel a lightyear in the average speed of a space shuttle, say 14000 miles an hour, it would take almost 49,000 years on Earth. If you walked at 1 mph nonspot, it would take 572 million years, or 5 million lifetimes.

I'd stay on earth if I was you...

Math and Arithmetic
Speed of Light

When did scientists attempt to measure the speed of light?

Ole Rømer, in 1676, by observing Jupiter's moons.

Speed of Light

What is the speed of light?

The speed of light in a vacuum is 299,792,458ms-1 or 183,000 miles per second.

In scientific notation and rounding for ease of memorization, this is usually written as 3.00 x 108 m/s

It's value however is different for other media and can be calculated using the following formula:


That depends on what it's traveling through.

When it's traveling through nothing ... empty space, vacuum ... the speed

is 299,792,458 meters per second.

The speed is somewhat less in any material that light might travel through ...

air, water, glass, diamond, jello, etc. It's different in each material.

186,282.397 miles per second or 299,792,458 metres per second

The number to remember is: 300,000 kilometres per second

Math and Arithmetic
Speed of Light

What is the speed of light divided by frequency?

It's wavelength.

Speed of Light

Is it possible to travel half the speed of light?

Yes it absolutely is, however, this is not for humans. At particle accelerators, like the Large Hadron Collider in Geneva, Switzerland, physics are able to send particles 99.99999% the speed of light. The particles are only 3 meters per second slower than light itself. In turn, physics are able to shoot a particle half the speed of light approx. 150,000 meters per second.

Speed of Light

Why does light go in the speed of 300000000 meter per second?

Because a photon is massless. Since photons are massless, by the laws of physics all massless objects MUST travel at the speed of light.

E=MC^2 "E" is Energy, "M" is Mass, and "C" is the speed of light squared or multiplied by its self.


Another contributor tried to respond to the same question:

Strange as it may seem, the speed of light was accurately predicted long before there was

any way to measure it. The prediction falls out naturally from the Physics and Math of

wave motion.

In the equation for a wave, a constant appears in one of the terms, and the speed of

the wave is the square root of the reciprocal of that constant.

When you take Maxwell's four equations of electricity and magnetism, and massage them

in such a way as to derive the equation of the electromagnetic wave that they would

produce, the constant in that particular place in the equation is the product of the

vacuum permittivity of space ... 8.854188 x 10-12 ... and the magnetic permeability

of space ... 1.256637 x 10-6 . Now watch closely as I handle these numbers, with

nothing up my sleeve:

The product is (8.854188 x 10-12) x (1.256637 x 10-6) = 1.11265 x 10-17 .

The reciprocal of that number is 8.987552 x 1016 ,

and the square root of that number, which should be the speed of the wave, is

299,792,462 meters per second ...

only 4 m/s different from the official, internationally accepted number !

Admittedly, this still doesn't answer your question of "why", but your question is now

"Why are the vacuum permittivity and the magnetic permeability of free space both

exactly the numbers that they are ?"

We hope you consider this to be some progress; but if you've simply lost all interest

by now, we'll understand.

A third response:

The speed of light is 300 000 000 m/s because we, as humans, defined what is 1 meter and what is 1 second. The speed of light itself is just a fundamental constant, a fact of this universe. It just "is."

In some branches of physics, the speed of light is just defined as 1 distance/time.

If you understand special relativity, consider what would happen if the speed of light were say, 10 m/s.

Speed of Light

What convinced scientists that light is a particle?

Nothing. Scientists believe that light can act as both a particle and a wave.

Look up the famous double-slit experiment.

Human and Animal Interaction
Sonic the Hedgehog
Speed of Light

How does sonic run fast?

He runs fast because of his sheos and plus he was born to run fast

Speed of Light

What is an object that allows light to pass through but not clearly?


Computer Printers
Speed of Light

How does laser speed guns use lasers?

Laser speed guns use lasers by Doppler shift measurement.

Speed of Light

Does the Milky Way travel at the speed of light?

No. No material object can travel at the speed of light.

Does the Milky Way galaxy move at all? An excellent, and perhaps unanswerable question. Movement exists only in reference to other things, and there is, according to Einstein, no "preferred frame of reference" that we might use to determine if the Milky Way galaxy is moving.

We see that other galaxies appear to be moving away from us. Are the other galaxies moving while we are standing still, or is the Milky Way moving away from THEM? We do not know. We may never know.

However there is evidence that, in general, galaxies are moving away from each other, and there are a few theories that go along with this. On that they will come back toward each other, they will slow and stop, no longer moving away nor towards each other, and finally that they will just continue moving away which is most widely accepted as evidence suggest that they are actually accelerating rather than decelerating.

Electricity and Magnetism
Speed of Light

Explain what the value of the index of refraction of a medium means?

It characterizes how much resistance the medium has to light that passes through it.

Speed of Light

What happens to the wavelength when light passes from air into water?

It gets shorter due to slower speed. Beware: color depends on frequency, which doesn't change.

The frequency of light is defined by its wavelength. When light travels from air into water, there is a phase shift in the electromagnetic ripple, not a frequency shift.

Science Experiments
Speed of Light

Does the angel of refraction depend on the wavelength?

Yes, the angle of refraction depend upon the wavelength.

Speed of Light

What determines the speed light?

The beam of light plays a major role in the speed of light.

The speed of any wave is determined by the characteristics of the medium

through which it's traveling. In the case of electromagnetic radiation, including

light, the medium is vacuum, and the relevant electrical characteristics are the

static permittivity and magnetic permeability. The reciprocal of the square root

of their product is the speed of light. The number was known long before the

speed of light could be measured. When it finally was, the close correlation

confirmed the hypothetical suspicion that light is electromagnetic radiation.

Speed of Light
Waves Vibrations and Oscillations

What is wave polarization?

Wave polarization is when you take light and separate it so it all travels on one plane. You can polarize light by means of transmission, reflection, refraction and scattering.

Speed of Light

Is there anything which travels faster than light?

It is generally accepted by physicists that nothing travels faster than light. There was a recent experiment in which neutrinos were observed to travel faster than light, but it is too early to give up on Einstein's theory of relativity just yet. That experiment will require closer examination.

Speed of Light
Metric System

What are the two most common systems of units?

The two most common systems of units are cgs and SI systems

Math and Arithmetic
Speed of Light

What is the speed of darkness?

The universe consists only of space, mass and energy. The word 'speed' is applicable only to mass (solid, liquid, or gas) or energy (heat, light, electromagnetic waves, etc.). Darkness is not a mass nor energy, so the word speed cannot be applied to darkness.

  • Darkness is the absence of light. There is no speed of darkness.
  • I guess you could say that darkness "arrives" when light departs. So the speed that light "leaves" the observed space would be the speed of darkness; meaning the speed of darkness would be the speed of light, about 3.00 × 108 m/s. The same reasoning is behind the statement "there is no 'cold,' only the absence of heat."
  • I think the contributor is asking a joke or riddle - a play on "the speed of light."
  • There is a "Speed of Darkness". It is the fifth studio album by the Celtic Punk band Flogging Molly. It was released May 31, 2011.
  • According to the Discworld series of books by Terry Pratchett, Darkness is faster than the speed of light as no matter how fast light tries to get there, darkness always got there first.
Speed of Light

What is the definition of speed of light?

The definition of the speed of light in a vacuum is 300,000,000 meters/sec, or 186,000 miles/sec.

Earth Sciences
Speed of Light

How fast is the speed of light in mph?

1mile = 1.609 km

Speed of light in kms-1 = 300000 kms-1

Speed of light in miles per sec = 300000/1.609 ms-1

= 186451.21 mps

In mph, speed of light = 671224362.958 mph

Speed of Light

How did Galileo was able to measure speed of the light?

Galileo spoke of how he tried to measure the speed of light by having two people with covered lanterns stand far apart, having one open the lantern, then having the other open HIS lantern when he saw the first lantern open, and finally having the first person measure the time it took between his initial opening of his lantern and his seeing of the second lantern.

Galileo could not measure any difference between the delay of the first person seeing the second lantern opening, whether the two people were very close to each other or half a mile apart. Galileo thus concluded that the speed of light was "if not infinite, then exceedingly fast."

Speed of Light

Why doesn't light always travel at exactly the same speed?

The speed of light in a vacuum is constant, it's value is c.

However c changes with different mediums (air, water...etc). It depends on the index of refraction, n. For example, here is how you would calculate the speed of light in air:

vair= c/nair

In other words, the speed of light changes depending on the medium that it travels through.


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