Published: 13 May, 2009

Density, temperature impact light's speed

Does temperature effect the speed of light?

This is a very interesting question! Some things, like chocolate syrup, flow much faster when they are warm compared to when they are cold. But light is not like that, its speed is independent of temperature. There is, however, an indirect way temperature can affect how fast light travels through a material, but before we discuss that, let's look at some basic facts.

The speed of light plays a very special role in physics. In 1861 James Clerk Maxwell first recognized the connection between electric induction and magnetic induction, and showed that the speed of light was determined by two well known constants of nature. Then in 1905, Albert Einstein dramatically raised the significance of the speed of light through his theory of relativity where he postulated that nothing in the universe could travel faster than the speed of light. He labeled the speed of light as c, and said that it was a constant of the universe. Furthermore, he stated that mass and energy are equivalent through the relation E=mc². So the speed of light, c, is a very important number in our world.

For this to be a good standard, it has to work everywhere and under all conditions. A meter in Ithaca, NY has to be identical to a meter in Istanbul, Turkey, or to a meter on the surface of the sun, or on the frozen planet Neptune. This can only be true if the speed of light is the same in all these places, and if it is not affected by external effects like temperature. So the value of c is independent of temperature.

What about that indirect impact of temperature on the speed of light that was mentioned at the beginning of this article? You might have noticed that the speed of light, c, is defined only for a perfect vacuum. If light travels through something that has mass, such as air or glass, the velocity is reduced. The velocity of light in these materials is given as c/n, where n is called the refractive index of the material. A perfect vacuum has an index of refraction equal to unity, but almost all other materials have an index greater than one. Window glass has a refractive index of approximately n=1.5, so light travels with a velocity of about 2/3 c in glass. Air has an index of 1.0003, so the speed of light in air is nearly the same as in vacuum, but just a little slower. As a rule of thumb, materials which have higher density have larger refractive indices.

If the density of a material changes with temperature, the speed of light in that material will also change. Have you ever seen "heat waves" coming off an object that is hot? This is due to hot air rising and bending the light rays that travel past the object. Hot air has a lower density than cold air, so light travels faster in hot air than in cold air. As the hot air mixes with the surrounding cold air, the light goes slightly faster in the hot regions, and this causes the distortion. So, the speed of light can be indirectly affected by temperature, but only if the medium the light is traveling through changes density.