The Speed of Light


"Light always travels at the same speed."
"Nothing can travel faster than the speed of light."

Have you heard these statements before? They are often quoted as results of Einstein's theory of relativity. Unfortunately, these statements are somewhat misleading. Let's add a few words to them to clarify:
 

"Light in a vacuum always travels at the same speed."
"Nothing can travel faster than the speed of light in a vacuum.
Vacuum cleaners are full of dust; a vacuum is not. Those additional three words in a vacuum are very important. A vacuum is a region with no matter in it. So a vacuum would not contain any dust particles (unlike the vacuum cleaner shown to the left, which is generally full of dust particles).

Light traveling through anything other than a perfect vacuum will scatter off of whatever particles exist.  This scattering slows light down when light travels through any medium other than the empty space of a vacuum, as illustrated below. 

<img SRC="light_scatter.jpg" BORDER=0 height=384 width=512>

Disclaimer:  This illustration is by necessity a simplification of the interaction between light and matter.  It is meant to help the reader remember that light in a non-vacuum medium slows down due to scattering, but it is not intended to represent an exact model of that scattering.
 

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How much does light slow down when it leaves a vacuum?
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Copyright © 1999-2004 Doris Jeanne Wagner and Rensselaer Polytechnic Institute.  All Rights Reserved.