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:
 

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.

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.
 

How much does light slow down when it leaves a vacuum? Continue to the next page to find out!