Predict why the refractive index of air is slightly higher than the refractive index of a vacuum?

1 Answer
Apr 7, 2017

The refractive index of a medium is defined as #n=c/v#, where #c# is the speed of light in a vacuum and #v# is the apparent velocity of light traveling through that medium.

The refractive index of a vacuum is #c/c=1#.

Since light seems to travel slower in air (but not by much since air is not as optically dense), the refractive index will be greater #c/v>1# since #0< v< c#.

Note that light does not actually slow down when traveling through a medium. Its actual speed is still #c#. Read more about this here on Wikipedia.