Index of Refraction

When light, or any EM wave, travels in a vacuum its speed is:
c = 3.00 x 108 m/s.

In any other medium light travels at a different (generally slower) speed. The speed of light v in a given material depends on the index of refraction, n, which is the ratio of the speed of light in vacuum to the speed of light in the medium:
index of refraction: n =

When light travels from one medium to another, the speed and the wavelength change, although the frequency remains the same. Because v = fλ, the index of refraction can also be stated in terms of wavelength:
n =

where λ is the wavelength in vacuum and λ/ is the wavelength in the medium.

Medium Index of Refraction
Vacuum 1.000
Air 1.000293
Water 1.333
Crown Glass 1.52
Salt 1.544
Diamond 2.419

The values above are valid for light with a wavelength of 589 nm in vacuum.