| c |
 |
| v |
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.