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- The
**index of refraction**of a medium in which the speed of light v < c isn ≡ c / v.

Note that the frequency is unchanged as light moves through different mediums.(For sound, c is usually taken to be the speed of sound in air, so that n for sound is often < 1.)

**Snell's law**for waves refracting at the interface of two media isn

where θ_{1}sin θ_{1}= n_{2}sin θ_{2},_{i}are measured relative to the normal to the interface between the media:Note that for refraction from a medium of greater index of refraction into a medium with a lesser index of refraction (n

_{1}> n_{2}), there are angles θ_{1}> θ_{critical}for which there is no solution:n

These angles result in_{1}sin θ_{critical}= n_{2}.**total internal reflection**back into the original medium.- Snell's law includes
**reflection**as a special case (n_{1}= n_{2}implies that θ_{1}= θ_{2}).Reflection results in a phase change of π when it is off the surface of a medium with a higher index of refraction.

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©2010, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.