Refraction
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Refraction is the change in direction of light when moving from one medium to another. Refraction produces rainbows and a similar effect in a prism. It is also the principle used in eyeglasses and the refracting telescope. The degree to which a medium refracts light is expressed as that medium's index of refraction.
The amount that the direction changes when moving between the two media is determined by Snell's Law.