Refraction

 Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth. Refraction is described by Snell's law, which states that the angle of incidence θ1 is related to the angle of refraction θ2 by

              sinθ1/sinθ2 = v1/v2 =n2/n1

 where v1 and v2 are the wave velocities in the respective media, and n1 and n2 the refractive induces. In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations.