Refraction

Refraction is the change in direction of wave propagation due to a change in its transmission medium.

The phenomenon is explained by the conservation of energy and the conservation of momentum. Owing to the change of medium, the phase velocity of the wave is changed but its frequency remains constant. This is most commonly observed when a wave passes from one medium to another at any angle other than 0° from the normal. Refraction of ligth 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.