Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy and momentum. The proper explanation would be that due to 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 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 for a given pair of media and a wave with a single frequency, the ratio of the sines of the angle of incidence θ₁ and angle of refraction θ₂ is equivalent to the ratio of phase velocities (v₁ / v₂) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n₂ / n₁):

In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations.