The bending of the normal to the wavefront of a propagating wave upon passing from one medium to another where the propagation velocity is different. The most common example is the refraction of light on passing from air to a liquid, which causes submerged objects to appear displaced from their actual positions. Refraction causes ocean waves to approach shorelines nearly perpendicularly as they reach shallow water and slow down when they "feel" the bottom. Refraction is also the reason that prisms separate white light into its constituent colors. This occurs because different colors (i.e., frequencies) of light travel at different speeds in the prism, resulting in a different amount of deflection of the wavefront for different colors.

The amount of refraction can be characterized by a quantity known as the index of refraction, commonly denoted n. The equation specifying the relationship between the angles to the normal in two media is called Snell's law (or sometimes simply the law of refraction).

Conical Refraction, Ewald-Oseen Extinction Theorem, Fermat's Principle, Index of Refraction, Law of Refraction, Least Time Principle, Malus and Dupin Theorem, Prism, Reflection, Refractivity, Snell's Law