Refraction

Refraction is the change in direction of a wave as it passes from one medium to another, caused by a change in the wave's speed. For light, this occurs where the optical density changes, such as at the boundary between air and glass. The bending results from the wavefront's altered velocity as it enters the new medium.

The speed of light in a medium is v = c/n, where c is the speed in vacuum and n is the refractive index of the medium. When a beam meets a boundary at an angle, the angles of incidence and refraction are related by Snell's law: n1 sin(theta1) = n2 sin(theta2).

Because the refractive index generally depends on wavelength, different colors are bent by different amounts, a phenomenon called dispersion. This produces prismatic separation of white light into colors and underlies chromatic aberration in lenses. If light travels from a denser to a rarer medium and the incidence angle exceeds a critical value, it can be totally internally reflected, trapping light inside the medium.

Refraction has many applications. Lenses use refraction to form images in glasses, cameras, microscopes, and telescopes. In telecommunications, light is guided through optical fibers by total internal reflection following a refraction at the core–cladding boundary. Refraction also explains atmospheric effects such as mirages and the apparent bending of stars near the horizon.

Refraction is a general wave phenomenon applicable to other waves, including sound and water waves, and is governed by the properties of the media and the wavelength of the wave.