The bending of light when it travels from one medium to another with different optical densities is called refraction. Refraction occurs because light changes speed as it passes from one medium to another. The index of refraction of a medium is a measure of how much the speed of light is reduced in the medium. When light travels from a medium with a lower index of refraction to a medium with a higher index of refraction, it bends toward the normal (the perpendicular to the surface). When light travels from a medium with a higher index of refraction to a medium with a lower index of refraction, it bends away from the normal.

The amount of bending depends on the difference in the indices of refraction of the two media. The greater the difference in the indices of refraction, the greater the bending.

Refraction is responsible for many optical phenomena, such as the formation of rainbows and the bending of light through lenses. It is also used in a variety of optical devices, such as telescopes, microscopes, and cameras.