Light is refracted at the boundary between two media differing in density. The amount of refraction at a boundary depends on three things: (1) The nature of the media (embodied in a characteristic quantity called the index of refraction for a medium). (2) The angle of incidence for the light ray on the boundary. (3) The wavelength of light.

The dependence of refraction on the wavelength is called dispersion. This dependence has both positive and negative implications for astronomy. On the positive side, it is the basis for the prism and its ability to separate light according to wavelength; on the negative side, it is the source of chromatic aberration in optical devices (the failure of different wavelengths to focus at the same point).