In the discussion of refraction we have seen that light may be refracted at the boundary between two media differing in density. The amount of this refraction 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, and the wavelength of the light.
The dependence of refraction on the wavelength of light 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).