Dispersion
The spreading of white light into its colours when it passes through glass, water or crystal.
Also: prism effect
Light bends when it passes from air into glass or water. Short wavelengths bend a little more than long ones, so blue light and red light leave at slightly different angles. The result is a spread of colours. This is dispersion. It makes rainbows, when sunlight passes through raindrops, and the flashes of colour, called fire, in a cut diamond. In cameras and telescopes it is a nuisance, causing coloured fringes around bright edges, which lens makers correct by combining different kinds of glass.