If a shaft of light entering a prism is sufficiently small such that the coloured edges meet, a spectrum results In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use "prism" usually refers to this type. Some types of optical prisms are not in fact in the shape of geometric prisms. Prisms are typically made out of glass, but can be made from any material that is transparent to the wavelengths for which they are designed. A prism can be used to break light up into its constituent spectral colours (the colours of the rainbow). They can also be used to reflect light, or to split light into components with different polarizations.

How prisms work

As light moves from one medium to another (for example, from air into the glass of the prism), it changes speed. As a result, the light's path is bent (refracted) and some of the light is reflected. The angle that the beam of light makes with the interface as well as the refractive indices of the two media determine how much of the light is reflected, and by how much its path is bent. The refractive index of most media varies with wavelength or colour of light due to dispersion, causing light of different colours to separate when refracted by the surfaces of the prism. Isaac Newton first thought that prisms split colours out of colourless light. Newton placed a second prism such that a separated colour would pass through it and found the colour unchanged. He concluded that prisms separate colours. He also used a lens and a second prism to recompose the rainbow into white light. This experiment has since become a famous example of new scientific methodology during the scientific revolution. The results of this experiment dramatically transformed the field of metaphysics, leading to John Locke's primary vs secondary quality distinction. Prisms are sometimes used for the reflection at the surfaces rather than for dispersion. If light inside the prism hits one of the surfaces at a steep angle, total internal reflection occurs and all of the light is reflected. This makes prisms a useful substitute for mirrors in some situations.
(Wikipedia contributors, "Prism (optics)," accessed 2/13/07)