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In geometry, a prism is a polyhedron made of two parallel copies of some polygonal base joined by rectangular or pallelogrammic faces. When these are rectangles, the figure is said to be a right prism. Right prisms with regular bases are one of the infinite series of vertex-uniform polyhedra, the other being the antiprisms. The cube is a particular type of right square prism which is also edge- and face-uniform and so counts among the Platonic solids. The dual of a prism is a bipyramid. Its volume is given by the product of the area of the base and the distance between them.

In optics, a prism is a device used to reflect light or to break it up (to disperse it) into its constituent spectral colors, traditionally built in the shape of a triangular prism.

As light moves from one medium (say air) to another denser medium (say the glass of the prism), it is slowed down and as a result either bent (refracted) or reflected. The angle that the beam of light makes with the interface as well as the refractive indices of the two media determine whether it is reflected or refracted, and by how much (see refraction, total internal reflection).

Prisms are used to reflect light, for instance in binoculars, since they are easier to manufacture than mirrors. Prisms can also be used to break up light into its constituent spectral colors because the refractive index depends on frequency (see dispersion); the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light.