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In geometry, a prism is a polyhedron made of two parallel copies of some polygonal base joined by faces that are rectangles or parallelograms. In the case these joining faces are rectangular, the object is said to be a right prism. The rectangular prism, or cuboid, and square prism are among the types of right prism, with a rectangular and square base, respectively.

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. The volume of a prism is the product of the area of one of the bases 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 (colors of the rainbow), traditionally built in the shape of a right prism with triangular base.

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.

Until Isaac Newton, it was thought that prisms added colors to white light. Newton placed a second prism such that a separated color would pass through it and found the color unchanged. He concluded that prisms separate colors. He also used a lens and a second prism to recompose the rainbow into white light.

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