In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. At least two of the flat surfaces must have an angle between them. 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 prism are not in fact in the shape of geometric prisms. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, plastic and fluorite.
A dispersive prism can be used to break light up into its constituent spectral colors (the colors of the rainbow). Furthermore, prisms can be used to reflect light, or to split light into components with different polarizations.
The term prism is a cool name for a special kind of three-dimensional solid.
A prism is a solid that has two faces that are parallel and congruent. These are called thebases of the prism. If you take any cross section of a prism parallel to those bases by making a cut through it parallel to the bases, the cross section will look just like the bases.
In the figure above, click 'show cross-section' and drag the cross section up and down. Note that it is always congruent to the bases; that is, it always has the same shape and size. This is true for right and oblique prisms.
Prisms are named for the shape of the base. In the figure above, select the various examples of a prism in the pull-down menu. Note the way the name of the prism depends on the shape of the bases.