Prism, in optics, a transparent object with at least two sides at angles to each other. Optical prisms have various shapes depending on their use. One of the most common shapes is that of a triangular geometric prism. A prism refracts (bends) light rays. The amount of refraction in a particular prism depends on the wavelength, and therefore the color, of the light and on the material of which the prism is made. Most optical prisms are made of glass.

Prisms produce a variety of effects. Because the amount of refraction depends on the wavelength of the light, a prism can separate white light into its component colors. This effect is used to produce spectra in spectroscopes.

Prisms can also be used to reflect light. When light reaches a boundary between two transparent substances, such as glass and air, it can, under certain conditions, be reflected instead of crossing the boundary. This effect, called total internal reflection, is due to refraction and depends on the angle at which the light strikes the boundary. Because of the shape of a prism, light that enters it may be reflected several times before emerging from the glass. Total internal reflection allows prisms to be used as mirrors in such instruments as binoculars and periscopes.