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A triangular prism, dispersing light

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 prism 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 colors (the colors of the rainbow). They can also be used to reflect light, or to split light into components with different polarizations.

If a shaft of light entering a prism is sufficiently narrow, a spectrum results.

Light changes speed as it moves from one medium to another (for example, from air into the glass of the prism). This speed-change causes light striking the boundary between two media at an angle to be refracted and enter the new medium at a different angle (Huyghens principle), or to be reflected away from it. The amount of reflected light and the degree of bending of the light's path will depend on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media (Snell's law). The refractive index of a medium varies with the wavelength or color of the light used, a phenomenon known as dispersion, and this causes light of different colors to be refracted differently and to leave the prism at different angles, creating an effect similar to a rainbow. This effect can be used to separate a beam of white light into its constituent spectrum of colors.

In Isaac Newton's time, some believed that prisms created new colors. Newton passed individual colors from one prism's spectrum through a second prism and found the color unchanged, and concluded from this that that these different colors must have already been present in the original light — the prism did not create new colors, but merely separated the colors that were already there. He also used a lens and a second prism to recompose the rainbow back into white light. This experiment has become a classic example of the methodology introduced 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 internal reflection at the surfaces rather than for dispersion. If light inside the prism hits one of the surfaces at a sufficiently steep angle, total internal reflection occurs and all of the light is reflected. This makes a prism a useful substitute for a mirror in some situations.

Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends on frequency; 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.

• Triangular prism
• Abbe prism
• Pellin-Broca prism
• Amici prism

Reflective prisms are used to reflect light, for instance in binoculars.

• Pentaprism
• Porro prism
• Porro-Abbe prism
• Abbe-Koenig prism
• Schmidt-Pechan prism
• Dove prism
• Dichroic prism
• Amici roof prism

There are also polarizing prisms which can split a beam of light into components of varying polarization. These are typically made of a birefringent crystalline material.

• Nicol prism
• Wollaston prism
• Rochon prism
• Glan-Foucault prism
• Glan-Taylor prism
• Glan-Thompson prism

Wikimedia Commons has media related to:

Prism

• Fresnel biprism
• Minimum deviation
• Prism compressor
• Prism (geometry)
• The Dark Side of the Moon, an album noted for the prism on its cover
• Theory of Colours
• Triangular prism (geometry)
• Superprism

• Hecht, Eugene (2001). Optics (4th ed.). Pearson Education. ISBN 0-8053-8566-5. 

• Geometry of Two Prism Spectroscopes
• Java applet of refraction through a prism