In optics, a prism is a device used to refract light, reflect it or 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 (e.g. air) to another denser medium (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).

Reflective prisms are used to reflect light, for instance in binoculars, since they are easier to manufacture than mirrors. Dispersive prisms are 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. There are also polarizing prisms which can split a beam of light into components of varying polarization.

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.

Types of prisms

Reflective prisms:

• Pentaprism
• Porro prism
• Porro-Abbe prism
• dove prism
• dichroic prism
• Amici prism

Dispersive prisms:

• Abbe prism
• Pellin-Broca prism
• triangular prism

Polarizing prisms:

• Wollaston prism

See also

• prism (geometry).