Prism (optics)

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For other uses, see Prism (disambiguation).

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

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.

1 How prisms work
2 Types of prisms
3 See also
4 References
5 External links

[edit] How prisms work

As light moves from one medium to another (for example, from air into the glass of the prism), it changes speed. As a result, the light's path is bent (refracted) and some of the light is reflected. The angle that the beam of light makes with the interface, as well as the refractive indices of the two media, determines how much of the light is reflected, and by how much its path is bent. The refractive indices of most media vary with wavelength or color of light, a phenomenon known as dispersion, causing light of different colors to separate when refracted at the surfaces of the prism.

Isaac Newton first thought that prisms split colors out of colorless light.^{[citation needed]} 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. This experiment has since become a famous example of the new scientific 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.

[edit] Types of prisms

A triangular prism, dispersing light

[edit] Dispersive prisms

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.

[edit] Reflective prisms

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

[edit] Polarizing prisms

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.