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

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). Prisms can also be used to reflect light, or to split light into components with different polarizations.

Contents

How prisms work

A triangular prism, dispersing light

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 the light to be refracted and to enter the new medium at a different angle (Huygens principle). The degree of bending of the light's path depends 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 many materials (such as glass) varies with the wavelength or color of the light used, a phenomenon known as dispersion. 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 can be used to separate a beam of white light into its constituent spectrum of colors. Prisms will generally disperse light over a much larger frequency bandwidth than diffraction gratings, making them useful for broad-spectrum spectroscopy. Furthermore, prisms do not suffer from complications arising from overlapping spectral orders, which all gratings have.

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.

Prisms and the nature of light

In Isaac Newton's time, it was believed that white light was colorless, and that the prism itself produced the color. Newton's experiments convinced him that all the colors already existed in the light in a heterogeneous fashion, and that "corpuscles" (particles) of light were fanned out because particles with different colors traveled with different speeds through the prism. It was only later that Young and Fresnel combined Newton's particle theory with Huygen's wave theory to show that color is the visible manifestation of light's wavelength.

Newton arrived at his conclusion by passing the red color from one prism through a second prism and found the color unchanged. From this, he concluded that the colors must already be present in the incoming light — thus, the prism did not create colors, but merely separated colors that are already there. He also used a lens and a second prism to recompose the spectrum 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.

Newton discussed prism dispersion in great detail in his book Opticks.[1] He also introduced the use of more than one prism to control dispersion.[2] Newton's description of his experiments on prism dispersion was qualitative, and is quite readable. A quantitative description of multiple-prism dispersion was not needed until multiple prism laser beam expanders were introduced in the 1980s.[3]

Types of prisms

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.

Grisms (grating prisms)

Diffraction gratings may be replicated onto prisms to form grating prisms, called "grisms". A transmission grism is a useful component in an astronomical telescope, allowing observation of stellar spectra. A reflection grating replicated onto a prism allows light to diffract inside the prism medium, which increases the dispersion by the ratio of refractive index of that medium to that of air.

Reflective prisms

Reflective prisms are used to reflect light, for instance in binoculars and prismatic sighting compasses.

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.

In optometry

By shifting corrective lenses off axis, images seen through them can be displaced in the same way that a prism displaces images. Eye care professionals use prisms, as well as lenses off axis, to treat various orthoptics problems:

See also

References

  • Hecht, Eugene (2001). Optics (4th ed.). Pearson Education. ISBN 0-8053-8566-5. 
  1. ^ I. Newton, Opticks (Royal Society, London, 1704).
  2. ^ The Discovery of the Spectrum of Light, http://www.juliantrubin.com/bigten/lightexperiments.html, retrieved 19 December 2009 
  3. ^ F. J. Duarte and J. A. Piper, Opt. Commun. 43, 303–307 (1982).

External links


Simple English

File:Prism rainbow
Drawing of a prism bending light

[[File:|right|128px|thumb|The glowing, colored strips are a rainbow.]] A prism is a special piece of glass, crystal, or plastic that bends light. The light bends (or refracts) because it moves slower in the glass, crystal or plastic than it does in air. If different colors of light move at different speeds, each color bends a different amount. This splits the light into lots of different colors called a spectrum. This spectrum has the same colors as a rainbow does. Rainbows are also made by bending light. They happen when light is bent by tiny drops of water floating in the air. If the light doesnt come out when you shine it into the prism, it is called internal reflection.

Uses

Prisms are used in binoSpectrumculars to bend the light. This lets the tubes of the binoculars be short. Sometimes prisms are used to reflect light instead of bending it. Some cameras use a prism to send light to the viewfinder, so the photographer can see what will be photographed.








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