prism

/priz"euhm/, n.

1. Optics. a transparent solid body, often having triangular bases, used for dispersing light into a spectrum or for reflecting rays of light.

2. Geom. a solid having bases or ends that are parallel, congruent polygons and sides that are parallelograms.

3. Crystall. a form having faces parallel to the vertical axis and intersecting the horizontal axes.

[1560-70; < LL prisma < Gk prîsma lit., something sawed, akin to prízein to saw, pristes sawyer]

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Piece of glass or other transparent material cut with precise angles and plane faces.

Prisms are useful for analyzing and refracting light (see refraction). A triangular prism can separate white light into its constituent colours by refracting each different wavelength of light by a different amount. The longer wavelengths (those at the red end of the spectrum) are bent the least, the shorter ones (those at the violet end) the most. The result is the spectrum of visible light, or the rainbow. Prisms are used in certain kinds of spectroscopy and in various optical systems.

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▪ optics

 in optics, piece of glass or other transparent material cut with precise angles and plane faces, useful for analyzing and reflecting light. An ordinary triangular prism can separate white light into its constituent colours, called a spectrum (see illustration—>, left). Each colour, or wavelength, making up the white light is bent, or refracted, a different amount; the shorter wavelengths (those toward the violet end of the spectrum) are bent the most, and the longer wavelengths (those toward the red end of the spectrum) are bent the least. Prisms of this kind are used in certain spectroscopes, instruments for analyzing light and for determining the identity and structure of materials that emit or absorb light.

 Prisms can reverse the direction of light by internal reflection (see illustration—>, right), and for this purpose they are useful in binoculars (q.v.).

      Prisms are made in many different forms and shapes, depending on the application. The Porro prism, for example, consists of two prisms arranged both to invert and to reverse an image and are used in many optical viewing instruments, such as periscopes, binoculars, and monoculars. The Nicol prism consists of two specially cut calcite prisms bonded together with an adhesive known as Canada balsam. This prism transmits waves vibrating in one direction only and thus produces a plane-polarized beam from ordinary light.

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