optical prism

optical prism [′äp·tə·kəl ′priz·əm]

(crystallography)

prism

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Optical prism

A simple component, made of a light-refracting and transparent material such as glass and bounded by two or more plane surfaces at an angle, that is used in optical devices, especially to change the direction of light travel, to accomplish image rotation or inversion, and to disperse light into its constituent colors. Once light enters a prism, it can be reflected one or more times before it exits the prism.

A variety of prisms can be classified according to their function. Some prisms, such as the dove prism, can be used to rotate an image and to change its parity. Image inversion by prisms in traditional binoculars is a typical application. Some prisms take advantage of the phenomenon of total internal reflection to deviate light, such as the right-angle prism and the pentaprism used in single lens reflex cameras. A thin prism is known as an optical wedge; it can be used to change slightly the direction of light travel, and therefore it can be used in pairs as an alignment device. Optical wedges are also use in stereoscopic instruments to allow the viewer to observe the three-dimensional effect without forcing the eyes to point in different directions. A variable wedge can be integrated into a commercial pair of binoculars to stabilize the line of sight in the presence of the user's slight hand movements. Other prisms such as corner-cubes can be used to reflect light backward, and are fabricated in arrays for car and bicycle retroreflectors. See Mirror optics, Reflection of electromagnetic radiation

An important application of a prism is to disperse light. When light enters at an angle to the face of a prism, it is refracted. Since the index of refraction depends on the wavelength, the light is refracted at different angles and therefore it is dispersed into a spectrum of colors. The blue color is refracted more than the red. When light reaches the second face of the prism, it is refracted again and the initial dispersion can be added to or canceled, depending on the prism angle. A combination of prisms in tandem can increase the amount of light dispersion. Dispersing prisms have been used in monochromators and spectroscopic instruments. With two prisms of different materials, it is possible to obtain light deviation without dispersion (an achromatic prism) or dispersion without deviation. See Dispersion (radiation), Refraction of waves, Spectroscopy