Dispersing Prism

an optical prism used to separate (decompose into a spectrum) radiation of different wavelengths in the optical region. The operating principle of dispersing prisms is based on the phenomenon of dispersion of light. The materials for such prisms are discussed in DISPERSING PRISMS.

The following are the most commonly used types of dispersing prisms (Figure 1):

(1) Simple triangular prism with refracting angle α = 60°.

(2) Cornu prism, which is a combination of two right-angle prisms in optical contact. The prisms are made from left-and right-handed quartz (see OPTICAL ACTIVITY and OPTICALLY ACTIVE SUBSTANCE), and the crystallographic axes are parallel to the bases of the prisms. In a Cornu prism the double refraction and the rotation of the plane of polarization are compensated; as a result, the quality of the spectrum is improved. In autocollimating devices the same effect is achieved by using half of a Cornu prism: the rear surface is coated with a reflecting layer.

(3) Pellin-Broca prism (the basic design is due to Abbe), in which decomposition into a spectrum is accompanied by deviation of the light beam by 90°.

(4) Rutherford prism, which consists of three prisms that are cemented together. The angular dispersion is increased because of the large refracting angle (100°), and the losses due to reflection are comparatively small.

(5) Amici, or direct-vision, prism, which consists of three or more prisms that are cemented together. The median spectral ray passes through the Amici prism without deviation; rays with a longer or shorter wavelength are deviated to either side of the median ray. In an instrument with an Amici prism, the optical axis does not have the break typical of most spectroscopic instruments.

Also classed as a dispersing prism is the Féry prism, which not only disperses a beam into a spectrum but also focuses the beam. The front and rear faces of the prism are curved, and the rear face is a mirror—that is, a metallic coating has been applied. If the radius of curvature of the front surface is R, the spectrum is located on a circle of radius R/2.

Dispersing prisms were extensively used in spectroscopic instruments until the 1970’s. At that time there developed a tendency to use other types of dispersing elements instead of prisms in many cases.