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In some colored transparent crystals, the effect wherein the color is quite different in different directions through the crystals. In such a crystal the absorption of light is different for different polarization directions. In colored transparent tourmaline the effect may be so strong that one polarized component of a light beam is wholly absorbed, and the crystal can be used as a polarizer. See Dichroism
the variation in color of a substance depending on the direction of propagation and the polarization of the light passing through the substance. It was first observed in 1816 by J. B. Biot and T. J. Seebeck.
Pleochroism is a result of the optical anisotropy of certain substances. Light is absorbed anisotropically in such substances, and the dependence of absorption on the wavelength of the radiation—that is, on color—leads to pleochroism. The form of pleochroism called circular dichroism, or the Cotton effect, results from a difference in absorption for right-handed and left-handed circularly polarized light. Pleochroism is most frequently observed in crystals, which also typically exhibit linear dichroism, which is a form of pleochroism wherein the absorption of the ordinary and the extraordinary rays is different. In the case of uniaxial crystals, two principal colors are distinguished when observation is carried out along the optic axis and perpendicular to the axis—that is, in the directions No and Ne. In biaxial crystals there are three principal colors in three directions, which may (in which case they are designated Ng, Nm, and Np) or may not coincide with the principal directions of the crystal. When viewed in other directions, the crystal exhibits mixed colors.
Tourmaline, which is a uniaxial crystal, and copper acetate, which is a biaxial crystal, are distinguished by strong pleochroism. The pleochroism of colored crystals is studied by examining thin sections with a polarizing microscope. As the microscope stage is rotated, the color of the crystal changes in accordance with the orientation of the section. This technique permits, for example, the identification of a mineral from published color tables.
Individual molecules may also possess anisotropy of absorption; the preferred orientation of such molecules causes pleochroism in the substances containing the molecules. Many dyes are of this type. The preferred orientation of anisotropically absorbing molecules that results in pleochroism may be natural or artificial. For example, it can be caused by an external field in colloids or by mechanical deformation in polymer films. Polaroid, the action of which is based on the phenomenon of linear dichroism, is a very important practical application of pleochroism.
REFERENCESBeliankin, D. S., and V. P. Petrov. Kristallooptika. Moscow, 1951.
Rostov, I. Kristallografiia. Moscow, 1965. (Translated from Bulgarian.)