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optical anisotropy[′äp·tə·kəl ‚an·ə′sä·trə·pē]
the difference in the optical properties of a medium as a function of the direction of propagation of optical radiation (light) in the medium and of the state of polarization of the radiation. Optical anisotropy, especially in crystal optics, is frequently understood to mean only the phenomenon of double refraction. However, it is more correct to also classify rotation of the plane of polarization, which occurs in optically active substances, as optical anisotropy.
The natural optical anisotropy of most crystals is due to the character of their structure (the difference in different directions of the field of forces binding the particles in the crystal lattice) and, in the case of some optically active crystals, also to the peculiarities of the excited state of the electrons and “ion cores” in the crystals. The natural optical activity (rotation of the plane of polarization) of substances that manifest it in any state of aggregation (crystalline, amorphous, liquid, or gaseous) is related to the asymmetric structure of the individual molecules of the substances and to the differences—resulting from this asymmetry—in the interactions of the molecules with variously polarized radiation.
Induced (artificial) optical activity arises in media that are by nature optically isotropic, upon exposure to external fields that single out certain directions in the media. These may be an electric field (the Kerr effect), a magnetic field (the Cotton-Mouton and Faraday effects), or a field of elastic forces (the phenomenon of photoelasticity). Double refraction in a fluid flow (the Maxwell effect) and in media through which light fluxes of superhigh intensity (usually laser radiation) are transmitted is also classified as artificial optical anisotropy.
S. G. PRZHIBEL’SKII