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critical opalescence[′krid·ə·kəl ‚ōp·ə′les·əns]
the abrupt intensification of scattering of light by pure substances in critical states, and also by solutions of liquids or gases when they reach the critical points of solubility.
Critical opalescence was explained in 1908 by M. Smoluchow-ski, who showed that the compressibility of a substance increases sharply at the critical temperature. As a result, the energy of thermal motion of the substance’s particles becomes sufficient for an “abrupt” intense increase in the number of microscopic regions in which the density of the substance deviates significantly from the average value (density fluctuations). Each such fluctuation is a disruption of the optical uniformity of the medium (a change in the index of refraction of the medium in a particular microvolume). The sharp increase in the number of fluctuations accompanying critical opalescence causes a medium that is virtually transparent at temperatures above and below the critical temperature to become a turbid medium.