Opalescence

Article By:

Chu, Benjamin Department of Chemistry, State University of New York, Stony Brook, New York.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.469800

The milky iridescent appearance of a dense transparent medium when the fluid or solid is illuminated by polychromatic radiation in the visible range, such as sunlight. Slight changes in the rainbowlike color of the system can occur, depending on the scattering angle, that is, the angle between the directions of incident radiation and of observation. All dense transparent fluids have local density fluctuations due to the thermal motions of molecules, or local concentration fluctuations due to the presence of a second component, such as colloidal suspensions or macromolecules in solution. Local fluctuations in density (or concentration) are accompanied by local fluctuations in the refractive index. Because the fluid is optically inhomogeneous, some of the light is scattered to the side. Normally, the amount of light scattered is very small, perhaps of the order of magnitude of 10⁻⁴ or less of its incident radiation. Whenever the amplitude of fluctuation becomes large, a significant portion of the incident light may be scattered. The transmitted light is then visibly weakened, and the fluid looks turbid. In solids, the local optical inhomogeneities may be frozen-in, making the system turbid (opalescent).

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