Turbid Media

media in which light scattering by constituent or generated irregular (randomly distributed) optical nonuniformities is of significant intensity. Scattering in turbid media leads to a change in the initial direction of the light irradiating the media. The disruption of optical uniformity of a medium is expressed in the nonuniformity of the index of refraction n throughout the medium. Optical nonuniformities in turbid media are often defects in the structure of the substance or inclusions of one substance within another with a different n (fog, smoke, suspensions, emulsions, colloidal solutions, and frosted glass). In other cases, turbid media may be pure substances in which the density, concentration (in solutions), and optical anisotropy, which is caused primarily by molecular orientation, experience brief deviations from a mean value (fluctuations), resulting in variations of n within a large number of microvolumes (for example, critical opalescence). Such deviations occur upon random thermal motion of particles in the microvolumes of the medium.

A light wave incident on a turbid medium may be scattered consecutively by a number of optical nonuniformities before leaving the medium. The number of such consecutive scatterings is called the scattering multiplicity. In the general case, the “glow” of turbid media consists of waves with differing scattering multiplicities. Single scattering is observed only for a low optical thickness τ of the turbid medium (τ ≦ 0.1). The scattering frequency increases with an increase in τ (the probability of irradiation of each optical nonuniformity by light that has already been scattered by other nonuniformities increases). Turbid media in which light is singly scattered (or the component of multiple scattering is low) include pure water in layers of moderate thickness (up to a few dozen meters). Examples of turbid media with predominantly multiple scattering (called highly turbid media) are dense clouds and fogs. The optical properties of these two classes of turbid media differ significantly. In dense clouds, the optical properties are determined by the relative index of refraction and the size of the optical nonuniformities (more precisely, by the ratio of the size to the wavelength of the scattered radiation), the shape of the nonuniformities, and the number of nonuniformities per unit volume. Light scattering in highly turbid media is caused not only by structure but also by such “external” factors as the length and the shape and nature of the boundary of the medium as a whole.

The problem of light scattering in highly turbid media of great optical thickness is dealt with in the theory of radiation transfer.