Light-scattering in homogeneous media regarded as reflexion from appropriate thermal elastic waves

Attributing the scattering of light in a homogeneous transparent liquid to the local fluctuations in density, and the latter to the superposition of the thermal elastic waves of different wave-lengths maintained in the enclosure, one may, following Einstein, evaluate the scattering coefficient of the liquid along any given direction. The expression for the scattering coefficient involves a triple infinite series, which Einstein evaluates by suitably replacing it by a triple integral. The series, however, can be directly summed, and the contributions from different progressive waves to scattering studied in detail. This method brings out prominently the appropriateness of regarding scattering as regular Bragg reflexions from suitable elastic waves, and also reveals some interesting features which are missed when the summation is replaced by integration. The intensities of the Brillouin components are calculated on this basis, both in a fluid medium and in a crystal; and in the latter case the expression for the overall intensity of the Brillouin components is shown to be identical with the well-known expression of Waller in X-ray scattering