ABSTRACT
Light scattering has been used to measure thermal properties of liquids and solids ever since Einstein showed that the intensity of light scattered by density fluctuations is proportional to the isothermal compressibility of the fluid. Density fluctuations, in turn, can be considered to be a sum of pressure fluctuations and entropy fluctuations. The theory of light scattering is phenomenological. It provides the connection between the spectrum of the scattered light and various thermodynamic and transport properties of the fluid. The use of the inherently macroscopic equations of hydrodynamics to describe events on the scale probed by light scattering may seem to be an unjustified extrapolation. The spectrum of the scattered light consists of a central, unshifted component with a width proportional to the thermal diffusivity and two Brillouin-Mandel'shtam components that are shifted in frequency by an amount proportional to the adiabatic sound speed.
