The Scattering of Light from Liquid Helium

A review is given of the scattering of light by superfluid helium. A brief discussion is given first of the theory of Brillouin and Rayleigh scattering from an ordinary simple pure liquid. It is shown how this discussion must be modified in the case of the superfluid phase of purc 4He, and experimental observations of Brillouin scattering from first sound in liquid helium are described. The discussion is extended to the case of a superfluid mixture of 3He and 4He, and a recent observation of Brillouin scattering from second sound in such a mixture is described. Recent work on Raman scattering from superfluid helium (two roton production) is then discussed, and in a final section a brief review is given of the special situation that obtains in the neighbourhood of the &point.