In 1957, Gor'kov and Pitaevskii showed that second sound in 3He–4He mixtures is strongly coupled into the concentration fluctuations, which, in turn, scatter light quite strongly (especially near the critical point). In this paper, we give a more complete analysis of the fluctuations in density, entropy, and concentration. Our discussion is based on the hydrodynamic equations of Khalatnikov, which assume that the 3He atoms move with the normal fluid, and for simplicity we omit all dissipative coefficients. If we limit ourselves to scattering from density and concentration fluctuations, our results for the first and second sound intensities agree essentially with those of Gor'kov and Pitaevskii. Near the critical point, second sound scatters light about six times more strongly than first sound does.
