Abstract
The backscattering of normally incident light by a medium in which there are two polariton branches is considered. The exciton boundary condition is written as u+ mu du/dz=0, and explicit results are given for this form. The phonon Green function which governs the scattering includes the reflection term which is of importance in opacity broadening. If the wavevector of the scattered light does not vary significantly over the linewidth, then the lineshapes are of the form derived by Dervisch and Loudon (1976); in general numerical evaluation is required. The opacity broadening means that the positions of the Brillouin peaks are shifted from those given by earlier theories. The equivalence between the differential formulation used here and an integrodifferential formulation is established and discussed.