Diffraction of light by a plane sound wave in an optically isotropic medium is described for arbitrary optical and acoustic polarizations. The index waves produced by a sound wave propagating along a crystallographic axis of a cubic crystal are determined. A partial wave analysis of the vector wave equation describing the propagation of a light wave is given, and analytic solutions of the equation are obtained in the Raman-Nath and Bragg regions of diffraction. The differences between diffraction of light by sound waves in solids and liquids are pointed out. It is shown that the solutions of the vector and scalar wave equations are significantly different only for large Bragg angles. The direction and phase dependence of a diffracted beam on the acoustic frequency and phase, respectively, is pointed out for real-time wavefront correction.