A theoretical model for the description of the optical diffraction phenomenon in an isotropic medium with ultrasonically induced anisotropy is presented. It is shown how to calculate the intensities and the state of light polarization in a specific order of diffraction and numerical computations are performed in the case of fused silica. The polarization effects are discussed for Raman-Nath and Bragg type diffraction, and for diffraction under intermediate conditions, the main interest being on thr latter case. It is seen that especially in the intermediate diffraction regime the polarization properties of the diffracted show a very wide diversity. Although the state of polarization in a particular order of diffraction is mainly elliptical, linear and circular polarization may occur at suitable combinations of the sound frequency and amplitude, especially in the zero order. The possibility to create Bragg effects at higher order Bragg angles is demonstrated as well, and the accompanying polarization properties are discussed.