The scattering of light by sound has been examined in the past mostly in terms of a single incident plane wave and approximations have often been used that are overly restrictive for many practical cases. To gain a better understanding of the basic diffraction process associated with realistically bounded light sources, we employ a rigorous approach to examine the irradiance from a line source or scatterer embedded in a medium traversed by an acoustic disturbance. As usual, we assume that the acoustic wave modulates the dielectric properties of the medium and thus produces a periodic stratification that serves to scatter the radiant energy. After deriving a formal solution for the scattered radiant flux produced by an arbitrary periodic modulation, we show that the principal diffraction aspects can be found by examining the simpler case of sinusoidal stratification. Since this particular case may be regarded as a prototype for the more general problem, we restrict our detailed quantitative analysis to the basic sinusoidal acoustic modulation. We find that the resulting illumination exhibits regions characterized by different diffraction effects that are intimately related to the Bragg directions prescribed by the acoustic wave fronts. These directions serve to classify the various diffraction regions into three basic varieties: (1) a set of wide angular domains wherein the radiant flux density is explainable by simple geometric-optical arguments, (2) another set of wide angular domains wherein the predominant diffraction effect is described in terms of a Bragg wave mechanism, and (3) a set of narrow angular domains that serve as transition regions between the other diffraction domains. Each transition region contains a caustic that accounts for multiple peaks of radiant energy density in the neighborhood of the Bragg directions. Physical interpretations and quantitative estimates for each diffraction domain are presented by employing asymptotic results that are very accurate for the sound intensities that are usually available.
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