Fresnel approximation applicability to ultrasonic diffraction.

A method based on Fresnel or parabolic approximation of the Helmholtz scalar wave equation is presented for calculating the diffracted amplitude when a plane monochromatic light wave is incident on a vessel filled with liquid irradiated by ultrasound. The intensities of the diffracted orders are calculated and compared with the Raman-Nath and Brillouin results and coincide for the range of validity of the latter. Good agreement between the theoretical curves and the experimental values of Klein and Hiedemann has been obtained even for a range of values where the Raman-Nath expressions are not used at all.