A theory is developed for the diffraction of light by ultrasonic waves of sufficiently high frequency, large amplitude, and/or large beamwidth that the emerging light wavefront is significantly amplitude modulated in addition to the phase modulation considered in the Raman‐Nath approach. The ultrasonic beam is considered to consist ofNadjacent sections and the final diffraction spectrum to result fromNsuccessive diffractions. The diffraction orders emerging from a given section are considered to be sources for further diffraction by the next section. Only phase modulation of the separate plane waves (diffraction orders) is considered within a section. Refraction of light is not considered as such; it is characterized by successive redistribution of light in the diffraction orders. Numerical results are obtained by iterations using an electronic computer. These results are compared with measurements.