A theory is developed for the farfield diffraction of a Gaussian light beam that passes axially through an ultrasonic cylindrical standing wave. The approach is applicable only in the range of parameters for which the ultrasonic perturbation may be characterized as an axially symmetric phase grating. Although the diffraction integral is not completely integrated in terms of known functions, some useful expressions for the average and time‐dependent light intensities are obtained. This acousto‐optic interaction geometry, with the light‐beam waist diameter equal to the first nodal diameter in the ultrasonic field, comprises an effective light modulator. Calculations of the on‐axis time‐dependent light intensity are presented and are confirmed by experimental results.