Diffraction of light by two superposed parallel ultrasonic waves,having arbitrary frequencies. General symmetry properties; method of solution by series expansion

In this study about the diffraction of light by superposed parallel ultrasonics, with frequency ration ₁:n ₂, we deduce a general symmetry property for the intensities of the diffraction pattern: if the intensities of the ordersn and ?n are equal the phase-difference must be of the form:

$$\delta = \frac{{n_1 - n_2 }}{{n_1 }} \begin{array}{*{20}c} \pi \\ 2 \\ \end{array} + p \frac{\pi }{{n_1 }}$$