IN Chapters I and II it was shown that the propagation of electromagnetic waves may be studied either by using Maxwell's equations, supplemented by the material equations, or by means of certain integral equations which utilize the polarization properties of the medium. In particular, either of these methods may also be applied to the study of the propagation of light through a medium whose density depends on space coordinates and on time. Though the former method has been used extensively in the past, the latter has only more recently been applied to such studies. In this chapter we shall apply the integral equation method to the problem of diffraction of light by a transparent homogeneous medium, disturbed by the passage of ultrasonic waves. It will be useful, however, to give first a qualitative description of this diffraction phenomenon and a brief summary of the theoretical work on this problem based on Maxwell's differential equations.

Qualitative description of the phenomenon and summary of theories based on Maxwell's differential equations

Qualitative description of the phenomenon

Ultrasonic waves are sound waves whose frequencies are higher than those of waves normally audible to the human ear. The angular frequencies of the ultrasonic waves produced in laboratories lie from about 105 s-1 to about 3 X 109 s-1, the former value representing the limit of audibility of the human ear. The corresponding range of wavelengths A of course depends on the velocity v of these waves in the medium in which they travel.