From monochromatic light diffraction to colour Schlieren photography

The early experiments of Hiedemann on light diffraction led to an understan ding of many experimental aspects of the subject, and defined many topics f or further investigation. The theory of Raman and Nath broadened the unders tanding and defined many concepts that were not apparent to the experimente r. Although this theory is basically correct, it has been revised and impro ved over the years. The transition between Raman-Nath and Bragg diffraction was illuminated by the definitions of Klein, Cook and Mayer. Initially, th e main focus of many investigations was on light diffraction by ultrasonic waves because there are many aspects of the subject. Later, it was realized that this interaction is an ideal way to study wave phenomena generally si nce optical diffraction does not disturb the ultrasonic wave. The unique pe rspective provided by acousto-optic interactions has made this a useful too l. By imaging ultrasonic waves one can make detailed studies of ultrasonic wave interactions and apply the results to many other types of waves. With acousto-optics one can study evanescent waves, ultrasonic finite amplitude effects, Bragg diffraction, Bragg imaging, etc. The application to other sy stems makes this a truly formidable study. The development of modem optics has made possible the description and understanding of wave interactions th at heretofore had been mysterious. This paper begins with a description of early experiments. Later, it deals with finite-amplitude effects, parametric oscillations, Bragg diffraction a nd Bragg imaging. The study of evanescent waves by schlieren photography is a special case. Some topics have been studied by theory and experiment; so me still await an acoustical theory although an optical theory exists, alon g with acoustical verification. Contributions of many citizens of the USA a re considered. The paper ends with the application of the modem concepts of spatial filtering and information content to the analysis of colour schlie ren photography.