ABSTRACT
A plane acoustic wave travelling in a transparent medium produces a periodic variation of refractive index which can act as a diffraction grating. The effects of acoustic waves on light transmission through transparent media have been known for a long time. They arise from refractive index variations associated with the pressure fluctuations of sound waves. Travelling ultrasonic waves are produced in a water tank by a piezoelectric transducer. Bragg diffraction from thermally excited ultrasonic waves in a transparent medium gives rise to a weak optical scattering mechanism first predicted by L. Brillouin. G. Alers have demonstrated the use of a scanned laser visualization technique for showing up surface and subsurface defects by the scattering of Rayleigh and Lamb waves on glass and various metal surfaces. The resolution of the spectral analysis is inversely related to the time window of the optical Fourier transformation.
