Diffraction of Laser Light by Ultrasound in Liquid

Abstract

Diffraction of light by a liquid carrying a sound wave enables a direct measurement of the acoustic wavelength in the liquid, and hence the acoustic velocity, if the frequency is known. The method of Debye and Sears for such measurements and an alternative method (striation or schlieren method) based on a visual display of standing wavefronts on a screen are reviewed. Measurements carried out with a He-Ne laser indicate that both methods work on the same diffraction phenomenon. Interpretation is given to the cause of image formation in the striation method, and a formula for the intensity distribution in the striation pattern is obtained. While utilizing a minimum of equipment, both methods illustrate some salient aspects of the compressional wave as well as physical optics and provide an excellent experiment for an advanced undergraduate laboratory or a lecture demonstration at an introductory level.