Diffraction of light at a standing ultrasonic wave (Debye-Sears effect) in a liquid
The experiment shows the diffraction of light at a standing ultrasonic wave (Debye-Sears effect) in a liquid. The sound velocity in the liquid (water) is determined using the dependence of the diffraction maxima on the wavelength of the diffracted laser light and the frequency of the ultrasonic wave.
Keywords: Debye-Sears effect, diffraction of light, diffraction grating, diffraction maxima, wavelength, sound velocity, standing and travelling wave
In 1932, Debye and Sears showed that light experiences a diffraction when passing through a liquid excited to high-frequency vibrations. Ultrasound can be made more or less “visible” using this effect. The density maxima and minima produced in the liquid by a standing or travelling ultrasonic wave here function like an optical diffraction grating. The grating constant of such a grating produced by an ultrasonic wave corresponds to the wavelength of this ultrasonic wave. It can be determined by means of the diffraction patterns of the light of a laser beam of a known wavelength. Because the wavelength is defined by frequency and sound velocity, the Debye-Sears effect can be used in this experiment structure in order to determine the sound velocity in the liquid being scanned with sound (e.g. water) with high accuracy.
The figures show typical diffraction patterns for green and red laser light at a standing ultrasonic wave in water at sound frequencies from 3 MHz to 10 MHz (increment: 1 MHz). As the ultrasonic frequency is raised, the distances between the individual diffraction maxima also increase, although the longer-wave red laser light is more strongly diffracted. The number of orders of diffraction is largely determined by the transmission characteristics of the sound probe and the frequency-dependent attenuation.