Acoustic grating method of ultrasonic study

Here ultrasonic waves are produced by inverse piezoelectric effect by using a quartz crystal. This ultrasonic waves are allowed to travel through a water column and then is allowed to reflect back by a mirror. This forms a grating because in the liquid column a stationary wave pattern is formed with nodes and antinodes. Nodal points blocks light or they are opaque to light. Antinodal points transmit the light. If you carry out diffraction study using a spectrometer and a source of monochromatic light, u can observe diffraction pattern. You find out the angle of diffraction, substitute it in grating equation to find out wavelength of ultrasonic waves. From wavelength, we can easily find out velocity if frequency is known.