Determination of Velocity of Ultrasonic waves Principle When ultrasonic waves passed through transparent liquid medium in a container, the waves get reflected. These reflected waves are called echos. The direct and reflected waves superimpose to form the stationary wave pattern. These waves give rise to a periodic vibration in the density of the liquid. The change density of the liquid in turn leads to vibration in the refractive index of the liquid. The density of the liquid would be maximum at nodal planes, while at the antinodal planes, it would be minimum. Such a liquid column subjected ultrasonic waves behaves like a grating. Under this condition, if a parallel beam of light is passed through the liquid at right angles to the waves, the liquid acts as a diffraction grating. Such a grating is known as acoustic grating. Experiment Thee experimental arrangement is shown in figure. There is a glass vessel containing tho liquid with a reflector fixed within the ...