Mechanism
When ultrasonic waves are generated in a liquid kept in rectangular vessel, the wave can be reflected from the walls of the vessel.These reflected waves are called echos. The direct and reflected waves get superimposed, which causes a standing wave to be formed. The density of the liquid at the node will be more than the density at an antinode. Under these conditions, if a parallel beam of light is passed through the liquid at right angles to the wave the liquid acts as a diffraction grating. Such a grating is known as an acoustical grating.
Here, the antinode acts as the transmitting slit and the node acts as the opaque part...thus resembling a normal ruled diffraction grating. This is obvious because the antinodes have points of minimum density and hence allow more amount of light to pass through them than those at nodes. Thus, the antinodes act like slits.
Read more about this topic: Ultrasonic Grating
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