Ultrasonic grating

An ultrasonic grating is a type of diffraction grating produced by interfering ultrasonic waves in a medium altering the physical properties of the medium, and hence the refractive index, in a grid-like pattern. The term acoustic grating is a more general term and can include operation at audible frequencies as well as ultrasonics.

An ultrasonic wave is a sound wave having a frequency greater than 20 kHz. The human ear cannot recognize ultrasonic waves, but animals such as bats and dogs can easily sense the ultrasonic wave. Ultrasonic waves can be produced by the Piezo-electric effect and magnetostriction.

When ultrasonic waves are generated in a liquid in a rectangular vessel, the wave can be reflected from the walls of the vessel. These reflected waves are called echoes. The direct and reflected waves are superimposed causing a standing wave to be formed. The density of the liquid at a node is more than the density at an antinode. Hence, the liquid acts as a diffraction grating to a parallel beam of light passed through the liquid at right angles to the wave.

The diffraction grating formed in this way is analogous to a conventional diffraction grating with lines ruled on a glass plate. The less dense antinodes refract light less and are analogous to the transmitting slits of a conventional grating. The denser nodes refract light more and are analogous to the opaque part of a conventional grating.

The grating element is equal to the wavelength of the ultrasonic waves. Let it be denoted by ${\displaystyle d}$. If ${\displaystyle \lambda }$ is the wavelength of the light passed through the grating which is diffracted by an angle ${\displaystyle \theta }$, then the nth order of the maximum is given by:

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