Describe the construction and working of acousting grating.

Construction and Working of Acousting Grating:

The liquid is acquired into a glass cell. The Piezo-electric crystal is fixed at one side of the wall into the cell and ultrasonic waves are produced. The waves travelling by the crystal get reflected through the reflector placed at the conflicting wall. The reflected waves get superimposed along with the incident waves generating longitudinal standing wave pattern termed as acoustic grating.

When light through a laser source as like He-Ne or diode laser is permitted to pass by the liquid in a direction perpendicular to the grating, diffraction obtains place and one can observe the higher order diffraction patterns onto the screen. The angle among the direct ray and the diffracted rays of different orders (θn) can be computed easily.

Construction and Working of Acousting Grating

With the help of the theory of diffraction,

d sin θ_n= nλ (1),

Here n = 0, 1, 2, 3, ... is the order of diffraction, d is the distance in between two adjacent nodal or anti-nodal planes and λ is the wavelength of light used.

Determining n, θ_n and λ, the value of d can be computed by eqn. (1). When λa is the wavelength of the ultrasonic waves by the medium, then d = λa/2 or λa = 2d (2)

When the resonant frequency of the Piezo-electric oscillator is N, in that case the velocity of ultrasonic wave is specified by

 v = N λa= 2Nd (3)

This way is useful into measuring the velocity of ultrasonic waves by liquids and gases at various temperatures. By these measurements, several parameters of the liquid as like free volume and compressibility can be computed.