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Velocity | Ultrasonic


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 vessel at its top.

A quarrz crystal Q placed between two metal plates A and B is mounted at the bottom of the vessel.
The metallic plates are connected to an oscillator whose frequency is so adjusted that the crystal vibrates in resonance with the frequency of the oscillator and thus produces ultrasonic waves within the liquid. Hence the liquid behaves like a greeting. This acoustic grating is mounted on the prism table of a spectrometer and a parallel beam of light from the monochromatic source S is passed through a liquid at right angle to the waves. Since the liquid is behaving like a greeting, the light beam on passing through it gets diffracted and produces a diffraction pattern. The diffraction pattern is viewed through the telescope.

The diffraction pattern consists of a central maxima with principal maxima of different orders on either side, If θ is the angle of diffraction for the n-th order principal maxima then,
dsinθ = nλ
Where λ is the wavelength of monochromatic light used and b is the grating element (The distance between adjacent nodal planes is defined as the grating element).

The grating element d=λ/2 , where λ is the wavelength of ultrasonic waves.

Or


Thus, knowing λ and n measuring θ, the wavelength of ultrasonic waves can be determined.

If the resonant frequency of ultrasonic generator is f, then the velocity of ultrasonic waves is given by
v=fλ

Using this acoustic diffraction method, the wavelength and hence the velocity of ultrasonic waves through liquids and gases at various temperature can be determined.






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