Determination of Ultrasonic Velocity in Liquid (Acoustical Grating Method)
When ultrasonic waves travel through a transparent liquid, due to alternate compression and rareaction, longitudinal stationery waves are produced. If monochromatic light is passed through the liquid perpendicular to these waves, the liquid behaves as diffraction grating. Such a grating is known as Acoustic Grating. Here the lines of compression and rareaction act as transparent light waves. It is used to find wavelength and velocity(v) of ultrasonic waves in the liquid.
It is consists of a glass tank, filled with the liquid. A piezo-electric (Quartz) is fixed at the bottom of the glass tank and is connected with piezo-electric oscillatory circuit as shown in the figure 1.7.
An incandescent lamp is used as a monochromatic source (S) and a telescope arrangement is used to view the diffraction pattern. A collimator consisting of two lenses L1 and L2 is used to ocus the light effectively in the glass tank.
(i) When the piezo-electric crystal is kept at rest:
Initially the piezo-electric crystal is kept at rest and the monochromatic at light is switched ON. When the light is focused in the glass tank filled with the liquid, a single image, a vertical peak is observed in telescope. i.e., there is no diffraction.
(ii) When the piezo-electric crystal is set into vibrations:
Now the crystal is put into vibrations using piezo-electric oscillatory circuit. At Resonance, Ultrasonic waves are produced and are passed through the liquid. These Ultrasonic waves are reflected by the walls of the glass tank and form a stationery wave pattern with nodes and antinodes in the liquid. At nodes the density of the liquid becomes more and at antinodes the density o the liquid becomes less. Thus, the liquid behaves as a directing element called acoustical grating element.
Now when the monochromatic light is passed the light gets directed and a diffraction pattern consisting of central maxima and principle maxima on either side is viewed through the telescope as shown in figure 1.7.2 as well as in 1.7.3.
Calculation of Ultrasonic Velocity
The velocity of Ultrasonic waves can be determined using the condition.
Thus, this method is useul in measuring the wavelength and velocity of ultrasonic waves in liquids and gases at various temperatures.