Correct Answer - Option 4 : Thermal detector
There are generally 4 methods for detection of Ultrasonic waves namely:
1) Piezo-electric detector
2) Sensitive flame method
3) Thermal detector method
4) Kundt's tube method
Thermal detector method:
- A platinum thermocouple connected to a balanced meter bridge acts as the detector for this method.
- When ultrasonic waves are passed through the medium, the pressure variations are adiabatic in nature, i.e., at nodes (regions of compressions), the air medium heats up and at (regions of rarefactions), the medium cools down.
- These variations in temperature are detected by the sensitive platinum thermocouple and thus changes its resistance.
- The change in resistance unbalances the bridge.
- Thus, the unbalancing of the bridge acts as a pointer for detecting the presence of ultrasonic waves.
Piezo-electric detector:
- The direct and inverse piezoelectric effects offered by crystals like quartz are used for detecting ultrasonic waves.
- We know that, in the direct piezoelectric effect, charges are produced in the crystal due to mechanical vibrations.
- When ultrasonic waves strike a quartz crystal, they charge on the opposite faces of the crystal.
- The charges thus developed are very feeble, and hence they are suitably amplified before the presence of ultrasonic waves is confirmed.
Sensitive flame method:
- This is a very simple method in which a flame is moved in the path of the ultrasonic waves for detecting them.
- When the flame is moved along the path of the ultrasonic waves, the flame flickers at the positions of the node, whereas the flame remains stationary at the positions of antinodes.
- This behavior of the flame confirms the presence of ultrasonic waves.
Kundt's tube method:
- Kundt's tube is a narrow long, hollow cylindrical tube, with one end open and the other end by a movable piston.
- Initially, lycopodium powder is uniformly smeared (sprinkled) inside the tube.
- Then ultrasonic waves are passed through the tube and the piston is adjusted to form a standing wave pattern.
- At this point, the nodes and antinodes formed by the standing waves wherein the lycopodium powder are heaped at the nodes and thrown off at the antinodes.
- At this point, the distance (d) between the successive nodes is measured from the heaps of the lycopodium powder, and the wavelength is calculated (d = λ /2).
- From the value of the wavelength, the presence of ultrasonic waves can be identified.
Light meter:
- It is used to measure the amount of light.
- In Photography it is designed either to measure light reflected from the subject or to measure the light falling on the subject.
