SONAR is a device that is used to measure the distance of underwater objects by a ship. Which of the following types of waves does it use for this purpose?
- A.Infrasonic waves
- B.Sound waves in audible range for human beings
- C.Ultrasonic waves
- D.All of the above
Correct answer
Explanation
The correct answer is C, Ultrasonic waves. SONAR sends out ultrasonic pulses and times their echo from underwater objects.
SONAR stands for Sound Navigation and Ranging. A transmitter on the ship sends ultrasonic waves, which have frequencies above 20,000 Hz, into the water. When they strike an object such as the sea bed, a submarine or a shoal of fish, they are reflected and picked up by a detector. If the echo returns after time t and sound travels at speed v in water, the distance is d = vt/2. Ultrasound is used because its short wavelength lets it travel in a narrow beam and reflect clearly from small objects without spreading much.
A is wrong: infrasonic waves, below 20 Hz, have long wavelengths and spread widely, so they cannot locate objects precisely. B is wrong: audible sound is less directional and would be masked by the noise of the sea and the ship. D is wrong because only ultrasonic waves are used.
Exam tip: SONAR = ultrasound and echo; distance = speed × time ÷ 2.
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