# A fishing vessel uses sonar to locate a shoal of fish beneath it. Ultrasound pulses are emitted downward from the hull of the boat. Explain how the sonar system detects the shoal of fish and determines how far below the surface the shoal is.

> OCR A-Level Physics A (H556) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Sonar systems work on the same principle as ultrasound imaging. A pulse of ultrasound is emitted from a transmitter and a receiver next to the transmitter detects any returning pulses. The speed of sound in seawater is approximately 1500 m/s.

## Mark scheme (5 marks)

1. Ultrasound pulses travel downward through the water from the transmitter
2. When the ultrasound reaches the boundary between water and the shoal of fish, the waves are partially reflected back
3. The reflected ultrasound pulses are detected by the receiver next to the transmitter
4. The time between emission and detection of the reflected pulse is measured
5. Because the speed of the waves is constant/known, the distance to the shoal can be calculated from the travel time (distance = speed × half the time taken)

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