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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Ultrasound pulses are emitted from the transmitter and travel downward through the water.
When the pulse reaches the boundary between water and the shoal of fish, part of the wave is reflected back.
The reflected pulse is detected by the receiver located next to the transmitter.
The time interval between the emission of the pulse and the detection of the reflected pulse is measured.
Because the speed of sound in seawater (≈1500 m s⁻¹) is known, the distance to the shoal is calculated as
distance = ½ × speed × time.
When the pulse reaches the boundary between water and the shoal of fish, part of the wave is reflected back.
The reflected pulse is detected by the receiver located next to the transmitter.
The time interval between the emission of the pulse and the detection of the reflected pulse is measured.
Because the speed of sound in seawater (≈1500 m s⁻¹) is known, the distance to the shoal is calculated as
distance = ½ × speed × time.
Examiner tips
- Use the exact phrase "time interval between emission and detection" to show understanding of the measurement step.
- Show the calculation formula with the ½ factor to demonstrate knowledge of round‑trip travel.
- Mention the known speed of sound (1500 m s⁻¹) to justify the calculation.
- Keep the answer concise – 5 points can be earned with 5 clear bullet‑style statements.
Common mistakes
- Forgetting the ½ factor in the distance calculation, giving the full distance instead of half the round‑trip.
- Using the wrong speed of sound or not stating it, which weakens the calculation.
- Not explicitly stating that the pulse is partially reflected at the boundary, implying total reflection.
Mark scheme (5 marks)
- Ultrasound pulses travel downward through the water from the transmitter
- When the ultrasound reaches the boundary between water and the shoal of fish, the waves are partially reflected back
- The reflected ultrasound pulses are detected by the receiver next to the transmitter
- The time between emission and detection of the reflected pulse is measured
- 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)
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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