A fishing boat uses sonar to locate a shoal of fish beneath it. A pulse of ultrasound is emitted from the boat's hull and a reflected pulse is detected 0.12 s later. Explain how the sonar system is able to detect the shoal of fish, and describe what happens to the ultrasound pulse at the boundary between the water and the shoal of fish.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ultrasound waves have a frequency higher than 20 kHz. The speed of sound in water is approximately 1500 m/s.
Model answer (5 marks)
1. Ultrasound waves are emitted from the hull of the boat and travel downward through the water.
2. At the boundary between the water and the shoal of fish, part of the ultrasound is reflected back towards the boat.
3. The remaining part of the wave continues to pass through the boundary and travels further into the shoal.
4. The time interval between emission and detection of the reflected pulse is measured.
5. Because the speed of the ultrasound waves (≈1500 m s⁻¹) is known and constant, this time can be used to calculate the distance to the shoal.
2. At the boundary between the water and the shoal of fish, part of the ultrasound is reflected back towards the boat.
3. The remaining part of the wave continues to pass through the boundary and travels further into the shoal.
4. The time interval between emission and detection of the reflected pulse is measured.
5. Because the speed of the ultrasound waves (≈1500 m s⁻¹) is known and constant, this time can be used to calculate the distance to the shoal.
Examiner tips
- Use the word ‘reflected’ to show understanding of partial reflection.
- Show the calculation: distance = ½ speed × time (0.12 s).
- Mention that the pulse travels twice the distance (down and back).
Common mistakes
- Forgetting that only part of the wave is reflected.
- Using the full time instead of half the time to calculate distance.
- Not recognising that the pulse travels through the water at a constant speed.
Mark scheme (5 marks)
- Ultrasound waves are emitted (from the hull of the boat) and travel downward through the water
- At the boundary between the water and the shoal of fish, the ultrasound is partially reflected back towards the boat
- The remainder of the wave continues to pass through / is transmitted beyond the boundary
- The time between emission and detection of the reflected pulse is measured
- Because the speed of the ultrasound waves is known (and constant), this time can be used to calculate the distance to the shoal
Key terms in this question
ultrasound · boundary · sonar
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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