# 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.

> AQA A-Level Physics (7408) — 3.3.1 Progressive and Stationary Waves · Explain · 5 marks

> Ultrasound waves have a frequency higher than 20 kHz. The speed of sound in water is approximately 1500 m/s.

## Mark scheme (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, the ultrasound is partially reflected back towards the boat
3. The remainder of the wave continues to pass through / is transmitted beyond the boundary
4. The time between emission and detection of the reflected pulse is measured
5. 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

- [ultrasound](https://www.gradenine.co.uk/glossary/ultrasound)
- [boundary](https://www.gradenine.co.uk/glossary/boundary)
- [sonar](https://www.gradenine.co.uk/glossary/sonar)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-fishing-boat-uses-sonar-to-bc681427) · Published by Druglandscape Ltd.