# A ship uses sonar to map the depth of the ocean floor. Explain how sonar works to determine the depth of the water beneath the ship, and describe what happens to the ultrasound waves at the boundary between the water and the seabed.

> OCR A-Level Physics A (H556) — 5.3 Astrophysics and cosmology · Explain · 5 marks

> Ships equipped with sonar devices emit ultrasound pulses downward from their hulls. The device both emits and detects ultrasound waves. The speed of sound in seawater is approximately 1500 m/s.

## Mark scheme (5 marks)

1. Ultrasound waves are emitted downward from the ship / sonar device
2. At the boundary between water and the seabed, the waves are partially reflected back
3. The remaining waves continue / are transmitted through the seabed (partial transmission)
4. A receiver / detector (on the ship) detects the reflected waves and measures the time between emission and detection
5. Because the speed of the waves is constant / known, the time measured can be used to calculate the depth of the water

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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-ship-uses-sonar-to-map-cdb0a05e) · Published by Druglandscape Ltd.