# A ship uses sonar to map the depth of the ocean floor. Explain how sonar waves are used to determine the depth of the ocean floor, and explain why the frequency of the sonar waves does not change as they travel from seawater into a denser layer of sediment on the ocean floor.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.3 Astrophysics and cosmology · Explain · 5 marks

> Sonar systems emit ultrasound waves from beneath a ship. These waves travel through seawater, reach the ocean floor, and return to a receiver on the ship. Scientists also study how the waves behave when they pass into different layers of material on the ocean floor.

## Mark scheme (5 marks)

1. Ultrasound/sonar waves are emitted from the ship and travel down through the water
2. Waves are (partially) reflected back when they reach the boundary/ocean floor
3. The speed of the sonar waves is constant/known, so the time taken for the reflected wave to return to the receiver is measured
4. The depth can be calculated because depth is proportional to the time taken (i.e. greater time = greater depth)
5. Frequency does not change because frequency is a property of the source/emitter, not the medium; when entering a denser medium the wave slows down and the wavelength decreases but frequency remains constant

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-1bd2b0b1) · Published by Druglandscape Ltd.