# A ship uses sonar to map the seabed. Pulses of ultrasound are emitted from beneath the ship, travel through seawater, and are partially reflected back when they reach the boundary between the seawater and the seabed. A receiver detects the returning pulses. Explain how sonar works and why ultrasound, rather than audible sound, is used for this purpose.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Sonar systems emit pulses of ultrasound from beneath a ship. The pulses travel through seawater and are partially reflected at the boundary between the seawater and the seabed. A receiver next to the emitter detects the reflected pulses, and the time between emission and detection is recorded.

## Mark scheme (5 marks)

1. Ultrasound pulses are partially reflected at the boundary between two media (seawater and seabed)
2. The remainder of the waves are transmitted (continue through) the seabed
3. The speed of the ultrasound waves is constant (in a given medium), so the time between emission and detection of the reflected pulse is used to determine the depth of the seabed
4. Ultrasound has a frequency above 20 kHz, which is above the upper limit of human hearing
5. Using ultrasound rather than audible sound means the sonar signal does not interfere with or cause disturbance to crew or marine life that can detect audible frequencies / ultrasound can be directed as a narrow pulse more effectively for imaging

## Key terms

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

## 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-01787cd5) · Published by Druglandscape Ltd.