# A doctor uses ultrasound waves to scan a patient for a kidney stone. The ultrasound pulse travels into the body and is partially reflected at the boundary between soft tissue and the kidney stone, returning to a receiver on the skin. Explain how the doctor is able to detect the kidney stone using this method, and explain why ultrasound is preferred over X-rays for this type of repeated scanning.

> OCR A-Level Physics A (H556) — 5.1 Thermal physics · Explain · 5 marks

> Ultrasound waves have a frequency higher than 20 kHz, which is above the upper limit of human hearing. When they reach a boundary between two different media inside the body, they are partially reflected back to a receiver placed on the skin surface.

## Mark scheme (5 marks)

1. Ultrasound waves are partially reflected at the boundary between two different media (e.g. tissue and kidney stone)
2. The speed of the ultrasound waves is constant, so the time between emission and detection of the reflected waves can be used to determine the depth / distance of the kidney stone
3. The reflected waves are detected by the receiver and used to build up an image / locate the kidney stone
4. X-rays can cause cell damage / mutations / cancer because they have high frequency and high energy
5. Ultrasound does not cause cell damage / is non-ionising, so it is safe to use repeatedly

## Key terms

- [ultrasound waves](https://www.gradenine.co.uk/glossary/ultrasound-waves)
- [boundary](https://www.gradenine.co.uk/glossary/boundary)
- [partially reflected](https://www.gradenine.co.uk/glossary/partially-reflected)
- [X-rays](https://www.gradenine.co.uk/glossary/x-rays)

## 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-doctor-uses-ultrasound-waves-to-a2dbc3e3) · Published by Druglandscape Ltd.