# An engineer is checking a metal pipeline for internal cracks using ultrasound. The ultrasound emitter is placed against the outside of the pipe and sends pulses into the metal. Explain how this method can detect a crack inside the pipeline, and why ultrasound is more suitable for this purpose than visible light.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.1 Thermal physics · Explain · 5 marks

> Ultrasound waves have a frequency higher than 20 kHz, above the upper limit of human hearing. When ultrasound reaches a boundary between two media, the waves are partially reflected back while the remainder continue through.

## Mark scheme (5 marks)

1. Ultrasound waves are partially reflected at the boundary between the metal and the crack (air gap)
2. The reflected waves are detected by a receiver / detector placed next to the emitter
3. The crack is detected because the reflected pulse arrives earlier than the pulse reflected from the far wall of the pipe
4. The speed of ultrasound is constant in the metal, so the time taken for the pulse to return indicates the distance to the crack
5. Visible light cannot pass through the metal to reach the internal crack, whereas ultrasound can travel through solid metal

## 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)
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