# A security engineer is testing whether cracks are present inside a metal railway track. Ultrasound pulses are sent into the metal from one end. Explain how ultrasound can be used to detect a crack inside the track, and why ultrasound is preferred over visible light for this purpose.

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

> Ultrasound waves have a frequency higher than 20 kHz, 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 when they reach a boundary between two different media
2. A crack creates a boundary inside the metal, so some ultrasound is reflected back earlier / before reaching the far end
3. The speed of ultrasound in the metal is constant, so the time between emission and detection of the reflected pulse indicates the distance to the crack
4. Visible light cannot pass through an opaque metal, so cannot be used to detect internal cracks
5. Ultrasound can travel through the solid metal, making it suitable for detecting internal features / it is non-invasive and does not damage the track

## 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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