# A quality control engineer uses ultrasound to check for internal cracks in a large metal casting. The engineer places an ultrasound emitter and receiver on the surface of the metal. A pulse is emitted, and two reflected signals are detected: the first arrives back after a short time and the second arrives back after a longer time. Explain how this technique detects the crack and why two reflected signals are received rather than one.

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

> Ultrasound pulses are transmitted into a metal casting from one surface. The speed of ultrasound in the metal is constant throughout.

## Mark scheme (5 marks)

1. Ultrasound is partially reflected at each boundary between two different media (or at any interface/surface it meets)
2. The first (earlier) reflected signal comes from the crack inside the metal, because the crack creates a boundary within the casting
3. The second (later) reflected signal comes from the far/back surface of the metal casting
4. The speed of ultrasound is constant, so the time between emission and detection of each signal indicates the depth/distance at which the reflection occurred
5. Two signals are received because the ultrasound pulse is only partially reflected at the crack, so the remainder continues through the metal and is then reflected from the back surface

## Key terms

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

## 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-quality-control-engineer-uses-ultrasound-f2d5baf1) · Published by Druglandscape Ltd.