# A student stands near a motorway and notices that the siren of an approaching ambulance sounds higher-pitched than when the ambulance is moving away. Explain why the observed frequency of the siren changes as the ambulance first approaches and then passes the student.

> Pearson Edexcel International GCSE Physics (4PH1) — 3.1 Properties of waves · Explain · 4 marks

> An ambulance travels along a straight motorway at high speed with its siren sounding at a constant frequency. A student standing at the roadside listens as the ambulance approaches and then moves away.

## Mark scheme (4 marks)

1. The ambulance (wave source) is moving relative to the student (observer), producing the Doppler effect.
2. As the ambulance approaches, the wavefronts are bunched together / compressed, so the observed wavelength decreases and the observed frequency increases.
3. As the ambulance moves away, the wavefronts are spaced further apart / stretched, so the observed wavelength increases and the observed frequency decreases.
4. The actual frequency emitted by the siren remains constant throughout; only the observed frequency changes.

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](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-student-stands-near-a-motorway-3feefb3a) · Published by Druglandscape Ltd.