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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

The ambulance is the source of sound waves and it is moving relative to the student, so the Doppler effect occurs.
1. When the ambulance approaches, each successive wavefront is emitted from a position closer to the student than the previous one. The wavefronts are therefore bunched together, the wavelength is reduced and the frequency heard by the student rises.
2. When the ambulance recedes, each wavefront is emitted from a position further from the student than the previous one. The wavefronts are stretched apart, the wavelength increases and the frequency heard by the student falls.
3. The siren’s own frequency does not change; only the frequency observed by the student changes because of the relative motion.

Examiner tips

  • Use the word ‘Doppler effect’ early. Show the change in wavelength for approaching and receding. Mention that the source frequency is constant. Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing the source and observer as moving. Forgetting to state that the emitted frequency is constant. Using vague terms like ‘speed up’ instead of ‘increase in frequency’.

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 in this question

observed frequency · frequency

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