Explain why a moving observer, travelling at constant velocity directly towards a stationary source of sound, detects a frequency that is higher than the frequency emitted by the source.

IB DP Physics Higher Level (2023 syllabus) — C.5 Doppler effect · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The observer moves towards the source, so the observer encounters wavefronts more frequently than if stationary.
The wavelength of the sound is unchanged because the source is stationary.
The effective speed of sound relative to the observer is increased (to v + v_observer), because the observer moves towards the source.
Since frequency equals wave speed divided by wavelength (f = v/λ), a greater relative speed with unchanged wavelength gives a higher detected frequency.

Examiner tips

  • Use the word ‘because’ to link cause and effect. Mention that wavelength is unchanged. Show the formula f = v/λ and explain the increase in v. Keep the answer concise and to the point.

Common mistakes

  • Confusing the speed of sound with the observer’s speed. Saying the wavelength changes. Forgetting to state that the source is stationary.

Mark scheme (4 marks)

  1. The observer moves towards the source, so the observer encounters wavefronts more frequently than if stationary.
  2. The wavelength of the sound is unchanged because the source is stationary.
  3. The effective speed of sound relative to the observer is increased (to v + v_observer), because the observer moves towards the source.
  4. Since frequency equals wave speed divided by wavelength (f = v/λ), a greater relative speed with unchanged wavelength gives a higher detected frequency.

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