A sound wave travelling through air reaches a boundary and enters water. Describe what happens to the speed, wavelength and frequency of the sound wave as it crosses this boundary, and explain why the wavelength changes.

Edexcel GCSE Physics (1PH0) — 4.1 Waves · Describe · 4 marks · View as Markdown

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

Sound waves travel through different media. The speed of sound in water is greater than the speed of sound in air.

Model answer (4 marks)

The speed of the sound wave increases when it enters water.
The frequency of the sound wave remains unchanged.
The wavelength of the sound wave increases.
Because the speed increases but the frequency stays the same, the wavelength must increase (v = fλ).

Examiner tips

  • Use the command word ‘describe’ – give a short statement for each property. Show the link between speed, frequency and wavelength with the wave equation. Keep the answer concise – 4 marks only.
  • Use correct UK spelling (e.g. ‘wavelength’).

Common mistakes

  • Saying the frequency changes. Forgetting to explain why the wavelength changes. Using the wrong direction for the change in wavelength (decreasing instead of increasing).

Mark scheme (4 marks)

  1. The speed of the sound wave increases as it enters water.
  2. The frequency of the sound wave remains constant / does not change.
  3. The wavelength of the sound wave increases.
  4. Because speed increases but frequency is constant, wavelength must increase (linking wave equation v = fλ or equivalent reasoning).

Key terms in this question

frequency · wavelength

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