Explain what happens to a water wave when it passes through a gap in a barrier that is approximately the same width as its wavelength, and describe how the pattern of the wave changes if the frequency of the wave is then increased while the gap stays the same size.

AQA A-Level Physics (7408) — 3.3.2 Refraction, Diffraction and Interference · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

When a water wave passes through a gap whose width is about one wavelength, the wave bends and spreads out around the edges of the gap – this is diffraction. Diffraction is most pronounced when the gap width ≈ λ.
If the frequency is increased (keeping the wave speed constant) the wavelength shortens (λ = v/f). The same gap is now many wavelengths wide, so the wave diffracts less and the spreading is reduced.

Examiner tips

  • Use the term ‘diffraction’ explicitly. Show the relationship λ = v/f to explain the change in wavelength. Explain how the ratio gap/λ changes when frequency rises. Keep the answer concise and use UK spelling.

Common mistakes

  • Confusing wave speed with frequency. Saying the wave ‘slows down’ instead of ‘shortens’. Forgetting to mention that diffraction is greatest when gap ≈ λ.

Mark scheme (5 marks)

  1. The water wave spreads out / bends around the edges of the gap after passing through it
  2. This effect is called diffraction
  3. Maximum diffraction / greatest spreading occurs when the gap width is approximately equal to the wavelength
  4. Increasing the frequency decreases the wavelength (at constant wave speed)
  5. With a shorter wavelength the gap is now much wider than the wavelength, so the wave diffracts less / spreads out less

Key terms in this question

wavelength · frequency

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