Explain why a sound wave and a light wave behave differently when they each pass through a gap of the same width, and explain how changing the frequency of the sound wave affects the amount of diffraction that occurs.

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).

A sound wave of frequency 500 Hz and a beam of visible light both pass through a gap of the same width in a barrier.

Model answer (5 marks)

Sound waves have a much longer wavelength than visible light. Diffraction occurs when a wave passes through a gap; it is greatest when the gap width is comparable to or smaller than the wavelength. Because the gap is close to the wavelength of the sound, the sound wave diffracts (spreads out) strongly, whereas the gap is much larger than the wavelength of light, so the light diffracts only slightly.

If the frequency of the sound is decreased, its wavelength increases (λ = v/f). A larger wavelength makes the gap relatively smaller, so diffraction becomes more pronounced. Conversely, increasing the frequency shortens the wavelength, making the gap relatively larger and reducing the amount of diffraction.

Examiner tips

  • Use the phrase ‘diffraction occurs when a wave passes through a gap’ to show understanding of the phenomenon.
  • Show the relationship λ = v/f when explaining frequency changes.
  • Explain the relative size of gap to wavelength for sound vs light to justify the difference in diffraction.
  • Mention that decreasing frequency increases λ, increasing diffraction; increasing frequency decreases λ, reducing diffraction.

Common mistakes

  • Confusing wavelength with frequency; students may say higher frequency gives more diffraction.
  • Failing to compare the gap size to the wavelength of each wave type.
  • Using vague terms like ‘spread out’ without linking to wavelength size.

Mark scheme (5 marks)

  1. Diffraction occurs when a wave passes through a gap (or around an obstacle)
  2. Diffraction is greatest when the gap width is approximately equal to (or smaller than) the wavelength of the wave
  3. Sound has a much longer wavelength than visible light (so the gap is closer to the wavelength of sound)
  4. Therefore sound diffracts (spreads out) much more than light through the same gap
  5. Decreasing the frequency of the sound wave increases its wavelength, making the gap comparatively smaller relative to wavelength — so diffraction increases; increasing frequency decreases wavelength, reducing diffraction

Key terms in this question

diffraction · frequency

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