A teacher demonstrates sound waves passing through a narrow gap in a wall. The teacher notices that students on the other side of the wall can hear the sound even when they are standing to the side, not directly in front of the gap. Explain why the students can hear the sound.

OCR GCSE Physics A: Gateway Science (J249) — P4.1 Wave behaviour · Explain · 4 marks · View as Markdown

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

A teacher in a school corridor demonstrates that sound can be heard through a narrow gap in a wall, even by students standing at the sides rather than directly opposite the gap.

Model answer (4 marks)

The sound waves bend and spread out after passing through the narrow gap – this is called diffraction. Diffraction is most pronounced when the width of the gap is comparable to the wavelength of the sound. Because the waves spread in all directions beyond the gap, students standing to the side of the gap can still hear the sound.

Examiner tips

  • Use the term "diffraction" explicitly. Mention that diffraction is greatest when gap ≈ wavelength. Explain that waves spread in all directions after the gap. Keep answer concise – 4 marks.
  • common_mistakes
  • :
  • Forgetting to mention diffraction or using vague terms like "spreading" without naming it. Failing to note that diffraction is strongest when the gap size is similar to the wavelength. Not explaining that the waves reach students to the side, not just in front.

Mark scheme (4 marks)

  1. The sound waves spread out / bend after passing through the narrow gap
  2. This spreading of waves is called diffraction
  3. Diffraction is greatest when the gap is similar in size to the wavelength of the wave
  4. Because the sound spreads out in all directions beyond the gap, students to the side (not directly in front) can still hear it

Key terms in this question

gap

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