Describe what happens to a wave when it undergoes diffraction through a gap, and state the conditions under which the greatest diffraction occurs.

Cambridge International IGCSE Physics (0625) — 3.1 General properties of waves · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

A wave that passes through a narrow gap spreads out or bends around the edges of the gap.
The frequency, wavelength and speed of the wave remain unchanged.
The greatest diffraction occurs when the gap size is approximately equal to the wavelength.
If the gap is much narrower than the wavelength, or the wavelength is much longer than the gap, the diffraction is even greater.

Examiner tips

  • Use the word "spreads out" or "bends" to show understanding of diffraction. Mention that frequency, wavelength and speed are unchanged. State the condition "gap ≈ wavelength" for maximum diffraction. Show the relationship that narrower gap or longer wavelength gives more diffraction.

Common mistakes

  • Confusing diffraction with reflection or refraction. Saying the wave’s speed changes. Forgetting to mention the gap size relative to the wavelength.

Mark scheme (4 marks)

  1. The wave spreads out / bends around the edges of the gap after passing through it
  2. The frequency, wavelength, and speed of the wave are unchanged / remain constant
  3. Greatest diffraction occurs when the gap size is of the same order of magnitude as the wavelength / when the gap is approximately equal to the wavelength
  4. The narrower the gap (relative to the wavelength) OR the longer the wavelength (relative to the gap size), the greater the diffraction

Key terms in this question

diffraction

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