Describe what happens to a water wave as it passes through a narrow gap in a barrier, and explain how the width of the gap affects the extent of this effect.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water waves in a ripple tank travel towards a barrier that has a narrow gap cut into it.
Model answer (5 marks)
The water wave spreads out after it emerges from the gap – it bends around the edges of the opening. This spreading of the wave is called diffraction. The amount of diffraction depends on the ratio of the gap width to the wavelength. When the gap width is close to the wavelength, the wave diffracts strongly and the spreading is most pronounced. If the gap is much wider than the wavelength, the wave passes through almost unchanged and the diffraction is small. Conversely, if the gap is much narrower than the wavelength, most of the wave is reflected and only a weak wave is transmitted through the gap.
Examiner tips
- Use the word ‘diffraction’ – it is the key term the scheme looks for.
- Explain the relationship between gap width and wavelength – state the three regimes (≈, >>, <<).
- Show the effect on the transmitted wave (strong, weak, little change).
- Keep the answer concise – 5 marks, so one sentence per point is enough.
Common mistakes
- Confusing diffraction with refraction – the wave does not change direction in the medium, only spreads. Failing to mention the three regimes of gap width relative to wavelength. Using vague language such as ‘bends’ without naming diffraction or describing the effect of width.
Mark scheme (5 marks)
- The wave spreads out (beyond the gap) / bends around the edges of the gap
- This effect is called diffraction
- The effect is greatest when the gap width is similar to (or equal to) the wavelength of the wave
- When the gap is much wider than the wavelength, the wave passes through with very little spreading / diffraction is reduced
- When the gap is much narrower than the wavelength, the wave is mostly reflected / very little energy passes through / the transmitted wave is very weak
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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