A harbour wall has a narrow opening that allows ocean waves to enter a sheltered bay. A boat captain observes that the waves spread out in a curved pattern after passing through the opening. Explain why the waves spread out after passing through the narrow opening, and describe how the amount of spreading would change if the opening were made much wider.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ocean waves approach a harbour wall. The harbour wall has a narrow opening. After passing through the opening, the waves spread out into the sheltered bay beyond.
Model answer (4 marks)
Diffraction
1. The spreading of waves through a gap is called diffraction.
2. Diffraction occurs because the narrow opening causes the waves to bend and spread around the edges of the opening.
3. The amount of diffraction is greatest when the gap width is similar to or smaller than the wavelength of the waves.
4. If the opening were made much wider, there would be less noticeable diffraction, so the waves would spread out less and would travel more like straight wavefronts.
1. The spreading of waves through a gap is called diffraction.
2. Diffraction occurs because the narrow opening causes the waves to bend and spread around the edges of the opening.
3. The amount of diffraction is greatest when the gap width is similar to or smaller than the wavelength of the waves.
4. If the opening were made much wider, there would be less noticeable diffraction, so the waves would spread out less and would travel more like straight wavefronts.
Examiner tips
- Use the word "diffraction" early. Explain the cause (bending around edges). Mention the relation to wavelength. State the effect of a wider opening.
Common mistakes
- Using "refraction" instead of "diffraction". Forgetting to relate the gap size to the wavelength.
Mark scheme (4 marks)
- The spreading of waves through a gap is called diffraction
- Diffraction occurs because the gap causes the waves to bend / spread around the edges of the opening
- The amount of diffraction / spreading is greatest when the gap width is similar to (or smaller than) the wavelength of the waves
- If the opening is made much wider, there is less / less noticeable diffraction, so the waves spread out less / the waves pass through more like straight wavefronts
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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