Explain what happens to a wave when it passes from deep water into shallow water, in terms of wave speed, wavelength, and direction of travel.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ocean waves travelling from the open sea towards a beach gradually move into shallower water. Surfers notice that the waves slow down and change direction as they approach the shore at an angle.
Model answer (4 marks)
When a wave moves from deep to shallow water the speed decreases because the water depth limits the motion of the water particles.
As the speed decreases, the wavelength shortens – the distance between successive crests becomes smaller.
The frequency, which is set by the source, does not change.
Because the speed changes across the wavefront, the wave is refracted: the part of the wave that enters the shallower water first slows down first, causing the wave to bend towards the normal (i.e. towards the direction of the shallower depth).
As the speed decreases, the wavelength shortens – the distance between successive crests becomes smaller.
The frequency, which is set by the source, does not change.
Because the speed changes across the wavefront, the wave is refracted: the part of the wave that enters the shallower water first slows down first, causing the wave to bend towards the normal (i.e. towards the direction of the shallower depth).
Examiner tips
- Use the word ‘decreases’ for speed and wavelength, ‘remains the same’ for frequency, and mention ‘refraction’ for direction change.
- Show the cause–effect chain: depth → speed ↓ → wavelength ↓, frequency unchanged, refraction.
- Use UK spelling (e.g. ‘behaviour’, ‘colour’).
Common mistakes
- Confusing wavelength with frequency – students say both change.
- Forgetting to mention that the frequency stays constant.
- Using vague terms like ‘slower’ without linking to wavelength change or refraction.
Mark scheme (4 marks)
- The wave speed decreases as the wave enters the shallower water
- The wavelength decreases as the wave slows down
- The frequency of the wave remains the same / does not change
- The wave changes direction / bends towards the normal as it enters the shallower water (refraction)
Key terms in this question
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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