The big idea: Watch sea waves roll through the narrow mouth of a harbour and fan out into curved ripples on the still water beyond. That spreading of a wave as it passes through a gap or around an edge is diffraction.
Instead of carrying straight on in a narrow beam, the wave fans out into the space beyond.
It happens to all waves — water, sound and light.
You hear it every day: You can hear someone talking around a doorway even when you can't see them.
The sound wave spreads through the doorway and bends into the room — that's diffraction.
Picture straight waves (like rows on the sea) heading towards a barrier with a gap in it. On the far side the waves bend round the edges of the gap and spread into the shadow region behind the barrier:
- Wide gap — the waves mostly carry straight on; only the very edges curl in. Little spreading.
- Narrow gap — the waves fan out in curved arcs on the far side. Lots of spreading.
New word — wavelength (λ): Wavelength λ is the length of one full wave — for example from one crest to the next.
Diffraction is all about how this wavelength compares with the size of the gap, so picture it first:
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Spot it: Waves spreading out after a gap or an edge = diffraction. The narrower the gap, the more they spread.
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The rule to remember: How much a wave spreads depends on the size of the gap compared with the wavelength λ.
Spreading is greatest when the gap is about the same size as the wavelength (gap ≈ λ).
If the gap is much wider than λ, the wave barely spreads at all.
Gap much bigger than λ
- Wave passes almost straight through
- Only the edges curl in
- Little diffraction
Gap about equal to λ
- Wave fans out widely
- Spreads into the shadow behind the barrier
- Most diffraction
The only equation that comes into diffraction is the wave equation, which links a wave's speed, frequency and wavelength. It is given in the data booklet:
- wave speed — how fast the wave travels (m s⁻¹)
- frequency — waves per second (Hz)
- wavelength — length of one full wave (m)
Why it matters here: Rearranged, λ = v ÷ f. For waves of the same speed, a lower frequency means a longer wavelength — and a longer wavelength spreads more through the same gap.
That's why a deep bass note (low frequency, long λ) carries around a corner better than a high note.
Two sounds travel through the same doorway at the same speed (340 m s⁻¹): a low note of 85 Hz and a high note of 3400 Hz. Find each wavelength and say which sound spreads more through the doorway.
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How this is tested — diffraction is usually a Paper 1A concept question (no past-paper calculation in the current pack — it's assumed background for double-slit questions):
Paper 1A — what they ask
- Which wave spreads most through a gap.
- How the spreading changes when you alter the wavelength, frequency, or gap width.
The rule
- Spreading is greatest when gap ≈ λ.
- A longer wavelength (lower frequency) spreads more through the same gap.
The classic trap: Thinking a higher frequency spreads more — it's the opposite. Higher frequency → shorter λ → less spreading.
Same gap, change the wavelength: Keep the gap the same and make the wavelength longer. Fewer waves now fit across the gap, so the gap-to-wavelength ratio falls toward 1 — and the wave spreads more.
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Straight water waves travel towards a barrier with a single gap of fixed width. Wave P has a wavelength of 2 cm; wave Q has a wavelength of 8 cm. Both pass through the same gap. State which wave spreads out more on the far side, and explain why.
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See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.