The big idea: Diffraction is the spreading out of a wave when it passes through a gap or around an edge.
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:
Spot it: Waves spreading out after a gap or an edge = diffraction. The narrower the gap, the more they spread.
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.
Worked example — which sound spreads more?
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.
Solution
- Start with the given wave equation, rearranged for λ:
- Low note (f = 85 Hz):
- High note (f = 3400 Hz):
- Bigger λ is closer to the doorway width, so it diffracts more:
Final answer
λ = 4.0 m (low note) and 0.10 m (high note). The low note spreads more, because its longer wavelength is closer to the doorway's size.
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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).
- What they ask: which wave spreads most through a gap, or how the spreading changes when you alter the wavelength, the frequency, or the gap width. - The rule: spreading is greatest when gap ≈ λ; a longer wavelength (lower frequency) spreads more through the same gap.
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.
IB-style question — which water wave diffracts more?
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.
Solution
- Spreading depends on the gap size compared with the wavelength — it is greatest when the gap is closest to λ.
- The gap is the same for both, so the wave with the longer wavelength is closer to the gap size:
- So Q's wavelength is nearer the gap width → Q diffracts more.
Final answer
Wave Q spreads out more, because its longer wavelength (8 cm) is closer to the size of the gap than P's (2 cm) — and diffraction is greatest when gap ≈ λ.