Diffraction

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:

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.

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:

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.

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):

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.