1. DiffractionWaves spreading out around obstacles
What is diffraction?
Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. It happens to every type of wave — light, sound, water, radio, even electrons (yes, really — see Section 4!).
Think of it like this: imagine water waves rolling toward a harbour wall with a narrow gap in it. On the other side of the gap, the water doesn't just carry on in a neat, narrow beam — it spreads out in circular ripples, as if the gap itself became a brand new source of waves. That's exactly what happens to light or sound passing through a narrow slit.
Key fact students often miss: when a wave diffracts, its wavelength and frequency stay exactly the same. The only thing that changes is its amplitude — and it usually gets smaller, because some of the wave's energy spreads out sideways instead of continuing forward (some energy is "dissipated").
What controls how much a wave diffracts?
The amount of spreading depends on how the gap size (or obstacle size) compares to the wavelength:
- Gap ≈ wavelength → strong diffraction, waves spread out a lot (almost like a semicircle).
- Gap >> wavelength → weak diffraction, the wave barely bends and just carries on mostly in a straight line.
Why this matters for exam questions
Examiners love asking "which of these waves would diffract noticeably through this gap?" Compare the wavelength of the wave to the size of the gap — if they're roughly the same order of magnitude, diffraction is significant. This is exactly why X-rays (wavelength ~10⁻¹⁰ m) diffract through the gaps between atoms in a crystal, but visible light (wavelength ~10⁻⁷ m) does not.
Huygens' Construction — explaining WHY diffraction happens
Huygens proposed that every point on a wavefront acts like its own tiny point source, sending out its own circular "secondary wavelet." The new wavefront a moment later is simply the curve that touches (is tangential to) all of these tiny wavelets.
When a wave meets a gap, only the wavelets from the points inside the gap continue on. Near the edges of the gap, these wavelets aren't cancelled out by neighbours on one side (because there's no wavefront there anymore) — so the wave curls around the edge into the "shadow" region. That's diffraction, explained from first principles.
Around an obstacle, it's the reverse: wavelets from the points either side of the obstacle bend inward behind it, but directly behind the obstacle there's a small region with no wavelets reaching it at all — a "shadow."
Practice Question 1
Sound waves have wavelengths roughly between 1.7 cm and 17 m. Explain, using ideas about diffraction, why you can hear someone talking around a corner in a corridor, but you usually can't see them.
Practice Question 2
A wave passes through a gap that is much wider than its wavelength. Sketch (in words) what happens to the wavefronts, and explain what happens as the gap gets narrower and narrower.