IB Physics SLTopic C.3 — How Waves BehavePaper 1 & 2λ ≈ gap~7 min read
Diffraction
Waves don’t travel in perfectly straight lines forever. Squeeze them through a gap or send them past an edge and they spread out — a behaviour called diffraction, and how much they spread comes down to one comparison: the wavelength versus the size of the gap.
📘 What you need to know
Diffraction is the spreading out of waves as they pass through a narrow gap or around an obstruction
Through a gap, the wavefronts curve and the wave’s amplitude drops (the barrier absorbs some energy)
Diffraction is most pronounced when the wavelength is close to the gap width
Gap much smaller than λ → the wave spreads out a lot; gap much bigger → barely any spreading
Around a barrier, a longer wavelength diffracts more and leaves a smaller “shadow”
Diffraction does not change the wavelength — keep the wavefront spacing constant when you draw
Diffraction Through a Gap
When a straight (plane) wave reaches a narrow gap, only the part lined up with the gap gets through — and on the far side it fans out into curved wavefronts, as if the gap were a new point source. The barrier on either side soaks up some of the wave’s energy, so the diffracted wave also has a lower amplitude than the wave that arrived.
Plane wavefronts hit the barrier; the part passing through the gap spreads into curved wavefronts, fanning out as if from a new source.
Gap Size vs Wavelength
Here’s the rule that exams test again and again: diffraction is strongest when the gap width is about the same as the wavelength. Make the gap much wider than the wavelength and the wave mostly carries straight on, bending only slightly at the edges. Make the gap comparable to (or smaller than) the wavelength and the wave spreads out dramatically.
Left: a gap close to the wavelength spreads the wave into wide arcs. Right: a gap much larger than the wavelength lets the wave pass almost straight, curving only at the edges.
Diffraction Around a Barrier
Waves also bend around the edge of an obstacle, curling in to fill the space behind it. How far they reach into that region depends on the wavelength: the greater the wavelength, the greater the diffraction. A long-wavelength wave wraps well around a barrier and leaves only a small shadow; a short-wavelength wave barely bends, leaving a large shadow region behind the obstacle where no wavefronts reach. This is why you can hear someone around a corner (long-wavelength sound) even when you can’t see them (short-wavelength light).
🧭 Judging how much a wave diffracts
Compare the wavelength with the gap (or obstacle) size
Gap ≈ wavelength → strong, obvious spreading
Gap much larger than wavelength → the wave stays nearly straight
Around a barrier, a longer wavelength means more bending and a smaller shadow
When sketching, keep the wavefront spacing constant — diffraction never changes the wavelength
Quick recap: diffraction is the spreading of waves through gaps or around edges, strongest when the gap matches the wavelength; longer wavelengths spread more, and λ itself stays unchanged.
WE 1
A student leaves a gap of about 10 cm between a door and its frame. Sound travels at 340 m s⁻¹.
Determine the frequencies of sound that are best diffracted through the gap.
Step 1
Diffraction is best when the wavelength is comparable to (or larger than) the gap, so λ ≈ 0.10 m
Step 2
Use v = fλ, rearranged to f = v/λ
f = 340 ÷ 0.10 = 3400 Hzf ≤ 3400 Hz are best diffractedLower frequencies mean longer wavelengths, which diffract even more — so the low, bassy notes escape the gap best.
WE 2
Which scenario best demonstrates diffraction? (A) UV light through a gatepost gap, or (B) X-rays passing between atoms in a crystalline solid.
Apply the rule
Diffraction is strongest when the wavelength ≈ the gap size
Compare
UV (λ ~ 10⁻⁷ m) is tiny next to a gatepost gap, so almost no diffraction. X-rays (λ ~ 10⁻¹⁰ m) closely match the atomic spacing in a crystal
Answer: B — X-rays in the crystalThis wavelength-matching is exactly why X-ray diffraction is used to study crystal structures.
💡 Top tips
Strongest diffraction when gap ≈ wavelength — this single idea answers most diffraction questions
Around an obstacle, longer wavelength = more diffraction and a smaller shadow
The wavelength doesn’t change on diffraction, so keep wavefront spacing equal in sketches
The diffracted wave has a lower amplitude because the barrier absorbs energy
Hearing round corners but not seeing round them is diffraction — sound has the far longer wavelength
⚠ Common mistakes
Changing the wavelength (wavefront spacing) in a diffraction sketch — it must stay the same
Thinking a very wide gap causes strong diffraction — wide gaps give little diffraction
Saying shorter wavelengths diffract more around a barrier — it’s the opposite
Forgetting the amplitude drops after diffraction through a gap
Drawing flat wavefronts on the far side of a narrow gap instead of curved ones
Up next: we return to bending light in detail, putting numbers to it with Refraction of Waves — Snell’s law, refractive index, and total internal reflection.
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