Multiple choiceQuestion 1 · Recall
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By the end you can define diffraction, explain why it happens to all waves, say which quantities stay the same when a wave diffracts, and give everyday examples.
Someone is talking in the corridor. You cannot see them — there is a wall and a doorway between you — but you can hear them perfectly well.
Sound waves reach you by spreading out as they pass through the doorway. They do not just travel in a straight line like a torch beam.
That spreading is diffraction. Light does it too: look at the soft edges of a shadow, or at the bright fringe that spills into the dark region behind a narrow slit.
Diffraction
The spreading of waves as they pass through an aperture (gap) or past an obstacle or edge.
When a wave passes through a gap in the same medium, these stay the same:
What changes is the direction the wavefronts travel in and how the wave's energy is spread. The wave fans out into a wider region, so its amplitude (and intensity) usually drops.
Careful with a common slip: the wave does not "slow down" in a gap unless it enters a different medium.
A useful model: treat every point on a wavefront as a tiny source of circular wavelets, and the next wavefront as the envelope of those wavelets.
At a wide gap, many such point-sources sit side by side. Their wavelets add mostly forward, so the wave continues almost straight.
At a narrow gap, there is effectively only one point-source left. It radiates circular waves in all forward directions, so the wave spreads out.
This model is a picture, not a proof — but it predicts the right behaviour, and it explains why the gap width compared with the wavelength is what matters.
Diffraction is only obvious when the gap or obstacle is similar in size to the wavelength.
Visible light has a wavelength of about — half a micrometre. Doorways, windows and letterboxes are millions of times wider, so a light beam passes through them almost unchanged.
Shrink the gap to a fraction of a millimetre and light does spread noticeably. That is why diffraction of light is demonstrated with a narrow slit or a diffraction grating, where the slits are comparable with the wavelength.
Sound has wavelengths of centimetres to metres, comparable with doorways, which is why you hear around corners every day.
Only light diffracts.
Every wave diffracts: sound, water waves, microwaves, radio waves, seismic waves. Light is simply the hardest to notice because its wavelength is so small.
Diffraction changes the wavelength of the wave.
The wavelength stays the same in the same medium. Diffraction changes the direction and spread of the wavefronts, not their wavelength or frequency.
A wave only diffracts if the gap is exactly one wavelength wide.
All waves diffract at every gap. The spreading is simply most noticeable when the gap is comparable with the wavelength.
Diffraction makes the wave speed up as it squeezes through.
The definition is worth two marks: the spreading of waves as they pass through a gap or past an obstacle.
If asked to explain an observation, name the wave, state that it spreads after the gap or edge, and compare the gap width with the wavelength.
Do not write "the waves bend" on its own — bending at a boundary is refraction. Diffraction is spreading after a gap or around an edge.
Speed depends on the medium, not on the gap. In the same medium it is unchanged.