Diffraction
Heard but not seen
- You can hear someone around a corner — but you can't see them.
- Both sound and light reach the gap, yet only sound bends around it.
- The difference is diffraction.
What diffraction is
- Diffraction is the spreading of a wave as it passes a gap or an obstacle.
- All waves do it — water, sound, light, microwaves.
Practice
Diffraction is the ____ of a wave as it passes through a gap.
Diffraction is the spreading (bending) of a wave at a gap or obstacle.
Practice
All waves can diffract.
Yes — water, sound, light and microwaves all spread at a gap or edge.
How much it spreads
- Gap much wider than $\lambda$ → the wave passes nearly straight through.
- Gap about the size of $\lambda$ → it fans out strongly.
Practice
A wave spreads out the most when the gap is:
When the gap ≈ the wavelength, the wave fans out strongly; a much wider gap lets it pass nearly straight.
Practice
Making the gap narrower makes the waves spread out ____.
A narrower gap (closer to one wavelength) gives stronger diffraction.
Why you hear but don't see
- Speech has $\lambda \approx 1\ \text{m}$ — close to a doorway's width, so it spreads round the corner.
- Light has $\lambda \approx 500\ \text{nm}$ — far smaller than the gap, so it barely spreads.
Practice
You can hear but not see around a corner because sound has a much larger wavelength than light.
Sound's wavelength (~1 m) is near the gap size, so it diffracts; light's (~500 nm) is far smaller, so it barely does.
You've got it
Key idea
- diffraction = a wave spreading through a gap or around an obstacle (all waves do it)
- most spreading when the gap is about one wavelength wide
- sound diffracts round corners (big $\lambda$); light hardly does (tiny $\lambda$)