Diffraction
| English | Chinese | Pinyin |
|---|---|---|
| gap | 缝 | fèng |
| diffraction | 衍射 | yǎn shè |
| obstacle | 障碍物 | zhàng ài wù |
| ripple tank | 水波槽 | shuǐ bō cáo |
| wavelength | 波长 | bō cháng |
Heard but not seen
- You can hear someone around a corner — but you can't see them.
- Both sound and light reach the gap 缝, yet only sound bends around it.
- The difference is diffraction 衍射.
What diffraction is
- Diffraction is the spreading of a wave as it passes a gap or an obstacle 障碍物.
- All waves do it — water, sound, light, microwaves.
A stationary wave forms where two opposite waves overlap (N marks a node, A an antinode)
Waves adding and cancelling
Two overlapping waves add where they are in phase and cancel where out of phase — change the phase to see the result. This is what makes diffraction patterns.
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.
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.
Diffraction in a ripple tank 水波槽 — a wide gap (a) spreads the waves little, a narrow gap (b) much more
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.
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.
Fundamental mode in a closed pipe — a node at the closed end, an antinode at the open end
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
- diffraction = a wave spreading through a gap or around an obstacle (all waves do it)
- most spreading when the gap is about one wavelength 波长 wide
- sound diffracts round corners (big $\lambda$); light hardly does (tiny $\lambda$)