Diffraction
| English | Chinese | Pinyin |
|---|---|---|
| diffraction | 衍射 | yǎn shè |
You hear around corners, but you can't see around them
- Someone talks in the next room and you hear them, even without a straight line to your ear.
- Yet you can't see around the same corner. Why the difference?
- Waves bend and spread as they pass through gaps and around edges — diffraction 衍射.
- How much they spread depends on the gap size compared with the wavelength.
Waves spread through gaps
- When a wave passes through a gap or past an edge, it fans out into the space beyond.
- This spreading is diffraction, and it happens for all waves.
- Straight (plane) waves become curved as they emerge from a narrow gap.
- It is why sound reaches you around a doorway or a wall.
Diffraction is the:
Diffraction is the spreading of waves as they pass gaps or edges.
Diffraction happens as waves pass through gaps or around ____.
Waves spread out at gaps and around edges.
Gap size matters most
- Diffraction is greatest when the gap is about the same size as the wavelength.
- A gap much wider than the wavelength barely spreads the wave.
- A gap around the wavelength's size spreads it almost in a semicircle.
- So a narrow gap (relative to $\lambda$) causes the most bending.
Diffraction is greatest when the gap width is:
Spreading is greatest when the gap is about the size of the wavelength.
Select all true statements about diffraction.
Diffraction spreads waves, is greatest at gap ≈ wavelength, and is why sound bends around corners. A huge gap diffracts least.
Why sound bends but light doesn't (usually)
- Sound has a wavelength of about a metre, similar to doorways — so it diffracts strongly.
- Light's wavelength is tiny (under a micrometre), so everyday gaps barely diffract it.
- That's why you hear around corners but light casts sharp shadows.
- Shrink a slit to light's wavelength, though, and light diffracts too (next lesson).
More or less diffraction?
Waves spread most when the gap is comparable to their wavelength. Sort each case.
Sound diffracts strongly around doorways because its wavelength is similar to a doorway's size.
Sound's metre-scale wavelength matches everyday gaps, so it diffracts strongly.
Why does light usually cast sharp shadows through everyday gaps?
Light's tiny wavelength means everyday gaps are far too big to diffract it much.
Diffraction is strongest when the gap width is comparable to the wavelength, not when the gap is huge. This is exactly why sound (long wavelength) diffracts around everyday obstacles but light (tiny wavelength) usually doesn't — the gap is far larger than light's wavelength.
Sound (wavelength ~$1\ \text{m}$) and light (wavelength ~$5\times10^{-7}\ \text{m}$) both pass through a $1\ \text{m}$ doorway.
- The doorway ≈ the sound wavelength, so sound diffracts strongly — you hear around it.
- The doorway is millions of times the light wavelength, so light barely diffracts.
Diffraction is the spreading of waves through gaps and around edges. It is greatest when the gap ≈ the wavelength. Because sound has a metre-scale wavelength it diffracts around everyday obstacles, while light's tiny wavelength means it usually casts sharp shadows.