Diffraction
Diffraction
- A wave does not always go straight. When it passes through a gap it spreads out into the space behind, as if it bends around the edge.
- This spreading is called . It is why you can hear someone through an open door before you can see them. How much it spreads depends on the size of the gap.
What diffraction is
- Here is the line to learn for the exam: diffraction is the spreading of a wave as it passes through a gap or around an edge.
- Straight water waves move towards a barrier with a gap in it. After the gap, the waves bend outwards and fill the space behind. Drag the gap width below to see the spreading change.
- This happens to every wave, not just water. Sound does it, light does it, microwaves and radio do it too.
straight wavesspread-out wavesgap = 3.0 λ·spread half-angle ≈ 19°·little diffraction
Gap size and wavelength
- How much a wave spreads depends on the gap size compared with the wavelength.
| Gap size | What the wave does |
|---|---|
| Much wider than the wavelength | stays almost straight; only the edges bend a little (little spreading) |
| About equal to the wavelength | spreads out the most, in wide arcs (strongest diffraction) |
- So the spreading is greatest when the gap is about the same size as the wavelength.
- To spread more, make the gap narrower or use a longer wavelength. Both bring the gap and the wavelength closer to the same size.
Worked example
Why you hear round a corner but cannot see round one
A doorway is about 1 m wide. Sound has a wavelength of about a metre. Light has a wavelength of about m. Which one spreads through the doorway?
- Sound: wavelength about 1 m, gap about 1 m. Gap and wavelength are close, so sound spreads a lot. You hear a person round the corner.
- Light: the doorway is millions of times wider than its wavelength. The gap is far too big, so light spreads very little. You do not see round the corner.
Answer
Diffraction is a wave spreading out at a gap or an edge. It is strongest when the gap is about the same size as the wavelength.
Your turn— tap to reveal the worked answer (9702/13/O/N/24 Q30)
Waves pass through a fixed gap. You want them to diffract more. Do you increase or decrease the frequency?
Answer: decrease the frequency. Lower frequency means a longer wavelength, which is closer to the gap size, so the waves spread more. (9702/13/O/N/24 Q30)