Interference and diffraction
When two waves overlap they add together point by point — interference — and a wave passing through a narrow opening spreads out into the region behind it — diffraction.
Practise this conceptInterference is what happens when two or more waves overlap: at each point, their displacements simply add together. Where two crests meet, they add to make a bigger crest — constructive interference. Where a crest meets a trough of equal size, they cancel out completely — destructive interference.
That cancellation doesn't destroy any energy, even though it might look that way at first. Energy that seems to vanish at a point of destructive interference reappears at nearby points of constructive interference, where the waves add up to more than either alone — interference redistributes energy, it never deletes it.
Diffraction is a related but distinct idea: it's the spreading of a wave as it passes through an opening or around an obstacle, most noticeable when the gap is comparable in size to the wavelength. It happens at edges and obstacles too, not only through gaps — sound bending around a corner so you can hear someone before you see them is diffraction at work.
Key ideas
Requires: Wave properties: amplitude, wavelength and frequency
Unlocks: Standing waves, The wave nature of light, Wave-particle duality
Formulas
| Symbol | Name | Unit |
|---|---|---|
| distance | m (metre) | |
| wavelength | m (metre) | |
| distance | m (metre) | |
| distance | m (metre) |
Common misconceptions
- Interference only happens when two waves are travelling in exactly opposite directions.
- Diffraction only occurs when a wave passes through a gap, never around an obstacle.
- When two waves interfere destructively, the energy of the waves is destroyed.