Coherent wave
If two waves are coherent, it means that they have a constant phase difference between them. This means that the peaks and troughs of the waves line up with each other in a consistent way.
They need to have:
- Same wavelengths
- Similar amplitudes
- The same plane of polarisation
Coherennce in diffraction
We need the waves used in diffraction to be coherent, because otherwise an integer path difference multiple of the wavelength would not always interfere constructively.
We can put a single slit in front of the two slits to make sure that the waves going into the two slits are coherent - the single slit creates a single wave that then splits into two waves when it passes through the double slit.
We only need to do that if the light source is not already coherent (for example, if it’s a laser, we don’t need to do that, because the light from a laser is already coherent).
flashcards
| Question | Answer |
|---|---|
| Coherent waves | Waves that have a constant phase difference, same wavelengths, similar amplitudes, and the same plane of polarisation. |
| Why do waves used in diffraction need to be coherent? | Otherwise an integer path difference multiple of the wavelength would not always interfere constructively. |
| How do you ensure coherence when using a double slit with a non-laser light source? | Place a single slit in front of the two slits; the single slit creates a single wave that splits into two coherent waves through the double slit. |
| When is a single slit not needed to create coherent waves for double slit diffraction? | If the light source is already coherent, e.g. a laser. |