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).