Question #7385a
1 Answer
Only coherent wave sources can provide regions of constant constructive/destructive interference.
Coherent wave sources have a constant phase difference. Without that the phase difference would be continually changing.
Take a two source interference experiment as an example. In that experiment there are regions of constructive interference and they do not move their position - we see them in the same place and their intensity does not change. This is because the waves form the sources arrive in phase with each other at all times (the phase difference is constant). The same can be said for the regions of destructive interference, except this time the waves arrive in anti phase at all times.
If the wave sources were not coherent in the two source experiment then there would be no positions where the waves would arrive in phase or anti phase all the time. Instead the waves would sometimes arrive in phase, sometimes in anti phase and sometimes in between. So there would be a constantly changing intensity across the whole screen, but we wouldn't be able to see it because it would happen too fast.
