About wave interference

[oupi]

hey guys,

for condtion of observable interference, why do the path difference should be smaller than the coherent length ??

thx

 

[LostConjugate]

Travelling over a larger distance changes the phase difference. Say your on a ski lift with your friend and he decides to take the longer route back down the mountain. You were in phase, constructively getting on the lift at the same time. Now you are out of phase because of the path difference and your getting on the lift by yourself.

 

[Doc Al]

for condtion of observable interference, why do the path difference should be smaller than the coherent length ??

You'll only get significant interference when the overlapping light is coherent (which means that it has some fixed, non-random phase relationship). So, if the path difference is more than the coherence length, there won't be enough coherence to produce a good pattern.