Sources in step

Two waves interacting: it is essential to know about the starting conditions

Wrong Track: To find out whether two sets of waves are in step at a certain common point, all we need to know is the trip time. If the trip time is the same for both waves they must arrive in step.

Right Lines: If you have two sets of waves from different sources they will arrive at a common point in step if they are in step with each other at their respective starting points; they are of the same frequency; and they have the same trip time. The technical term for two sources being in step is that they are coherent. To be coherent, the two sources need not be exactly in step: they might be exactly out of step or somewhere in between. The critical point is that there is a constant, unchanging phase relationship between the two sources.