Coherence

Coherence means constant phase difference between two sources. If you beat two spheres out of step(180 degrees out of phase) instead of instep(in phase) on the water surface, the spot where you expect to see constructive interference will turn into destructive interference. The sources have a phase difference but it still produce interference pattern that are permanent except that it is opposite to what you would get if the sources were in phase. Therefore sources need only satisfy the condition of constant phase difference. If the phase difference is constant , this imply that the frequency is constant. Sources having constant frequency may not have a constant phase difference. For example , if I two spheres moving up and down at the same rate (same freq) but are sometimes in step and sometimes inserted out of step , than a fixed location on the water surface changes from constructive to destructive intermittently and therefore an observable pattern cannot be seen. Coherence is the condition for observable inteference pattern.