Hello, I have a simple question. Suppose a perfect point source in front of a mirror. The virtual image of the point source acts like a second point source. Now lets look at the interference of the direct point source light and the virtual point source, at some position. Since the path length difference between direct light and mirrored light is constant, the interference is going to be stationary. Now, if the mirror is moved such that the path difference changes in time, what can I say about the interference or spatial coherence? How does the degree of coherence / quality of interference depend on the speed of the movement? I assume an incoherenct interference due to doppler shift. But if the speed of the path-difference-change is constant, then the doppler shift would give rise to a constant frequency difference. Therefore the interference would be a particular beat, right? Is it then possible to observe a stationary interference pattern?