# If we shoot to photons to a surface, in which conditions we can differentiate the points where the photons collided with the surface?

Now, I heard this example from one of my professor 4 years ago when I first entered university, and at that I had some objections (and I still do), because, I think, if we flick our wrist fast enough, the spot on the moon will not be a continuous spot, but that spot will be like "$$. \quad .\quad . \quad . \quad .$$", as if we as turning on and off the light while moving it, and these ideas led me to the following question:

Lets shoot a two photon onto a reflective surface, and observe where the photons hit the surface;

What conditions should be satisfied so that we can differentiate the points where the photons collided with the surface ?

• Yes light contains a zillions of photons, but instead of using mars, we can use another celestial object, so that $R$ can get big as much as we want; these are just experimental technicalities. This means even a small change in the angle that the photons are sent from the source can collide with that celestial objects far apart, so at some point, that motion of that spot on that celestial object cease to have a continuos motion. – Our Jan 23 '19 at 14:15