There is something (among many things of course) that confuses me with the uncertainty principle as it pertains to position and momentum of a photon. If one shoots a photon of red light, for example, at a screen, one can see where exactly the photon hits the screen so position is known. Also, because the speed of light in vaccuum is uniform at c, one can calculate, using simple algebra, the location of the photon in every step of the way. Where is the uncertainty here? I am not measuring the location of the photon inbetween but why should I, if c is a universal constant then c is a universal constant. Obviously, my reasoning is wrong, but I can't decide where my mistake is. 1. Is it something to do with measuring the exact time of the impact? But why should I care, again c tells me how long it took for the light to hit the screen. 2. Or maybe I cannot measure the exact distance to the screen? But then I should never be able to measure that distance, regardless of whether I am performing experiments with light or not.