I'm trying to calculate the uncertainty in momentum/direction resulting purely from the certainty in position caused by a small slit, as a function of the slit width. Can anyone tell me if I'm doing this right? Starting with the HUP: dX * dP >= h/4pi dX = the slit width w P = h/lambda * sin(theta) where theta is the angle that the photon might be travelling after the slit relative to the center of the slit so dP = 2 * h/lambda * sin(theta) A factor of 2 because the uncertainty can extend in either direction from the center of the slit so w * 2 * h/lambda * sin(theta) >= h / 4pi sin(theta) >= lambda / (8pi * w) Is that right? My only concern with the calculation is that "w" could be so small that sin(theta) could be greater than 1, which is of course impossible, so that's why I think I made an error somewhere. Thanks for any ideas!