Consider the famous experiment of electron diffraction. electrons are accelerated by an anode. After this anode there is a graphit target (to create an interference pattern). In this case one has to take the Bragg condition to calculate the angle constructive interference actes. The Bragg condition is given by: 2*d*sin(w)=n*lambda For the first maxima we get: 2*d*sin(w)=lambda Now my problem is the following. Look at the picture (shows electron diffraction). The angle the electrons leave the graphit is 2w. Thus, 2w is "the angle of the first maxima". But why the hell is this angle 2w. I don't get this. I don't know, arghhh. I could derive the Bragg condition and calculate the probability distribution on the wall (in this experiment). But I cannot derive this angle of 2w. Would be great if someone could help quickly.