Trying to find the angle of the velocity vector from a recoiled electron after an impact from a photon. I have already found the initial energy of the photon, and the energy of the scattered photon, and so can calculate the momentums. I know the photon is scattered at an angle of 110 degrees, and I know from the Compton effect and the difference in the wavelengths what the electron's energy is. Here is a relevant sketch: http://upload.wikimedia.org/wikipedia/commons/e/e3/Compton-scattering.svg Note that theta here is the 110 degrees. I want to know the angle of the velocity vector of that recoiled electron. Please, if anyone can point a finger at where to go I'd be happy. I've been working on this for quite a long time, trying to turn the initial and final momentum into i and j components, but it is getting me nowhere. I have the feeling the answer is a lot more simple than what I've made it out to be!