1. The problem statement, all variables and given/known data photon scatter angle is theta, electron recoil angle is phi prove tan(phi) = (1+hf/mc^2)^-1 cot(theta/2) 2. Relevant equations 3. The attempt at a solution Conservation of energy is the same as in here: http://en.wikipedia.org/wiki/Compton_scattering Conservation of Momentum In x : p_gamma = p_gamma' cos(theta) + p_e' cos(phi) In y: 0 = p_gamma' sin(theta) - p_e' sin(phi) p_gamma'=p_e' sin(phi)/sin(theta) Let's solve for p_e'^2 since we can directly substitute it in the energy term. p_e'^2 = p_gamma^2 sin^2(theta)/ [sin^2(phi)cos^2(theta) + 2 sin(phi)cos(theta)sin(theta)cos(phi) + sin^2(theta)cos^2(phi)] I'm not sure where the cot(theta/2) comes from which we want to show.