So, while working some problems in my modern physics books, I met this question: A photon whose energy equals the rest energy of the electron undergoes a Compton collision with an electron. If the electron moves off at an angle of 40 degrees with the original photon direction, what is the energy of the scattered photon? The Compton effect equation is very simple, but it feels like you're missing a variable, since there's not a simple relation between the electron's angle (given) and the photon's angle (unknown). You can solve for one in terms of the other, but to do so you have to deal with the electrons momentum. What am I missing?