[Mentor's note: This thread does not use the template because it started in one of the non-homework forums. I moved it here instead of deleting it and asking the poster to repost here, because it had accumulated several useful replies.] Hi. I have the exact same problem that ZachWeiner had in here: 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? Unfortunately the answers don't help me much. I would need more information how to get the energy of the scattered photon. Thanks.