Hi! Just wanted to say that this is my first post here, and I thank in advance anyone who spends time on this. Hopefully I will be able to return the favor someday. Anyway..., I thought I knew how to do this, but apparently I was wrong... I need to analyze the collision of a photon at an electron. 1. The problem statement, all variables and given/known data A photon hits an electron in it's rest system. After the collision, the photon's direction is perpendicular to it's prior direction, having half the starting energy (first it was going x+, now it is going y+). After the collision, the electron is traveling at some speed v at an angle -a to the x axis (i.e. x+ y-) 1. write the relevant preservation equations. 2. Find the hitting photon's energy ... (if I'll understand these I'll do fine with the rest) The "knowns" are C - the speed of light and E0 - the electron's rest energy. 2. Relevant equations 3. The attempt at a solution I have to say that I'm new to using the Lorentz transformation and it's consequences so I likely made a few mistakes, but..: I had the conservation of energy as: (E1 is the photon's starting energy) E_0 + E_1 = E_0 * \omega + (E1 / 2); I wrote the conservation of momentum as follows: (E_1 / C) = (E_0 / C) * \beta * cos a (X axis) 0 = (E_1 / 2C) - (E_0 / C) * \beta * sin a (Y axis) Then (after hours of thought and tackling this) I realized that E=pc is only true for particles traveling very close to (or at) the speed of light, and revised it to: P_1 = P_e * cos a (X axis) 0 = P_2 - P_e sin a (Y axis) and used: (E_1 / 2) = P_2 * C P_2 = ([P_e]^2 + [m_e]^2 * C^2)^(1/2) However, it got somewhat messy and then I got an impossible answer, so I suspect something is wrong here as well... I would really appreciate if someone explains the general way of solving this kind of problems (or rather how to treat the momentum conservation, I guess) and/or what is wrong about my way. Thanks in advance, Shwouchk.