1. The problem statement, all variables and given/known data Hello everybody , a/There's an elastic collision between a photon of energy E and an electron at rest. After the collision, the energy of the photon is E/2 and propagates in a direction making an angle theta=60° with the initial direction. Find E. What kind of photon is this ? 2. Relevant equations 3. The attempt at a solution I really have difficulties with this problem, for example in question a/ How can I find the energy if i don't have the velocity of the electron ..? a/c=1 Conservation of energy gives: E+mo=E/2+gamma*mo E(1-1/2)=mo(gamma-1) If I use conservation of momentum I have : h/λ=h/λ'cosθ+mv But then ? Thank you !