1. The problem statement, all variables and given/known data In a Compton scattering experiment, a photon is scattered through an angle of 90.0deg, and the electron is scattered through an angle of 21.4deg. Determine the wavelength of the scattered photon 2. Relevant equations Conservation of momentum and energy. p = h/lambda, E = hc/lambda 3. The attempt at a solution Since the angle of scattering is 90.0 deg for the photon the compton wavelength would just be 2.43*10^-12. I know that the momentum is conserved in both directions (x and y) and i got two equations for it but I don't know what to do for energy conservation. What do we use for energy?