1. The problem statement, all variables and given/known data Hello, I need to find which values for θ and ψ are not defined for the Compton scattering. Where θ is the angle of the electron and ψ is the angle for the photon. 2. Relevant equations I have attached the formulas I´m using. http://tinypic.com/view.php?pic=v4z2qf&s=5 3. The attempt at a solution According to me ψ≠0 , since the photon´s angle would not be modified and therefore there would be no scattering. I have also done some graphs using Matlab and they show that θ ≠ ∏/2 and θ≠∏. I´m using radians. I have determined algebraically that tan(ψ/2)= sen (ψ/2)/cos(ψ/2). Therefore cos(ψ/2)≠0. ψ/2≠ ∏/2. So ψ≠∏. However I have heard about "backscattering" happening in the practice, so I´m a bit confused since the math shows it is not possible but yet it is. P.D. English is not my mother language so I apologize for any grammar or any other kind of mistake.