Quantum Mechanics Help. Compton Scattering Use.

[Billy12]

Homework Statement

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

Homework Equations

Conservation of momentum and energy. p = h/lambda, E = hc/lambda

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?

 

[voko]

Energy and momentum are linked through the relativistic energy-momentum equation.

 

[Billy12]

I solved it without the relativistic equations.