# Question on Momentum/Energy transferred in photon Elastic scattering

1. Nov 5, 2012

### konsensei

Hi. Im new to the forum, If I am missing something, please kindly remind me.
1. The problem statement, all variables and given/known data
Given/Known:
- A photon with Energy Eγ, momentum pγ hit an atom with mass M.
- The photon is perfectly elastic scattered at angle $\theta$

Problem: Find recoil energy

2. Relevant equations
Relevant Knowledge:
- Due to elastic scattering, the photon's energy remain the same (almost), only direction is changed. To my understanding, the electron in the atom absorbs the initial photon's energy. However, the energy is not enough to excite or knock out the electron, the electron then emits a photon with similar wavelength (same energy), but in random direction.

3. The attempt at a solution
- Using q2 = k2 + k'2 - 2kk' cos $\theta$
I could find the recoil momentum (momentum transfer). But then I don't know how to find energy