# What is the max E that teh electron can obtain?

1. Sep 27, 2011

### thatoneguy123

A photon having 20 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain?

2. Relevant equations
$\lambda$' - $\lambda$ = (h/mc)(1-cos(theta)

3. The attempt at a solution
i just dont know where to get started or anythign can someone show me how to do this?

2. Sep 27, 2011

### daveb

Do you know what happens when the scatter angle is 0 degrees, 90 egrees, and 180 degrees?

3. Sep 27, 2011

### thatoneguy123

no thats why i dont get how to do this.