What is the max E that teh electron can obtain?

[thatoneguy123]

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

Homework Equations

$\lambda$' - $\lambda$ = (h/mc)(1-cos(theta)

The Attempt at a Solution

i just don't know where to get started or anythign can someone show me how to do this?

 

[daveb]

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

 

[thatoneguy123]

no that's why i don't get how to do this.