# Compton scattering, formulae question

1. Nov 17, 2013

### channel1

1. The problem statement, all variables and given/known data
An X-ray source of wavelength gamma is directed at a sample; determine the maximum speed of scattered electrons.

2. Relevant equations

KE=($\gamma$-1)mc2=hc/$\lambda$ - hc/$\lambda$'

3. The attempt at a solution

I've been provided with the solution using the formula above, however it looks like it's Eincident - Escattered ---or in other words initial minus final...isn't convention final-initial? Why do we subtract initial minus final in this case?

2. Nov 18, 2013

### gildomar

The final is subtracted because the scattered photon will have a lower energy than the incident photon (since some of it has gone to the electron).

3. Nov 18, 2013

### vela

Staff Emeritus
Apply conservation of energy to the situation.

This question really doesn't belong in the advanced physics section.