# Compton scattering, formulae question

## Homework Statement

An X-ray source of wavelength gamma is directed at a sample; determine the maximum speed of scattered electrons.

## Homework Equations

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

## 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?

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).

vela
Staff Emeritus