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=([itex]\gamma[/itex]-1)mc2=hc/[itex]\lambda[/itex] - hc/[itex]\lambda[/itex]' 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?