1. The problem statement, all variables and given/known data 2. Relevant equations <S>=c[itex] \epsilon [/itex]E2/2 [itex] \Delta P = A \Delta t <S>/c [/itex] 3. The attempt at a solution My question is if we need the "box height"? This is my work which I'm assuming is wrong. :C If there's another problem like this on here please put a link. I didn't see any, but my search terms may not have been ideal "Pressure of light in a box".