1. The problem statement, all variables and given/known data A mirror moves perpendicular to its plane with speed (beta)c. A light ray is incident on the mirror from the \forward" direction (i.e., vm dot vl < 0, where vm is the mirror's 3-velocity and vl is the light ray's 3-velocity) with incident angle µ (measured with respect to the mirror's normal vector). find cos (phi) where phi is the reflected angle. 2. Relevant equations Unsure, I believe the law of cosines is involved, but I am not sure how that would fit in to any other equation. 3. The attempt at a solution We had an example of when the mirror is moving parallel, using the 4-momentum. This is very similar (I think), but I can't seem to get my head around it. I figure for the mirror, the velocity in the x direction will be zero, because it is moving up the y with speed (beta)c. But that is where we get stuck... can anyone point me in the right direction?