1. The problem statement, all variables and given/known data A mirror moves perpendicular to its plane with speed βc. A light ray is incident on the mirror from the “forward” direction (i.e., vm · vl < 0, where vm is the mirror’s 3-velocity and vl is the light ray’s 3-velocity) with incident angle θ (measured with respect to the mirror’s normal vector). (a) Find cos φ, where φ is the angle of reflection. (b) By what factor does the frequency of the light change upon reflection? 2. Relevant equations None that I know of. 3. The attempt at a solution P~ in = E c (sin θin , − cos θin , 0 , 1) P~ ′ in = E c [γ(sin θin − β) , − cos θin , 0 , γ(1 − β sin θin)] I don't know what else.