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**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?