\fWe know that the angle of reflection equals to the angle of incidence for a stationary mirror, but is it true for a moving mirror? To find out, let's consider a flat mirror parallel to the yz plane moving in positive r-direction (i. e., perpendicular to its plane) with velocity v. An observer located in front of the mirror, but not on the same axis, points a laser pointer to the mirror. The angle of incidence is 07. What is the angle of reflection, OR? Hint: work with the 4-vector k" = (w/c, kr, ky, kz). Transform it to the mirror rest frame S, consider reflection there (i. e., change the k"-vector accordingly, kr > -kr) and transform back to the observer's rest frame S. As a 4-vector, k" is transformed according to usual transformation rules: W - UKI w = y(w - Bckx) = V1- v2 / c2 kI - ZW kx = 1(kx - Bw/c) = V1 - v2/c2 (1) ky = ky, kz = kz. Keep an eye on signs, they might be tricky here