Hello, There are two FoR S and S'. A and B in S FoR, A' and B' in S' FoR. A is a source of photons and B is a mirror. A' is a transparent glass and B' is a mirror. S' is moving to the right direction. Think that there is very negligible distance between A and A'. Now, A fires a photon beam such that the beam passes though middle of the transparent glass A'. S' is moving with high speed, so mirror B' does not stop the beam. Observer at A sees that the beam reaches to mirror B. But, observer at A' sees that S is moving and the beam passes his glass A' and reaches to mirror B'. The beam does not reach to mirror B from his perspective. The question is which mirror would be hit by the beam?