Mirror B Hit by Photon Beam from A

[mananvpanchal]

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?

 

[HallsofIvy]

I don't understand what you mean by "S' is moving at very high speed so mirror B' does not stop the beam". What does S's motion have to do with whether or not the mirror stops the beam? Are you saying that mirrors B and B' are of "relatively" small width and so S' has passed the position by the time the beam reaches there? If so then the beam will not hit S' from any frame of reference.