Paradoxical relativity.

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Has anyone come across the following argument somewhere? Can anyone suggest a relativistic solution to the paradox? I've done my best for the diagrams. Regards. The standard imaginary model of special relativity (without the mirror) is that of a beam of light (AB) seen as perpendicular to the motion inside the reference frame. While the observer outside the moving reference frame sees the beam of light (AC) pointing ahead of the perpendicular.