In this image, we have a bar, on the lower mark we have placed a lamp and on the upper a mirror as shown. At some point the lamp goes on and the light reaches the mirror, it reflects and goes back from where it started. The events A = light emition and B = return at starting point are two events which take place in the same reference frame (Σ). For Σ the light travelled a distance equal to 2(AB) = 2d with speed C. So Δt = 2d/c Observer Σ along with the bar, move with velocity v in relation to Σ' to the right. Therefore Σ' will notice that the light follows the track Α'Β'Γ' with velocity c............ Can somebody please explain to me, why the light follows the track Α'Β'Γ', I can't seem to "feel" the above image. P.S. If you have any links that will help me with the understanding of special relativity or have some exercises (With solutions or answers) i would be very thankful.