I am a new fish to SR and I have some questions about light clock. Hope someone to clear my concepts. Q1. As we all know, the speed (scalar) of light is "c" and is independent to the motion of the light source, but what about the velocity (vector) of light (with magnitude "c" but what direction ? )? Q2. Suppose the direction of a light ray is prependicular to the velocity of the light source (like the one in light clock.), can the light ray get the horizontal speed from the light source? Q3. In the frame of a person who stand beside a light clock on a moving train which having speed "u" to right, why the path of a photon that he observed is vertically up and down ? Is it because the photon got the horizontal speed ("u" to right) .If yes, then it become a classical veloctiy addition ?; If no, the photon got no horizontal speed from the source, then how can the person on the train see a vertical up and down path instead of diagonal path? I hope you can understand what I am talking about. Thank you!!