If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would click a 'go' button and a beam of light would bounce between the mirrors. So far so good. Now, we introduce the very speedy observer to system. Lets say he/she gets to observe the moment I click the 'go' button. Which direction was the laser pen pointing for the observer? If perpendicular to the mirrors, why does the light beam strike the 2nd mirror at all?
closed as unclear what you're asking by Bill N, user36790, Daniel Griscom, Gert, Sebastian Riese Feb 18 '16 at 21:46
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The laser pen will point in the same direction for the moving observer, but the light ray will emerge at an angle. So the direction of the light ray won't be the same as the direction of the laser tube.
To see why this is imaging looking at a single pulse of light as it travels along the laser pen towards the aperture:
From your perspective, with the laser pen moving towards you, as the pulse of light travels horizontally along the pen it also travels vertically so it moves in a diagonal line. When the light emerges from the light pen it carries on moving in the same diagonal direction.