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I'm studding special relativity and there is a famous example where there is a moving train witch has a light source on its floor and a mirror attached to its roof.

an observer in the train sees a pulse of light leaves the source vertically and goes to the mirror and comes back to the source.

the example says that an observer on the earth should see the path of light as a triangle like figure "b" in this image: enter image description here

but I don't understand why?! If someone shoot a ball like this, it's logic because the ball will have an extra velocity in the direction of x-axis but here we know that the pulse is going upward with the velocity c, so if the observer on the earth see the light is going right with the train , it should have an extra velocity Vx in the direction of x, and so the total velocity will be equal to (c^2 + Vx ^2)^(1/2) witch is larger than c.

it will be solved with saying that the light will go on the L direction (shown in the figure) with the total c velocity, but how we know that the observer on the earth should see this? why not this : " the light goes upward with the velocity c and so wont come back to the light source in the train. "

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solved by the links by @joshphysics . thanks. –  Ramtin Didab Oct 16 '13 at 11:39

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