In the picture above, the horizontal black lines represent mirrors, and the red lines represent the path of a beam of light. The mirror system is moving from left to right. In the view to the left of the dotted line, we are moving with the mirror system; and so we see the beam of light go up and down vertically. In the situation on the right, we are not moving; and the mirror system passes through our view from left to right, which causes the path of the light beam to trace out a triangle.
This is a paradox. There is only one mirror system. How can the beam of light travel both straight up and down and diagonally? Which is it? This paradox was resolved by Albert Einstein in the year 1905. In his famous paper on special relativity, Einstein put forth the idea that the speed of the light beam was a constant but that the flow of time changes depending on whether you are moving or not.
