ABSTRACT
The first postulate of special relativity states that the laws of physics are identical in all inertial (non-accelerated) frames of reference. The second postulate states that the speed of light in vacuum (c) is the same for all observers. Now consider the motion as viewed from a frame S' that moves with a speed v in the -x direction. In this frame both points A and B move with speed v in the x direction. In the time that light is traveling, point B is moving and the light has a longer distance to travel to get there. Its speed is fixed so it takes a longer time to get from A to B. Since the speed of light is fixed, the two frames are related by the velocity triangle.
