In this problem we calculate the length of a light clock that moves at speed v parallel to its axis in the moving system. A light clock ticks every time light makes a round trip between two mirrors separated by a distance L. The key point is that we can deduce the length of this clockdirectly from the postulates of relativity:
1. The laws of physics are the same in any coordinate system that moves at constant velocity (i. e., any inertial reference frame).
2. The speed of light is c when measured with respect to any coordinate system moving at a constant velocity.