A train of proper length L has clocks at the
A train of proper length L has clocks at the front and back. A photon is fired from the back of the train to the front. Working in the train frame, we can easily say that if the photon leaves the back of the train when the clock there reads zero, then it arrives at the front when the clock there reads L/c. Now consider this setup in the ground frame, where the train travels by at speed v. Rederive the above result (that the difference in the readings of the two clocks is L/c) by working only in the ground frame.