 So here we have Alice, she's standing inside a train, and with her is a light clock of some length l with a photon bouncing up and down at the speed of light. So if we take one tick of this clock to be a round trip of the photon, how long will Alice measure this to be? Well, a round trip is a length 2l, and the photon travels that at speed c, so the total time is 2l divided by c. Now let's look at this from Bob's point of view. So Bob is standing on the tracks watching this train go past at some speed v. So from Bob's point of view, things look a bit different. So the photon starts off over here, but the light clock is carried along by the train as it moves. So it hits the upper mirror here, and then returns to the lower mirror completing the tick here. So from Bob's point of view, the path of the photon through the clock looks like this. So here we think about the second postulate of relativity. The speed of light is the same in all reference frames. So even though the light is moving diagonally, Bob should measure it to have the same value c that Alice measures. So in this case, what is the vertical velocity of the photon?