 Now suppose that whenever Bob receives a blue flash from Alice he immediately sends his own red flash towards Dave, once every formula_10 seconds (by Bob's clock).Einstein's second postulate, that the speed of light is independent of the motion of its source, implies that Alice's blue flash and Bob's red flash both travel at the same speed, neither overtaking the other, and therefore arrive at Dave at the same time.So Dave receives a red flash from Bob every formula_8 seconds, by Dave's clock, which were sent by Bob every formula_10 seconds by Bob's clock.