Light, unlike sound or water ripples, does not propagate through a medium, and there is no distinction between a source moving away from the receiver or a receiver moving away from the source. Fig. 3-6 illustrates a relativistic spacetime diagram showing a source separating from the receiver with a velocity parameter β β , so that the separation between source and receiver at time w is β β w . Because of time dilation, w = γ γ w ′ . Since the slope of the green light ray is −1, T = w + β β w = γ γ w ′ ( 1 + β β ) . Hence, the relativistic Doppler effect is given by