Is the Doppler shift an independent principle of wave mechanics? Or we can deduce it from other principles of wave mechanics? The article  gives an analogy of throwing and receiving balls. But it may be inappropriate. We are discussing wave mechanics? Why bother an analogy of balls/particles? Also could anyone discuss the relation between Doppler shift and Galilean relativity and Einstein's relativity principle? Because the Doppler effect happens both in the case of water or sound wave and light wave.