Major Differences between Doppler Effects of Sound and Light

Despite their shared qualitative features, sound's and light's Doppler effects differ in several important, specific ways. (A) Light's speed is ABSOLUTE (the same in every reference frame, by the postulates of relativity) -- so for light, only the RELATIVE velocity between observer and source matters; it is physically meaningless to ask 'which one is really moving'. (B) Because of this, light has genuinely different classical and relativistic Doppler formulas, whereas for sound -- which travels through an ordinary physical medium at everyday, non-relativistic speeds -- only the CLASSICAL treatment given in this chapter is needed; there is no separate 'relativistic sound Doppler effect' to speak of.

(C) Consequently, for an EXACT sound-Doppler calculation, it is absolutely essential to know specifically WHO is moving -- the source, the listener, or both -- since (as sections 8.9.1-8.9.3 showed) the source-moving and listener-moving cases give genuinely different-looking formulas, even for the same relative speed, unless that speed is small enough for the simplified relative-velocity form of section 8.9.4(D) to apply. …