When you have a listener and a source, and when one of the two move relative to the other, the frequency perceived by the listener will be different.
- If the listener travels toward the stationary source at 0.5 times the speed of sound, he will perceived a frequency 1.5 times higher.
- If the source travels toward the stationary listener at 0.5 times the speed of sound, the listener will perceived a frequency 2 times higher.
I perfectly understand why the frequency is different in both cases (1.5 and 2 times): if you draw lines representing the crest of the cos wave emitted at a given frequency, you will see that the lines comes closer following the number I gave previously in the two cases.
What I don't understand is the Doppler effect for electromagnetic waves (like the light). Apparently the Doppler effect is there independent of who is moving and the only thing that matter is the relative velocity between the source and the receiver. I here only speak of non-relativistic Doppler effect so the relative speed is small with respect to the speed of light.
It can explain the red-shift of the star light due to the expansion of the universe.
Why is it only the relative speed that matter here? I read that it was because in the sound cases, the sound propagate in a medium (air) and that the light propagate in nothing ... but I do not see how it explains it ?
Thank you for any help !