Doppler effect for sound waves is asymmetric i.e. for the same relative motion in the cases (i) and (ii), the apparent change in the frequency is not the same. It is because, sound waves require a medium for their propagation and the motion of the observer relative to the medium is different in two cases.
Doppler effect for light waves is symmetric i.e. for the same relative motion in the case
(i) and (ii), the apparent change in the frequency is same. For light waves in vacuum, there is no difference in the two cases. It is because, the relative motion between the source and the observer is not affected, whether the source is in motion or the observer is in motion. However, for the light waves in medium, the situations in the two cases are different as in the case of Doppler effect for sound waves. Likewise the formulas for Doppler effect in the two cases will be different.
