Q.What is Doppler effect?

The Doppler effect (Christian Doppler, 1842) is the apparent change in a wave's frequency, perceived by a listener, whenever there is relative motion between the wave's source and the listener -- it applies to both sound and light, though this chapter develops it for sound. Three cases are distinguished, all built from tracking how the spacing between successively-emitted wave crests changes due to relative motion.

Source moving, listener stationary: $n=n_0\dfrac{v}{v\mp v_s}$ (minus for approach, plus for recede) -- an approaching source compresses the wavelength ahead of it, raising the apparent frequency. Listener moving, source stationary: $n=n_0\dfrac{v\pm v_L}{v}$ (plus for approach, minus for recede) -- here the wavelength in the medium is unchanged, but the listener's own changing rate of encountering crests shifts the apparent frequency. Both moving: the general formula $n=n_0\dfrac{v\pm v_L}{v\mp v_s}$ applies, with each sign decided independently (upper for that party's own relative approach, lower for recede, always judging the other party as if momentarily at rest).

For small relative speeds ($v_s,v_L\ll v$), all these collapse to a single relative-speed form, $\Delta n/n_0\approx v_r/v$. Sound's Doppler effect differs from light's in that WHO is moving genuinely matters (unlike light, whose speed is absolute so only relative velocity counts), sound's treatment is purely classical (no relativistic correction needed), and wind blowing along the source-listener line shifts the effective sound speed used throughout, $v\to v\pm v_w$.