what do you mean by doppler;s effect ?

TheDoppler effect(orDoppler shift), named after theAustrianphysicistChristian Doppler, who proposed it in 1842 inPrague, is the change infrequencyof awave(or other periodic event) for anobservermoving relative to its source. It is commonly heard when a vehicle sounding asirenor horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.