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Dopplers effect is the change that seems to happen in the rate of sound or light wave production of an object when it's movement changes in relation to other object.

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Doppler's effect is the apparent change in frequency in waves due to relative motion between observer and source of waves.  A scientist called Doppler found it in 19th century.

Doppler shift in sound waves is observable easily when a train or zeep with a siren  sounds and moves away. Or, even when there is a fast wind blowing.

apparent frequency = ν' = ν₀ [ ν + v_observer ] / [ v - v_source ]
           where v₀ = original frequency,
             v = velocity of sound in still air + velocity of wind, if blowing
                       v_observer = velocity of observer
                       v_source = velocity of the source of sound
Doppler effect is also observed in light waves.