The Doppler effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or 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.

Doppler Effect Approaching Source:
New Frequency of Approaching Source:
f'＝ v / λ_f
where,
f' =New Frequency of Approaching Source
v =Wave Velocity
λ_f = Wavelength in Front of Source.