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 Wavelength Behind:
Wavelength in Behind of a Moving Source:
λ_b = (V-u_s) / f₀
where,
λ_f = Wavelength in Behind of a Moving Source
v = Wave Velocity
u_s =Source Velocity
f₀ = Source Frequency.