The Doppler effect/shift is the change in frequency of a wave moving relative to its source. the received frequency is higher during the approach.
Wavelength in Behind of a Moving Source: λb = (V-us) / f0
where, f0 = Source Frequency, us =Source Velocity, v = Wave Velocity, λf = Wavelength in Behind of a Moving Source.
For example, when Wave Velocity(v) = 10m/s, Source Velocity(us) = 2m/s, Source Frequency(f0) = 2Hz, Then Wavelength in Front of a Moving Source(λf) = 2.67m.