Doppler Effect
The apparent shift in observed wave frequency caused by relative motion between the wave source and the observer.
Governing formula
f_obs = f_src (v ± v_obs) / (v ∓ v_src)
SI unit
Hz
In depth
As a source moves toward an observer, wave crests compress, increasing observed frequency (higher pitch / blueshift). As it recedes, wave crests stretch out, decreasing observed frequency (lower pitch / redshift).
Examples in the real world
The pitch drop of an ambulance siren as it speeds past, or cosmological redshift of light from distant expanding galaxies.