The difference between the frequency fo of radiation received at a point and the frequency fs of the radiation when it was emitted, when observer and source are moving with respect to each other. If relativistic effects are ignored, the Doppler shift is given by Δf ≡ f _o - f _s = f _s ((c + v) / (c - v)) ½ , in which c is the speed of the radiation and v is the speed of the observer with respect to the source (positive for the two points moving apart).