Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves 'bunch together'. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. Therefore, each wave takes slightly less time to reach the observer than the previous wave.

The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. 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. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. The Doppler effect is named after the physicist Christian Doppler, who described the phenomenon in 1842. The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave.