Doppler Effect: Motion and Frequency Shifts

Definition of Doppler effect and Doppler shift

The Doppler effect (also called the Doppler shift) is the change in the observed frequency (equivalently, the period or wavelength) of a wave due to relative motion between an observer and the wave’s source. A common example is sound: when a vehicle approaches, the received sound has a higher pitch than the emitted sound; when it recedes, the pitch is lower. This happens because successive wave cycles are emitted from positions that are progressively closer to the observer during approach (reducing the time between arrivals and increasing frequency) and progressively farther during recession (increasing the time between arrivals and decreasing frequency). For waves in vacuum (such as electromagnetic waves), only the relative velocity between observer and source matters; for waves in a medium (such as sound), the velocities relative to the medium also matter, so motion of the source, observer, and/or medium can all contribute. In general, the Doppler effect can be described by a relationship between emitted and observed frequency that depends on the wave propagation speed in the medium and the relative speeds of the source and receiver. The effect varies with direction: if the source approaches at an angle, the observed frequency changes gradually, reaching equality when the source is moving perpendicular to the line of sight, and then decreasing as it recedes. The Doppler effect has important consequences and applications, including explaining the pitch slide of passing sirens, measuring velocities in radar, and using frequency/phase shifts in medical ultrasound and other sensing methods. In astronomy, it underlies redshift/blueshift measurements used to infer whether stars and galaxies are approaching or receding.

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Approaching versus receding frequency change

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