Definition of Doppler effect and Doppler shift

Summary

The Doppler effect (also called the Doppler shift) is the change in the observed frequency (and thus the period) of a wave due to relative motion between an observer and the wave’s source. A common example is the change in pitch of a siren: as the vehicle approaches, the received sound has a higher pitch (higher frequency), and as it recedes, the pitch becomes lower (lower frequency). 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 traveling through a medium (such as sound), the relevant velocities are relative to the medium, so the total Doppler effect can depend on motion of the source, the observer, and/or the medium. The topic also notes that when relative speeds approach or exceed the wave speed in the medium, the standard Doppler equation can break down (e.g., leading to effects like sonic booms for sound).

Key Points

• 1

  Doppler effect is the observed change in wave frequency caused by relative motion between observer and source.

• 2

  Approaching source increases observed frequency (higher pitch); receding source decreases observed frequency (lower pitch).

• 3

  In vacuum, only relative observer–source velocity matters; in a medium, velocities relative to the medium determine the shift.

• 4

  For sound, if the source moves faster than the speed of sound, the usual Doppler predictions fail and shock-wave effects (sonic boom) occur.