Doppler effect

Science

Definition

The Doppler effect is the change in frequency or wavelength of a wave as perceived by an observer moving relative to the wave source. When the source and observer move toward each other, the frequency increases; when they move apart, the frequency decreases.

Examples

  • An ambulance siren sounds higher-pitched as it approaches and lower-pitched as it drives away
  • Astronomers use redshift (Doppler effect for light) to determine that distant galaxies are moving away from Earth
  • Police radar guns use the Doppler effect to measure the speed of a moving vehicle
Key Fact

f_observed = f_source × (v ± v_observer) / (v ∓ v_source), where v is the wave speed in the medium.

Related Terms

Sound waves Wave properties Electromagnetic spectrum Stars and galaxies

Study This Concept

Practice Doppler effect with free review games in these units:

Physics Unit 6: Waves and Sound