What is the Doppler Effect?

Glossary Definition

The Doppler effect, or Doppler shift, describes the changes in frequency of any kind of sound or light wave produced by a moving source with respect to an observer.

Waves emitted by an object traveling toward an observer get compressed—prompting a higher frequency—as the source approaches the observer. In contrast, waves emitted by a source traveling away from an observer get stretched out, prompting a lower frequency.