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Overview
The Doppler effect occurs when the source of a wave and the observer move relative to one another. If the source and observer move closer together, the observed frequency increases. If they move farther apart, the observed frequency decreases. This effect appears in sound, light, radar, astronomy, medical imaging, weather tracking, and many communication systems.
Relative Motion
The Doppler effect depends on relative motion between the wave source and the observer. The wave itself is produced at a source frequency, but the observer may detect a different frequency if the spacing between arriving wavefronts changes.
Approaching Source
When a wave source moves toward an observer, successive wavefronts are compressed closer together in front of the source. The observer receives more wave cycles per second, so the observed frequency is higher.
Receding Source
When a wave source moves away from an observer, successive wavefronts are spread farther apart. The observer receives fewer wave cycles per second, so the observed frequency is lower.
Sound Example
A familiar example is the changing pitch of a siren as an emergency vehicle passes. The pitch sounds higher as the vehicle approaches and lower as it moves away.
Light Example
Light also shows Doppler shifts. When a light source moves away, its observed wavelength can shift toward red. When it moves closer, the observed wavelength can shift toward blue. Astronomers use these shifts to study the motion of stars, galaxies, and gas clouds.
Medium Considerations
For sound, the motion of the source and observer is measured relative to the medium, such as air. For light in vacuum, the effect is handled through electromagnetic wave behavior and relativity.
Applications
The Doppler effect is used in radar speed detection, weather radar, medical ultrasound, astronomy, navigation, flow measurement, and motion sensing.
Common Mistakes
A common mistake is thinking the actual emitted frequency changes. In many cases, the source emits at the same frequency, but the observer detects a shifted frequency because of relative motion.
Why This Matters in Physics
The Doppler effect links wave behavior to motion. It shows how waves can carry information about speed, direction, expansion, circulation, and moving systems.