Why does the observed frequency of a wave change when the source or observer moves?
The observed frequency of a wave changes when the source or observer moves because motion affects how often wavefronts reach the observer. Waves—whether sound, light, or water waves—spread out from a source in regular intervals. If the source moves toward the observer, each successive wavefront is emitted from a position closer than the previous one. This compresses the spacing between wavefronts, making them arrive more frequently and increasing the observed frequency. If the source moves away, the spacing stretches, making wavefronts arrive less frequently and lowering the observed frequency. This change in perceived frequency is the Doppler effect.
When the observer moves instead of the source, the effect follows the same principle. Moving toward incoming wavefronts causes the observer to encounter them more quickly, raising the observed frequency. Moving away means wavefronts take longer to reach the observer, lowering the frequency. What changes is not the wave itself but the relative speed at which the observer meets the wavefronts. This shows that frequency is not just a property of the wave; it also depends on the motion of both source and observer.
In sound waves, this frequency shift changes pitch. A passing siren sounds higher as it approaches because the wavefronts are compressed, and lower as it moves away because they are stretched. The medium (air) carries the wave, but motion changes how wavefront spacing is perceived. In electromagnetic waves such as light, the same principle applies, but the shift is expressed as changes in wavelength rather than pitch. Moving toward a light source creates a blueshift (shorter wavelengths), while moving away creates a redshift (longer wavelengths).
The Doppler effect reveals that wave measurements depend on relative motion, not absolute motion. Since waves propagate at their own fixed speeds—sound at the speed of sound in air and light at the speed of light in vacuum—changes in observed frequency come solely from differences in motion between source and observer.
Frequently Asked Questions
Does the Doppler effect change the actual frequency of the wave?
No. The source emits waves at a fixed frequency. Only the observer’s perception of how often wavefronts arrive changes due to motion.
Why does the Doppler effect apply to both sound and light?
Because both involve waves. Although sound requires a medium and light does not, the underlying principle of wavefront spacing still applies.
Is the Doppler effect useful in astronomy?
Yes. Redshifts and blueshifts reveal whether stars and galaxies are moving toward or away from Earth, helping map the universe’s expansion.
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RevisionDojo breaks down concepts like the Doppler effect into simple, intuitive explanations so you can master wave behavior with confidence.
