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The Doppler effect
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When a source of waves moves, the waves are squashed in front and stretched behind. You hear this as a change of pitch.
What happens
A source moving towards you squashes the waves in front of it: the observed wavelength is shorter and the observed frequency is higher.
A source moving away stretches the waves: the observed wavelength is longer and the frequency is lower.
This change is called the Doppler effect.
A source moving away stretches the waves: the observed wavelength is longer and the frequency is lower.
This change is called the Doppler effect.
Sound
An ambulance siren sounds higher pitched as it approaches, then lower pitched as it moves away.
The siren itself does not change. Only the frequency you receive changes.
The siren itself does not change. Only the frequency you receive changes.
Light
The same thing happens with light. A light source moving away has its wavelength stretched towards the red end of the spectrum: red-shift.
Red-shift from distant galaxies shows they are moving away from us.
Red-shift from distant galaxies shows they are moving away from us.
A car sounds its horn as it drives towards you and past you. Describe what you hear.
- Approaching: waves bunched, shorter wavelength, higher frequency.
- Moving away: waves stretched, longer wavelength, lower frequency.
Answer: Higher pitch as it approaches, then lower pitch as it moves away.
If a wave source moves towards an observer, the observed wavelength is shorter and frequency higher. If it moves away, wavelength is longer and frequency lower. This is the Doppler effect; red-shift of light is an example.
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