Physics / Unit 7: Waves and Sound
7.4 The Doppler Effect
Notes
The Doppler effect is the change in the observed frequency of a wave when the source and the observer move toward or away from each other.
- Moving toward each other: waves bunch up, so the observed wavelength is shorter and the frequency (pitch) is higher.
- Moving apart: waves spread out, so the pitch is lower.
- The source's actual frequency does not change; only what the observer hears changes.
- Uses: police radar, weather radar, medical ultrasound, and redshift of light from galaxies moving away from us.
Practice
An ambulance with its siren on drives past you. What do you hear?
Approaching sources are heard at higher frequency; receding ones at lower.
A car horn sounds while the car drives toward a listener. Compared with a listener behind the car, the listener in front hears:
The waves are squeezed together in front of a moving source.
Light from a distant galaxy is shifted toward the red end of the spectrum. What does this tell astronomers?
A redshift (lower frequency, longer wavelength) means the source is receding.
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