The Doppler Effect
Sound and light change frequency depending on whether the source is moving toward or away from you.
The Doppler effect was described by Austrian physicist Christian Doppler in 1842. When a wave source approaches an observer, the waves are compressed and the frequency increases. As it recedes, the waves are stretched out and the frequency decreases. This effect applies to all waves: sound, light, and radio. For light, these shifts are called redshift and blueshift, respectively, and they are among the most important tools in astronomy.
An ambulance drives past. The siren sounds higher-pitched as it approaches and suddenly drops to a lower pitch once it has passed. The sound itself hasn't changed at the source—it is the distance between you and the source that is changing rapidly.
By measuring how much light from distant galaxies is redshifted, scientists discovered that the universe is expanding.
The effect doesn't originate in the sound source itself, but in the relative motion between the source and the observer. The driver in the ambulance never hears the frequency shift.
Next time you hear a passing siren, listen closely for the shift in pitch at the exact moment the vehicle drives by.
Relative motion causes waves to behave differently for a sender and a receiver.
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