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Doppler effect
Intermediate
Frequency shifts when source or observer moves.
f' = f(v±v_o)/(v∓v_s)If source and observer move relative to each other, the observed frequency changes: f' = f(v ± v_o)/(v ∓ v_s). Approaching raises, receding lowers pitch.
An ambulance siren dropping in pitch as it passes.
Signs: approach means higher frequency.
In more detail
The Doppler effect is the change in observed frequency when a source and observer move relative to each other. Approaching sources sound higher, and receding sources sound lower.
Real-world uses
- Police radar uses the Doppler shift of radio waves to measure speed.
- Weather radar measures wind speeds using the Doppler effect.
- Police radar measures the frequency shift of a radio wave reflected from a car to find its speed.
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Frequently asked questions
Why does a siren drop in pitch as it passes?
As it approaches, the waves are compressed and the pitch rises. After it passes, the waves are stretched and the pitch falls.
Does the Doppler effect apply to light?
Yes. Light from moving stars shifts in colour, which reveals their motion.
Why does a passing ambulance siren drop in pitch?
The sound is compressed as the source approaches and stretched as it recedes, so the observed frequency changes.
Related topics in Waves & Sound
Ultrasound · Infrasound · Sonar and echoes · Doppler with moving source · Doppler with moving observer · Doppler in light