Doppler Effect
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
When a wave source and an observer move toward each other, the observer hears a higher frequency. Moving apart, a lower one. The wave’s speed doesn’t change: the crests just arrive more or less often.
1. Why it happens
2. Who’s moving matters only a little
Moving source: the wavelength really is shorter in front and longer behind.
Moving listener: the wavelength is normal, but the listener runs into crests more often (moving toward) or less often (moving away).
An ambulance’s siren sounds high as it approaches, then drops as it passes. The pitch stays steady while it approaches at a steady speed; it doesn’t keep rising.
Redshift: light from distant galaxies is shifted to longer wavelengths, so they’re moving away from us. Blueshift means moving toward us.
3. Example, then try it
A car approaches you with its horn on, then passes. Compared with the horn’s real frequency, what do you hear?
Approaching: a higher, steady pitch. After it passes: a lower, steady pitch.
The sound reaches you at 340 m/s either way.
A speaker emitting 340 Hz moves at 34 m/s through still air (sound travels at 340 m/s). Find the wavelength in front of it and behind it, and the frequency a person hears in front and behind.
At rest: λ = 340 ÷ 340 = 1.0 m. Crests leave every 1/340 s.
Moving: between crests, the speaker moves 34 ÷ 340 = 0.10 m. So λ in front = 1.0 − 0.10 = 0.90 m; behind = 1.0 + 0.10 = 1.10 m.
Heard in front: f = v/λ = 340 ÷ 0.90 ≈ 378 Hz.
Heard behind: 340 ÷ 1.10 ≈ 309 Hz.
Answer: 0.90 m and 378 Hz in front; 1.10 m and 309 Hz behind.
4. Common mistakes
Spectral lines from a star are shifted toward shorter wavelengths. What does this tell us?
Shorter wavelengths (a blueshift) mean the source is approaching. A redshift means it’s moving away.
Ready to practice?
5 questions on Doppler Effect, each with a hint and an explanation for every wrong answer.