Doppler Effect
Motion of source or observer shifts perceived frequency.
About this topic
The Doppler effect is the change in observed frequency when a source and observer move relative to each other. Approaching sources have compressed wavefronts and sound higher; receding ones sound lower, as with an ambulance siren passing. For a moving source, f′ = f·v/(v ∓ v_s).
The minus sign is for a source approaching and the plus sign for one receding. The source's own pitch doesn't change; it is the spacing of the wavefronts reaching you that does. The same effect occurs with light, where it appears as a redshift for objects moving away and blueshift for those approaching.
Worked example: a siren of 700 Hz on a vehicle moving at 30 m/s toward you, with sound at 343 m/s, is heard at 700 × 343 / (343 − 30) ≈ 767 Hz. After it passes, you hear 700 × 343 / (343 + 30) ≈ 644 Hz. The sudden drop is the familiar neeeeow sound.
Police radar guns use it to measure car speeds, doctors use Doppler ultrasound to measure blood flow, and astronomers use redshift to show galaxies are moving apart, supporting an expanding universe. A common misconception is that the pitch changes gradually as the source approaches; in fact it is steady while approaching and steady, lower, after passing. Another is that the effect depends on distance, which it does not.
Key ideas
- Approaching: higher pitch
- Receding: lower pitch
- Used in radar
Quick check
Use cases: where Doppler Effect helps
- Homework: review the key idea behind Doppler Effect before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Doppler Effect without notes.
- Visual learning: play with the interactive lab to see how Doppler Effect changes when you adjust its inputs.
- Teaching a friend: explain Doppler Effect in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Doppler Effect?
Motion of source or observer shifts perceived frequency.
What will I learn in Doppler Effect?
Approaching: higher pitch. Receding: lower pitch. Used in radar.
Is Doppler Effect part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Doppler Effect lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Doppler Effect?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.