Science
What is The Doppler effect?
The Doppler effect is the change in a wave's frequency when its source moves relative to you. It's why a siren sounds higher as it approaches and lower as it passes — and it lets astronomers tell if stars are moving toward or away from us.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains the doppler effect.
Key things to understand
- 1A moving source changes the frequency you detect.
- 2Approaching waves bunch up (higher pitch); receding waves stretch (lower pitch).
- 3It explains the changing pitch of a passing siren.
- 4Astronomers use it (redshift) to measure how galaxies move.
Frequently asked questions
- What is the Doppler effect?
- The change in a wave's frequency caused by the source moving toward or away from the observer.
- Why does a siren change pitch as it passes?
- Sound waves bunch up as it approaches (higher pitch) and stretch as it leaves (lower pitch).
- How do astronomers use the Doppler effect?
- Light from galaxies shifts red or blue, revealing whether they're moving away from or toward us.