Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

What is the Doppler effect?

ScienceLesson on requestFree to read

Quick answer

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.

Key things to understand

  1. A moving source changes the frequency you detect.
  2. Approaching waves bunch up (higher pitch); receding waves stretch (lower pitch).
  3. It explains the changing pitch of a passing siren.
  4. Astronomers use it (redshift) to measure how galaxies move.

People also ask

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.

Part of the guide: Everyday Science: Light, Heat & Matter →