The Doppler Effect

A train whistle is an everyday example of a Doppler shift Click on image for full size (36K JPEG) The Doppler effect was named after Christian Doppler, who first came up with the idea in 1842. He thought that sound waves would have a higher frequency if the source was moving toward the observer and a lower freqency if the source was moving away from the observer. A commonly used example is a train. When a train is approaching, the whistle has a higher pitch than normal. You can hear the change in pitch as the train passes. The same is true with sirens on police cars and the engines of race cars. Think of sound waves as pulses emitted at regular intervals. Imagine that each time you take a step, you emit a pulse. Each pulse in front of you would be a step closer than if you were standing still and each pulse behind you would be a step further apart. In other words, the frequency of the pulses in front of you is higher than normal and the frequency of the pulses behind you is lower than normal. The Doppler effect doesn't just apply to sound. It works with all types of waves, which includes light. Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light from distant galaxies was shifted toward lower frequencies, or the red end of the spectrum. This is known as a red Doppler shift, or a red-shift. If the galaxies were moving toward Hubble, the light would have been blue-shifted.

How does the Doppler effect help weather forecasts?

Last modified prior to September, 2000 by the Windows Team

The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). © The Regents of the University of Michigan. Windows to the Universe® is a registered trademark of UCAR. All Rights Reserved. Site policies and disclaimer