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Doppler Effect and Red Shift 2009

Page history last edited by Caitlin 13 years, 5 months ago

Doppler Effect


          The Doppler Effect, also known as the Doppler Shift, was named after an Austrian physicist and mathematician named Christian Doppler who proposed it in 1842. He, like many others, noticed there is a change in frequency/wavelength of a wave for an observer moving relative to the source of the waves. This is most commonly illustrated with passing vehicles or passing sirens although it can be transposed to other applications such as light wavelengths. As the car or siren approaches the frequency is higher than normal, at the exact moment of passing it is the normal frequency, and as it recedes the noise is lower than the original frequency. One of the big supporting pieces of evidence to proving the Doppler effect was an experiment done by Christoph Hendrik Diederik Buys Ballot in 1845. He gathered some horn players and put them in an open cart attached to a train. He then had the conductor drive the train back and forth along the tracks. As the musicians were being pulled, they played a single note on their horns. Ballot sat next to the track and listened carefully as the train approached and receded. He found that the notes he heard were slightly different than the notes being played and heard by the musicians. Although unusual, Ballot's experiment clearly demonstrated one of the most important wave phenomena known to scientists - the Doppler effect.


      Often the Doppler effect is explained through a series of pictures like the one following.


This is modeling what the waves look like. They stretch out and loose energy, resulting in a red shift

This is modeling what the waves would look like a different way. This image also shows how movement changes how the waves hear/look.

The image farthest to the left would be the observer moving left. The image in the middle would be a stationary observer. And the image farthest to the right would be an observer moving right.


          * Here is a cool animation that may help illustrate the Doppler Effect.  http://www.lon-capa.org/~mmp/applist/doppler/d.htm



          The Doppler effect can be applied to many different wave experiments such as light, sound, water, but for our purposes we will connect it to astronomy/cocmology and light wavelengths. 

                The Doppler effect in astronomy and cosmology uses light wavelengths which result in either a redshift or blueshift. This is because in the visible part of the electromagnetic spectrum the red wavelengths are longer (lower energy) and the blue wavelengths are shorter (higher energy), an increased wavelength is called a redshift even if it is not visible, just as a decreased wavelength is called a blueshift even if it is not visible. This is commonly used to measure the speed that stars and galaxies are approaching or receding from us. This also ties into the cosmic microwave background, which supports the big bang theory. The cosmic microwave background radiation can be gathered by radio telescope which shows a dull glow. This can be explained because there was radiation released during the big bang and it takes years and years and years to get to earth. We then can see the radiation through special scopes, and tell, because of the Doppler effect, how long far away other things are in our solar system and also what they looked like tons of years ago.











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