| Contents for this page | Related topics |  |
|---|---|---|
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Introduction Red shifts and blue shifts Additional questions |
Doppler effect with sound Light and colour 2-D and 3-D wavefronts Wave nature of matter |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) understand what is meant by "red shifts" and "blue shifts" of astronomical objects, and (b) know how one can calculate the velocity of these objects relative to the earth by using the Doppler Effect of light. | ||
The Doppler effect is observed not only with sound waves, but also with electromagnetic waves. However, since these waves (light and radio waves, for example) do not require a medium in which to travel, the frequency difference that are observed are based on the relative velocities of the source and observer. The Doppler effect for electromagnetic waves is used in astronomy, traffic control and satellite tracking.
From Einstein's Theory of Relativity we get that the observed frequency of light, fobserver, (or wavelength, λobserver), is related to the source frequency, fsource, (or wavelength, λsource) by:
where c is the speed of light in a vacuum, and u is the RELATIVE SPEED of the source moving away from the observer. If the source is moving towards the observer, use the ratio c + u/c - u.
The above formula predicts that the observed frequency of visible light will be lower, that is, shifted towards the red region of the spectrum, when the distance between the source and the observer is increasing. This is found in practice, and is called a RED SHIFT (see diagram above). Conversely, if the distance between the source and the observer is decreasing, the frequency of the light will be shifted towards the blue end of the spectrum, resulting in a BLUE SHIFT.
Light from most distant galaxies is red-shifted, indicating that these galaxies are receding from the earth. This is taken as evidence that the universe is expanding.
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