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speed of light: Quick Facts

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Facts

Also Known As electromagnetic constant • velocity of light • c

Photos and Videos

Two-panel illustration comparing classical and relativistic velocity addition: The top panel shows an archer on a moving train and a stationary archer both shooting arrows, with their speeds adding. The bottom panel shows a person on the train and one on the ground both shining light toward a target, but the speed of light is the same for both observers, illustrating the constancy of the speed of light in special relativity.
Two graphs illustrate effects of relativity as velocity approaches the speed of light: the left graph shows time dilation increasing sharply, while the right graph shows length contraction decreasing, both as functions of velocity.
Figure 6: Fizeau's method for measuring the velocity of light.
Figure 7: Michelson's Mount Wilson experiment, 1927.
relativistic velocity combination
Understand refraction and why the speed of light changes when it travels through glass
relativistic mass
What would traveling at light speed really look like?
See how to break the “cosmic speed limit” of light by using a laser
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Related Topics and References

Topics
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Key People

A.A. Michelson.
A.A. Michelson
German-born American physicist
James Bradley
James Bradley
English astronomer
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Harlow Shapley
American astronomer
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Louis Essen
British physicist
Ole Rømer
Ole Rømer
Danish astronomer
Léon Foucault
Léon Foucault
French physicist
Armand-Hippolyte-Louis Fizeau.
Armand-Hippolyte-Louis Fizeau
French physicist

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Italian physicist Guglielmo Marconi at work in the wireless room of his yacht Electra, c. 1920.
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