Velocity of Light in Vacuo

c = 2.997924580x108 ms-1

The speed of light is the same whether the observer measuring it is stationary or in constant motion. This was first proved experimentally by Michelson and Morley from 1881 onwards, in their failure to detect the ether. The ether was the hypothetical medium which pervaded the whole of space and through which electromagnetic radiation was thought to have propagated. In an attempt to measure the difference in light's velocity through this ether, it was shown that all experiments return the same value for the speed of light, regardless of the relative velocity of the experiment which researchers expected to be superimposed upon that of light. This disproved that the ether existed and paved the way for Einstein to develop the theory of special relativity.

It takes roughly 8 minutes and 20 seconds for the light from the sun to reach the Earth.

Historical Notes

  1. 1667 Galileo just deduced that light travels at least ten times faster than sound.
  2. 1675 Ole Roemer came to the conclusion that light must have traveled at 2x108 ms-1.
  3. 1728 James Bradley estimated the speed of light to be around 3.01x108 ms-1.
  4. 1849 Hippolyte Louis Fizeau measured the speed of light at 3.133x108 ms-1.
  5. 1862 Leon Foucault measured the speed of light at 2.99796x108 ms-1.

See also: Einstein′s Formula, Fizeau Method, Group Index, Physical Constants, Tachyon.

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