what is the speed of light ?
Article ID : 41
Created on 2007-07-20 at 9:39 AM
Author : Rahul Bhanot [firstname.lastname@example.org]
Online URL : http://www.knowledge-gallery.com/question.php?ID=41
The speed of light in a vacuum is an important physical constant denoted by the letter
c for constant or the Latin word celeritas meaning
"swiftness". It is the speed of all electromagnetic radiation in a vacuum, including visible light, and more generally it is the speed of anything with zero rest mass.
In metric units,
299,792,458 metres per second
(1,079,252,848.8 km/h) but 3×108 m/s is commonly used in rough estimates. Note that this speed is a definition, not a measurement. Because the fundamental SI unit of length, the metre, has been defined since
October 21, 1983
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travels in a vacuum in 1/299,792,458 of a second. Thus, any further increase in the precision of the measurement of the speed of light will actually change the length of the metre; the speed of light will remain precisely
m/s. In imperial units, the speed of light is about
186,282.397 miles per second
, that is about one foot per nanosecond.
A line showing the speed of light on a scale model of Earth and the Moon
When passing through a transparent or translucent material medium, like glass or air, light will have a slower speed than in a vacuum; the ratio of
c to the observed phase velocity is called the refractive index of the medium. In general relativity, a gravitational potential can affect the speed of distant light in a vacuum, but locally light in a vacuum will always pass an observer at a rate of
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