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Line 1: − The '''speed of light''' in a vacuum (postulated to be constant for all inertial observers by the [[Special Theory of Relativity]]) is 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is now used to define the [[SI]] meter, this is now the value ''by definition''.+ − In physics, it is often represented in equations by the letter '''c,''' as in − :λ = c / f − ([[wavelength]] of an electromagnetic wave in vacuum = the speed of light divided by the wave's frequency). − The speed of light is about one foot per nanosecond. The late computer pioneer Admiral Grace Hopper was fond of keeping foot-long lengths of wire in her purse; she used them as props for her talks, referring to them as "nanoseconds," and using them to explain how the speed of light set limitations on [[computing]] systems: no signal could possibly propagate in any wire faster than the speed of light.<ref>Chiarella, Donald Joseph Gray (2002), ''Life in God's Management Corps,'' [http://books.google.com/books?vid=ISBN0595256430&id=VhM9jDwbywUC&pg=PA14&lpg=PA14&ots=ZXHirs9jQI&dq=grace+hopper+nanosecond&ie=ISO-8859-1&sig=XmbYtV2laxCMIKMLzR1eXHP1Eok p. 14]</ref> − The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]]. − Since the speed of light in a vacuum is observed to be constant, it can be used to define distances as well. The distance that light travels in one year is known as a [[light-year]], which is about 6 million million (6x10<sup>12</sup>) miles. − The speed of light raises questions regarding the age of the universe, which are usually summed up under the term "[[starlight problem]]". − ==Notes and references== − <references/> − [[category: astronomy]]+ − [[category: physics]]+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
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