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:<math> c </math> is the speed of light

:<math> c </math> is the speed of light

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 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,'' [https://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 (as a result of Fermat's principle of least time) to 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]].

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise (as a result of Fermat's principle of least time) to 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]].

In 1676{{#tag:ref|Some sources state 1675.<ref name="Moreux"/>|group=note}}, the Danish astronomer [[Ole Rømer]] became the first person who attempted to quantitatively estimate the speed of light. His method was based on observation of the orbit of the satellites of [[Jupiter]] such as [[Io]] and evaluating their [[eclipse]] data as seen from the Earth. The Dutch scientist [[Christiaan Huygens]]  was first who applied the arithmetic onto Rømer’s estimate for the maximum time delay between periodical observations of Io's eclipse by Jupiter at the Earth's nearest and farthest position with respect to Jupiter.<ref>{{cite web |editor=Steven Soter and Neil deGrasse Tyson|title=Cosmic Horizons: Astronomy At The Cutting Edge|publisher=New Press, American Museum of Natural History|year=2000|url=http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html|accessdate=August 4, 2013}}</ref>

In 1676{{#tag:ref|Some sources state 1675.<ref name="Moreux"/>|group=note}}, the Danish astronomer [[Ole Rømer]] became the first person who attempted to quantitatively estimate the speed of light. His method was based on observation of the orbit of the satellites of [[Jupiter]] such as [[Io]] and evaluating their [[eclipse]] data as seen from the Earth. The Dutch scientist [[Christiaan Huygens]]  was first who applied the arithmetic onto Rømer’s estimate for the maximum time delay between periodical observations of Io's eclipse by Jupiter at the Earth's nearest and farthest position with respect to Jupiter.<ref>{{cite web |editor=Steven Soter and Neil deGrasse Tyson|title=Cosmic Horizons: Astronomy At The Cutting Edge|publisher=New Press, American Museum of Natural History|year=2000|url=http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html|accessdate=August 4, 2013}}</ref>

In 1849, the French physicist Fizeau developed method which enabled to measure the speed of light between two points at the Earth by means of using the light source, rotating strobe disc (spinning toothed wheel), and the mirror. He shone a light ray between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back through the same gap between the teeth of the wheel over the round-trip distance of 17&nbsp;km. By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap, and the rotational speed at which the light returned through the ''next'' gap. The experiment showed that the light traveled over the known distance in 1/18000 second. The resulting speed of light through air was obtained by dividing the known distance by time.   

In 1849, the French physicist Fizeau developed method which enabled to measure the speed of light between two points at the Earth by means of using the light source, rotating strobe disc (spinning toothed wheel), and the mirror. He shone a light ray between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back through the same gap between the teeth of the wheel over the round-trip distance of 17&nbsp;km. By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap, and the rotational speed at which the light returned through the ''next'' gap. The experiment showed that the light traveled over the known distance in 1/18000 second. The resulting speed of light through air was obtained by dividing the known distance by time.   

The more precise methods, involving a rapidly rotating mirror, have been used by Americans Michelson and Newcomb.<ref name="Moreux">{{cite book |author=Théophile Moreux|title=Pour comprendre la physique moderne|year=1948|url=http://books.google.com/books?id=CetIPAAACAAJ&dq=Th.Moreux+Pour+comprendre+la+physique+moderne&hl=sk&sa=X&ei=lvL9UcmvKcfY4QTvm4DwBA&redir_esc=y|language=French}}</ref>

The more precise methods, involving a rapidly rotating mirror, have been used by Americans Michelson and Newcomb.<ref name="Moreux">{{cite book |author=Théophile Moreux|title=Pour comprendre la physique moderne|year=1948|url=https://books.google.com/books?id=CetIPAAACAAJ&dq=Th.Moreux+Pour+comprendre+la+physique+moderne&hl=sk&sa=X&ei=lvL9UcmvKcfY4QTvm4DwBA&redir_esc=y|language=French}}</ref>

==Does light travel at the "speed of light"?==

==Does light travel at the "speed of light"?==