Editing Speed of light (section)

=== Time of flight techniques ===
[[File:Michelson speed of light measurement 1930.jpg|thumb|center|One of the last and most accurate time of flight measurements, Michelson, Pease and Pearson's 1930–1935 experiment used a rotating mirror and a one-mile (1.6&nbsp;km) long vacuum chamber which the light beam traversed 10 times. It achieved accuracy of ±11&nbsp;km/s.|600x600px]]
[[File:Fizeau-int.svg|thumb|Diagram of the [[Fizeau's measurement of the speed of light in air|Fizeau apparatus]]:{{image key|list type=ordered
|Light source
|Beam-splitting semi-transparent mirror
|Toothed wheel-breaker of the light beam
|Remote mirror
|Telescopic tube}}|alt=A light ray passes horizontally through a half-mirror and a rotating cog wheel, is reflected back by a mirror, passes through the cog wheel, and is reflected by the half-mirror into a monocular.]]

A method of measuring the speed of light is to measure the time needed for light to travel to a mirror at a known distance and back. This is the working principle behind experiments by [[Hippolyte Fizeau]] and [[Léon Foucault]].

The [[Fizeau's measurement of the speed of light in air|setup as used by Fizeau]] consists of a beam of light directed at a mirror {{convert|8|km|mi|0}} away. On the way from the source to the mirror, the beam passes through a rotating cogwheel. At a certain rate of rotation, the beam passes through one gap on the way out and another on the way back, but at slightly higher or lower rates, the beam strikes a tooth and does not pass through the wheel. Knowing the distance between the wheel and the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light can be calculated.<ref name=How>
{{Cite web
 |last=Gibbs
 |first=P.
 |year=1997
 |title=How is the speed of light measured?
 |url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html
 |work=Usenet Physics FAQ
 |publisher=University of California, Riverside
 |access-date=13 January 2010
 |url-status=dead
 |archive-url=https://web.archive.org/web/20150821181850/http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html
 |archive-date=21 August 2015
}}</ref>

The [[Foucault's measurements of the speed of light|method of Foucault]] replaces the cogwheel with a rotating mirror. Because the mirror keeps rotating while the light travels to the distant mirror and back, the light is reflected from the rotating mirror at a different angle on its way out than it is on its way back. From this difference in angle, the known speed of rotation and the distance to the distant mirror the speed of light may be calculated.<ref>
{{Cite web
 |last=Fowler |first=M.
 |title=The Speed of Light
 |url=http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html
 |publisher=University of Virginia
 |access-date=21 April 2010
}}</ref> Foucault used this apparatus to measure the speed of light in air versus water, based on a suggestion by [[François Arago]].<ref name="Hughes2012">{{Cite book|last1=Hughes|first1=Stephan|url=https://books.google.com/books?id=iZk5OOf7fVYC|title=Catchers of the Light: The Forgotten Lives of the Men and Women Who First Photographed the Heavens|date=2012|publisher=ArtDeCiel Publishing|isbn=978-1-62050-961-6|pages=210}}</ref>

Today, using [[oscilloscopes]] with time resolutions of less than one nanosecond, the speed of light can be directly measured by timing the delay of a light pulse from a laser or an LED reflected from a mirror. This method is less precise (with errors of the order of 1%) than other modern techniques, but it is sometimes used as a laboratory experiment in college physics classes.<ref>See, for example:
* {{Cite journal
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 |last2=Martin |first2=M.
 |last3=McCartney |first3=H.
 |last4=Wilf |first4=B.
 |year=1968
 |title=Direct determination of the speed of light as a general physics laboratory experiment
 |journal=[[American Journal of Physics]]
 |volume=36 |issue=9 |page=847
 |doi=10.1119/1.1975166
 |bibcode = 1968AmJPh..36..847C
}}
* {{Cite journal
 |last1=Aoki |first1=K. |last2=Mitsui |first2=T.
 |year=2008
 |title=A small tabletop experiment for a direct measurement of the speed of light
 |journal=[[American Journal of Physics]]
 |volume=76 |issue=9 |pages=812–815
 |doi=10.1119/1.2919743
 |arxiv=0705.3996
 |bibcode = 2008AmJPh..76..812A |s2cid=117454437
}}
* {{Cite journal
 |last1=James |first1=M. B. |last2=Ormond |first2=R. B. |last3=Stasch |first3=A. J.
 |year=1999
 |title=Speed of light measurement for the myriad
 |journal=[[American Journal of Physics]]
 |volume=67 |issue=8 |pages=681–714
 |doi=10.1119/1.19352
 |bibcode = 1999AmJPh..67..681J
}}</ref>