Editing Speed of light (section)

=== Interferometry ===
[[File:Interferometer sol.svg|thumb|An interferometric determination of length. Left: [[constructive interference]]; Right: [[destructive interference]].|alt=Schematic of the working of a Michelson interferometer.]]
[[Interferometry]] is another method to find the wavelength of electromagnetic radiation for determining the speed of light.<ref name=Vaughan>
{{Cite book
 |last=Vaughan |first=J. M.
 |year=1989
 |title=The Fabry-Perot interferometer
 |url=https://books.google.com/books?id=mMLuISueDKYC
 |pages=47, 384–391
 |publisher=CRC Press
 |isbn=978-0-85274-138-2
}}</ref> A [[Coherence (physics)|coherent]] beam of light (e.g. from a [[laser]]), with a known frequency (''f''), is split to follow two paths and then recombined. By adjusting the path length while observing the [[interference (wave propagation)|interference pattern]] and carefully measuring the change in path length, the wavelength of the light (''λ'') can be determined. The speed of light is then calculated using the equation&nbsp;''c''&nbsp;=&nbsp;''λf''.

Before the advent of laser technology, coherent [[radiowave|radio]] sources were used for interferometry measurements of the speed of light.<ref name=Froome1858>
{{Cite journal
 |doi=10.1098/rspa.1958.0172
 |title=A New Determination of the Free-Space Velocity of Electromagnetic Waves
 |first=K. D.
 |last=Froome
 |journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences
 |volume=247
 |year=1958
 |pages=109–122
 |issue=1248
 |bibcode = 1958RSPSA.247..109F
 |jstor=100591 |s2cid=121444888
}}</ref> Interferometric determination of wavelength becomes less precise with wavelength and the experiments were thus limited in precision by the long wavelength (~{{cvt|4|mm|in}}) of the radiowaves. The precision can be improved by using light with a shorter wavelength, but then it becomes difficult to directly measure the frequency of the light.<ref name="NIST_pub"/>

One way around this problem is to start with a low frequency signal of which the frequency can be precisely measured, and from this signal progressively synthesize higher frequency signals whose frequency can then be linked to the original signal. A laser can then be locked to the frequency, and its wavelength can be determined using interferometry.<ref name="NIST_pub">
{{Cite book
 |title        = A Century of Excellence in Measurements, Standards, and Technology
 |editor-last  = Lide
 |editor-first = D. R.
 |contribution = Speed of Light from Direct Frequency and Wavelength Measurements
 |last         = Sullivan
 |first        = D. B.
 |year         = 2001
 |pages        = 191–193
 |publisher    = CRC Press
 |isbn         = 978-0-8493-1247-2
 |url          = http://nvl.nist.gov/pub/nistpubs/sp958-lide/191-193.pdf
 |url-status   = dead
 |archive-url  = https://web.archive.org/web/20090813061018/http://nvl.nist.gov/pub/nistpubs/sp958-lide/191-193.pdf
 |archive-date = 13 August 2009
}}</ref> This technique was due to a group at the National Bureau of Standards (which later became the [[National Institute of Standards and Technology]]). They used it in 1972 to measure the speed of light in vacuum with a [[Measurement uncertainty|fractional uncertainty]] of {{val|3.5|e=-9}}.<ref name="NIST_pub"/><ref name="NIST heterodyne">
{{Cite journal
 |last1=Evenson |first1=K. M. |year=1972
 |title=Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser
 |journal=Physical Review Letters
 |volume=29
 |issue=19 |pages=1346–1349
 |doi=10.1103/PhysRevLett.29.1346
 |bibcode=1972PhRvL..29.1346E
 |s2cid=120300510 |display-authors=etal
}}</ref>