**Increased Accuracy of c and Redefinition of The Metre**

In the second half of the 20th century much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. In 1972, using the latter method and the 1960 definition of the metre in terms of a particular spectral line of krypton-86, a group at NBS in Boulder, Colorado determined the speed of light in vacuum to be *c* = 299,792,456.2±1.1 m/s. This was 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre. Since similar experiments found comparable results for *c*, the 15th Conférence Générale des Poids et Mesures (CGPM) in 1975 recommended using the value 299,792,458 m/s for the speed of light.

In 1983 the 17th CGPM redefined the metre thus, "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." As a result of this definition, the value of the speed of light in vacuum is exactly 299,792,458 m/s and has become a defined constant in the SI system of units. Improved experimental techniques do not affect the value of the speed of light in SI units, but instead allow for a more precise realization of the definition of the metre.

Read more about this topic: Speed Of Light, History

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