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Metre

The metre (m) is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Expressed as a mathematical equation, this definition becomes

1 m = c·t, with t = 1/299 792 458 s.

From this can be derived c = 299 792 458 m/s.

The symbols c, t and s mean speed of light in a vacuum, time and second, respectively.

The metre definition assigns a fixed value to the speed of light c. This fundamental constant can therefore no longer be measured; it has been fixed by definition. From this can be concluded that the unit of length is dependent on the unit of time, the second.

The physical realisation of the unit of length is usually done by laser of a known and highly stable frequency. From the frequency n and the speed of light c, the wavelength of the stabilised laser can be calculated from the known relationship λ = c/n. This wavelength then serves directly as a reference in interferometric length measurements. The primary standard is usually a helium-neon laser, whose optical frequency is stabilised to an atomic transition - in this case, an absorption line of iodine gas. The metre at METAS is based on a group of three iodine-stabilised helium-neon lasers. They are compared on a regular basis, one to each other and against similarly stabilised lasers of foreign metrological institutes; e. g. at the Bureau International des Poids et Mesures (BIPM). For the application of the metre definition in dimensional metrology, refer to the relative technical field (Length).