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Quantity |
Length |
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Unit |
Meter |
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Symbol |
m |
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Definition |
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. |
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Comments |
This definition of the metre was adopted in 1983. Note that it fixes the speed of light in vacuum exactly at 299 792 458 m s-1 . |
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| Before this definition was adopted, the metre was defined based on a wavelength of krypton-86 radiation (1960), and the length of an international prototype metre (1889). |
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| The Comité International des Poids et Mesures (CIPM) recommend that the metre can be measured in one of three ways: |
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| i) by means of the path length l travelled by light in vacuum in a time t and using the relation l = ct , where c is the exactly defined speed of light, |
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| ii) by means of the wavelength λ of a light wave of measured frequency f, and the relationship c = fλ, |
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iii) by means of one of the approved radiations, a set of atomic transitions for which the wavelengths and frequencies are well known.
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References |
(A) The International System of Units , Bureau International des Poids et Mesures, 7 th edition, 1998 |
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