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The base unit of length in the SI (metric) system is the metre.

In 1960, the metre was defined as equal to 1,650,763.73 wavelengths of the

orange-red emission line in the electromagnetic spectrum of the krypton-86 atom

in a vacuum.

In 1983, the definition of the metre was changed to be the path travelled by light

in vacuum during a time interval of 1/299,792,458 of a second.

I was taught that radon gas or the wavelength of radon was considered to be one meter one cycle only equals one meter they chose radon because the wavelength does not change with temperature and pressure fluctuations

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13y ago

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