Measurement of Length

One of the seven fundamental quantities, length, is measured in the SI unit metre (m). The definition of the metre has itself evolved over time as measurement technology improved. In 1960, the metre was redefined in terms of a specific number of wavelengths of the orange-red light emitted by krypton atoms — a far more precise and reproducible standard than the earlier physical platinum-iridium bar. By 1983, an even more precise definition was adopted: once the speed of light in vacuum could be measured with extreme precision as $c = 299\,792\,458\ \text{m/s}$, the metre was redefined as the length of the path travelled by light in vacuum during a time interval of $1/299\,792\,458$ of a second. This effectively fixes the speed of light as an exact defined constant and derives the metre from it (consistent with the general 2019 SI redefinition strategy described in section 1.2.1). …