How was the length of a meter redefined in the year 1983?
On October 20, 1983, the meter was officially redefined as the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.
Light travels at 299792.458 km/sec, so the period was chosen as 0.00000000335641 second, or 3.35641 x 10-9 second.
The original definitions (1799 and 1899) depended on a measured standard bar. In 1960, it was redefined as a relationship to the wavelength of light from ions of the element krypton. In 1980, the standard was based on the unusually cohesive wavelength of an iodine-stabilized helium-neon laser.