SI unit - metre
SI base unit: metre (m)
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs.
This definition implies the exact relation c =
The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of