Definition Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of of a second". A millimetre, of a metre, is therefore the distance travelled by light in of a second.
DefINITion SInce 1983, tHE meTrE h.AS been Defined As "the lengTh Of the Path tRaVelLed by lIgHt in vacuuM during A TIm,e iNterval of of a sEcOnd". a miLLIMetre, oF a metre is theRefoRE the diStAnce Travelled by lIgHt iN oF a secoNd