Kacapatan cahayo, atau bahaso Inggirih: lightspeed, di ruang hampa udaro , umumnyo dilambangkan c,[1] adalah universal konstan fisika pantiang dalam banyak bidang fisika. Nilai eksaknyo didefinisikan sabagai 299792458 meter per detiak (sakitar 300 000 km/s , atau 186000 mi/s ). Hal iko topek kareno, talah sapakek jo internasional, meteran didefinisikan sabagai panjang jalan nan dituju oleh cahayo dalam ruang hampa salamo selang waktu dari 1/299 792 458detik.
↑"The origins of the letter c being used for the speed of light can be traced back to a paper of 1856 by Weber and Kohlrausch [...] Weber apparently meant c to stand for 'constant' in his force law, but there is evidence that physicists such as Lorentz and Einstein were accustomed to a common convention that c could be used as a variable for velocity. This usage can be traced back to the classic Latin texts in which c stood for 'celeritas', meaning 'speed'."