Kng-sokkng ê sok-tō͘, te̍k-pia̍t sī kng tī chin-khong lāi (vacuum) ê sok-tok. Kng-sok sī chin tiōng-iàu ê bu̍t-lí tiāⁿ-sò͘ (physical constant), koàn-sì chioh ēng Latin-gí celeritas (khoài-sok) ê thâu-bûn-jī c lâi piáu-sī. Choân-pō͘ ê tiān-chû hok-siā (electromagnetic radiation) pau-hâm khoàⁿ-ū-kng, tī chin-khong lóng sī ēng kng-sok leh cháu.

SI tn̂g-tō͘ ê tan-ūi tō sī tēng-gī chò kng 1 bió cháu ê kī-lî ê 1/299,792,458; iā-chiū-sī kóng kng-sok sī 299,792,458 m/s.

Chham-khó siu-kái