The speed of light in vacuum is an universal physical constant denoted by c. The value of the constant is 299792458 metres per second (approximately 3.00×10^{8} m/s). The light speed at which the light rays propagates through transparent materials, such as glass or air, is less than c.

In the year 1975, the speed of light was known to be 299792458 m/s with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second.

In the year 1975, the speed of light was known to be 299792458 m/s with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second.