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.

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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.