Physics > Speed of Light

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All electromagnetic radiation travels through free space at the speed of light and regardless of its colour or brightness, visible light will always travel at this speed through free space.

The speed of light was officially included in the International System of Units on October 21, 1983 and is denoted as the lower case letter 'c'.

The refractive index of a transparent material indicates how much slower light travels through that medium than in a vacuum. This slowing causes refraction.

See our Refractive Indices page for more information.

Speed of light in different units

Metres per second 299,792,458
km per hour 1,079,252,848.8
Miles per hour ≈ 670,616,629.4
Miles per second ≈ 186,282

Approximate length of time for light to travel...

One foot 1.0 nanosecond
One metre 3.3 nanoseconds
One km 3.3 microseconds
One mile 5.4 microseconds
Around Earth's equator 0.13 seconds
From Earth to geostationary orbit and back 0.24 seconds
From Earth to the moon 1.3 seconds
From Earth to the sun 8.3 minutes
To Earth from Alpha Centauri 4.4 years
From edge to edge of the Milky Way 100,000 years