How fast does light travel in one second?

Light travels about 186,000 miles per second in a vacuum. This is the commonly used imperial-unit approximation of the speed of light, whose exact value is 299,792,458 metres per second.

The speed of light is represented by the letter c and is a fundamental constant of physics. In 1983, the metre was officially defined using this constant: one metre is the distance light travels in a vacuum during a precisely specified fraction of a second. Because of that definition, the value in metres per second is exact rather than experimentally rounded.

Light moves slightly more slowly through materials such as air, water, and glass. The difference is especially important in lenses, fibre-optic cables, microscopes, and telescopes. In everyday conversation, however, “186,000 miles per second” almost always means the speed in empty space.

This limit also explains why sunlight takes about 8 minutes 20 seconds to reach Earth. A light-year is not a unit of time; it is the distance light travels in one year—nearly 5.88 trillion miles.