Speed of light

The speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum. The speed of light in vacuum is a universal physical constant denoted c, exactly equal to 299792458 m⋅s−1.

It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second.
The value 299,792,458 metres per second is approximately 1 billion kilometres per hour; 700 million miles per hour.
For other approximations of c, valid for various units and size scales see the infobox.

Back to Light Science →

Source: Wikipedia · Image: Wikimedia Commons · Public domain

LUMINOSIT

The science of light, made practical — photometry in plain English, plus the lumen, lux, candela, and energy calculators every lighting decision needs.

Get new guides by emailOccasional emails. No spam. Unsubscribe anytime.

InformationAbout Contact Privacy Policy Terms of Service Disclaimer Affiliate Disclosure Cookie Policy

Every species, image, fact, and figure is drawn from Wikipedia and Wikimedia Commons, used under public-domain and Creative Commons licenses. No content is AI-generated.