Speed of light in vacuum

speedOfLight, c, is the exact speed at which light propagates in vacuum, expressed in metres per second. It defines the metre and connects space, time, mass, and electromagnetic waves.

Practical use

Use c to convert wavelength and frequency or mass and rest energy. Pair it with planckConstant for photon energy and planckTime for natural units.

Caveats

Light travels more slowly in material media. Do not confuse c with a signal speed in every system; use divide with explicit units and pow for c² in E = mc².