Symbol c. The speed at which electromagnetic radiation travels. The speed of light in a vacuum is 2.997 924 58 × 108 m s−1. When light passes through any material medium its speed is reduced (see refractive index). The speed of light in a vacuum is the highest speed attainable in the universe (see relativity; Cerenkov radiation). It is thought to be a universal constant and is independent of the speed of the observer. Since October 1983 it has formed the basis of the definition of the metre.
Subjects: Astronomy and Astrophysics.