The speed of light in vacuum, often called simply speed of light and commonly denoted c, is a universal physical constant exactly equal to 299,792,458 metres per second (approximately 1 billion kilometres per hour; 700 million miles per hour). speed of light, speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second. The speed of light in vacuum is 3 × 108 m/s. The vacuum is a space where there is no matter or in which the pressure is so low that any particles in the space do not affect any processes being carried on there. Around 1676, Danish astronomer Ole Roemer became the first person to prove that light travels at a finite speed. The speed of light in vacuum, c, forms the basis for the definition of the unit of speed, the metre per second. The fixed numerical value of the speed of light in vacuum, c, is defined exactly:
