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, 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: Frequently Asked Questions The speed of light in a vacuum is approximately 299,792,458 meters per second. The speed of light in a vacuum is approximately 299,792,458 meters per second (m/s). When comparing the speed of light in vacuum with the speed of light in air, the speed of light in air is 1.0003 times slower than the speed of light in a vacuum.
