What is the constant speed of light in a vacuum?

The constant speed of light in a vacuum is exactly 299,792,458 m/s. Physicists usually denote this universal constant by the letter c.

This value is exact rather than a rounded measurement because the metre has been defined using the distance light travels in a vacuum during a specified fraction of a second. In everyday language, the speed is often rounded to 300,000 km/s or 300 million m/s, but the quiz answer gives the precise SI value.

The speed applies to all electromagnetic radiation in a vacuum, including radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Special relativity states that every inertial observer measures the same vacuum speed, regardless of the motion of the light source or observer.

A frequent mistake is assuming that light always travels at this speed. Light moves more slowly through materials such as glass, water, and air because it interacts with matter. Also, objects with nonzero rest mass can approach c but cannot be accelerated to or beyond it. In roughly one second, light could circle Earth about 7.5 times in vacuum.