The speed of light (c) is a primary physical constant for the speed of propagation oflight and electromagnetic waves.
The speed of light (c0) in a vacuum is exactly 299,792.458 km/s. In calculations, 300,000 km/s is used. In denser media such as metals or glass, this speed is reduced to the signal propagation time "c". This is calculated from the shortening factor, the Nominal Velocity of Propagation (NVP), which is multiplied by the speed of light.