What is the approximate speed of electromagnetic waves in a vacuum?

Electromagnetic waves travel in a vacuum at exactly 299,792,458 metres per second.

This value is exact because the metre has been defined since 1983 using the distance light travels in vacuum during a specified fraction of a second. The speed is commonly represented by c. It is a fundamental constant connecting space and time in special relativity and appears throughout electromagnetism.

James Clerk Maxwell’s equations predicted that changing electric and magnetic fields propagate as waves at a speed determined by electrical and magnetic constants. The agreement with measured light speed helped establish that visible light is electromagnetic radiation.

Light travels more slowly through transparent materials because of interactions with matter. The reduced speed is described using refractive index. Radio waves, infrared radiation, visible light, and X-rays all share the same vacuum speed, although their wavelengths and frequencies differ.