Physics – 3.3 Electromagnetic spectrum | e-Consult

The speed of electromagnetic waves in a vacuum is constant because they are disturbances in the electromagnetic field itself. Electromagnetic waves are self-propagating; a changing electric field creates a changing magnetic field, which in turn creates a changing electric field, and so on. This self-sustaining process occurs regardless of any external medium. Unlike mechanical waves (e.g., sound waves) which require a medium to propagate, electromagnetic waves do not. The speed is determined by fundamental constants (permittivity of free space, ε₀, and permeability of free space, μ₀) and is a property of the electromagnetic field, not a property of a medium.