The equation for the speed of an electromagnetic wave is

c = 1 / √ (ε₀*μ₀)

Explanation:

The speed of the electromagnetic wave in free space is given in terms of the permeability and the permittivity of free space by

c = 1 / √ (ε₀*μ₀)

where the permeability of free space (μ₀) is a physical constant used often in electromagnetism and ε₀ is the permittivity of free space (a physical constant).