The speed of light in free space (c), which is the phase velocity of any electromagnetic radiation in free space, is ≅2.9979×108 m/s. The wave impedance of free space (η0)...The speed of light in free space (c), which is the phase velocity of any electromagnetic radiation in free space, is ≅2.9979×108 m/s. The wave impedance of free space (η0) is the ratio of the magnitude of the electric field intensity to that of the magnetic field intensity in free space and is √μ0/ϵ0≅376.7Ω.