^{2}η_{0} = , μ_{0} = 4π10^{-7} H/m is the permeability of vacuum, and ϵ_{
0} is the permittivity
of vacuum; ϵ_{0}μ_{0}c^{2} = 1, so with an approximate c ≈ 3.00 ⋅ 10^{8} m/s, ϵ_{
0} ≈ (1∕36π)10^{-9} F/m, and
η_{0} ≈ 120π ≈ 377 Ω.