From (4) we obtain the phase velocity ‍ c / n = 1 / μ μ ϵ ϵ . For a vacuum this reduces to ‍ c = 1 / μ μ 0 ϵ ϵ 0 . Dividing the second result by the first gives n = μ μ rel ϵ ϵ rel . For a non-magnetic medium (the usual case), this becomes ⁠ n = ϵ ϵ rel ⁠. (Taking the reciprocal of (5), we find that the intrinsic impedance is Z = μ μ / ϵ ϵ . In a vacuum this takes the value Z 0 = μ μ 0 / ϵ ϵ 0 ≈ ≈ 377 Ω Ω , ‍ known as the impedance of free space. By division, Z / Z 0 = μ μ rel / ϵ ϵ rel . For a non-magnetic medium, this becomes Z = Z 0 / ϵ ϵ rel = Z 0 / n . )