In order to compute meaningful Fresnel coefficients, we must assume that the medium is (approximately) linear and homogeneous. If the medium is also isotropic, the four field vectors ‍ E, B, D, H are related by D = ϵ ϵ E B = μ μ H , where ϵ and μ are scalars, known respectively as the (electric) permittivity and the (magnetic) permeability of the medium. For a vacuum, these have the values ϵ 0 and μ 0, respectively. Hence we define the relative permittivity (or dielectric constant) ϵ rel = ϵ / ϵ 0 , and the relative permeability μ rel = μ / μ 0.