Since the Fresnel equations were developed for optics, they are usually given for non-magnetic materials. Dividing (4) by (5)) yields Y = n c μ μ . For non-magnetic media we can substitute the vacuum permeability μ 0 for μ, so that Y 1 = n 1 c μ μ 0 ; Y 2 = n 2 c μ μ 0 ; that is, the admittances are simply proportional to the corresponding refractive indices. When we make these substitutions in equations (13) to (16) and equations (21) to (26), the factor cμ 0 cancels out. For the amplitude coefficients we obtain: