We assume that the media are non-magnetic (i.e., μ 1 = μ 2 = μ 0), which is typically a good approximation at optical frequencies (and for transparent media at other frequencies). Then the wave impedances are determined solely by the refractive indices n 1 and n 2 : Z i = Z 0 n i , where Z 0 is the impedance of free space and i = 1, 2. Making this substitution, we obtain equations using the refractive indices: R s = | n 1 cos ⁡ ⁡ θ θ i − − n 2 cos ⁡ ⁡ θ θ t n 1 cos ⁡ ⁡ θ θ i + n 2 cos ⁡ ⁡ θ θ t | 2 = | n 1 cos ⁡ ⁡ θ θ i − − n 2 1 − − ( n 1 n 2 sin ⁡ ⁡ θ θ i ) 2 n 1 cos ⁡ ⁡ θ θ i + n 2 1 − − ( n 1 n 2 sin ⁡ ⁡ θ θ i ) 2 | 2 , R p = | n 1 cos ⁡ ⁡ θ θ t − − n 2 cos ⁡ ⁡ θ θ i n 1 cos ⁡ ⁡ θ θ t + n 2 cos ⁡ ⁡ θ θ i | 2 = | n 1 1 − − ( n 1 n 2 sin ⁡ ⁡ θ θ i ) 2 − − n 2 cos ⁡ ⁡ θ θ i n 1 1 − − ( n 1 n 2 sin ⁡ ⁡ θ θ i ) 2 + n 2 cos ⁡ ⁡ θ θ i | 2 .