Now we suppose that the transmitted wave is evanescent. With the correct sign (+), substituting (9) into (13) gives r s = n cos ⁡ ⁡ θ θ i − − i n 2 sin 2 ⁡ ⁡ θ θ i − − 1 n cos ⁡ ⁡ θ θ i + i n 2 sin 2 ⁡ ⁡ θ θ i − − 1 , where n = n 1 / n 2 ; that is, n is the index of the "internal" medium relative to the "external" one, or the index of the internal medium if the external one is vacuum. So the magnitude of r s is 1, and the argument of r s is − − 2 arctan ⁡ ⁡ n 2 sin 2 ⁡ ⁡ θ θ i − − 1 n cos ⁡ ⁡ θ θ i , which gives a phase advance of ‍