With the same choice of sign, substituting (9) into (15) gives r p = cos ⁡ ⁡ θ θ i − − i n n 2 sin 2 ⁡ ⁡ θ θ i − − 1 cos ⁡ ⁡ θ θ i + i n n 2 sin 2 ⁡ ⁡ θ θ i − − 1 , whose magnitude is 1, and whose argument is − − 2 arctan ⁡ ⁡ n n 2 sin 2 ⁡ ⁡ θ θ i − − 1 cos ⁡ ⁡ θ θ i , which gives a phase advance of ‍