In the case of TIR, the angle θ t does not exist in the usual sense. But we can still interpret (8) for the transmitted (evanescent) wave, by allowing cos θ t to be complex. This becomes necessary when we write cos θ t in terms of sin θ t ‍, ‍ and thence in terms of sin θ i using Snell's law: cos ⁡ ⁡ θ θ t = 1 − − sin 2 ⁡ ⁡ θ θ t = 1 − − ( n 1 / n 2 ) 2 sin 2 ⁡ ⁡ θ θ i . For θ i greater than the critical angle, the value under the square-root symbol is negative, so that