In the above formula for r s ‍, if we put n 2 = n 1 sin ⁡ ⁡ θ θ i / sin ⁡ ⁡ θ θ t (Snell's law) and multiply the numerator and denominator by ⁠ 1 / n 1 ⁠ sin θ t ‍, we obtain r s = − − sin ⁡ ⁡ ( θ θ i − − θ θ t ) sin ⁡ ⁡ ( θ θ i + θ θ t ) .