Note: cos ⁡ ⁡ θ θ 1 must be positive, which it will be if n → → is the normal vector that points from the surface toward the side where the light is coming from, the region with index n 1 . If cos ⁡ ⁡ θ θ 1 is negative, then n → → points to the side without the light, so start over with n → → replaced by its negative.