By the Snell law of reflection, a light ray incident upon a reflective surface will be reflected at an angle equal to the incident angle. Both angles are measured with respect to the normal to the surface. If a light ray emanates from O in the direction x ∈ S n−1 , and A is a perfectly reflecting surface, then the reflected ray has direction: () x∗ = T (x) = x − 2 x, ν ν, where ν is the outer normal to A at the point where the light ray hits A.