When light travelling in a denser medium strikes the surface of a less dense medium (i.e., n 1 > n 2), beyond a particular incidence angle known as the critical angle, all light is reflected and R s = R p = 1. This phenomenon, known as total internal reflection, occurs at incidence angles for which Snell's law predicts that the sine of the angle of refraction would exceed unity (whereas in fact sin θ ≤ 1 for all real θ). For glass with n = 1.5 surrounded by air, the critical angle is approximately 42°.