The arcsin function yielding θ c is defined only if ‍ n 2 ≤ n 1 ( v 2 ≥ ≥ v 1 ) . Hence, for isotropic media, total internal reflection cannot occur if the second medium has a higher refractive index (lower normal velocity) than the first. For example, there cannot be TIR for incidence from air to water; rather, the critical angle for incidence from water to air ‍ is the angle of refraction at grazing incidence from air to water (Fig. 6).