Eq. (3) is the law of refraction for general media, in terms of refractive indices, provided that θ 1 and θ 2 are taken as the dihedral angles; but if the media are isotropic, then n 1 and n 2 become independent of direction while θ 1 and θ 2 may be taken as the angles of incidence and refraction for the rays, and Eq. (4) follows. So, for isotropic media, Eqs. (3) and (4) together describe the behavior in Fig. 5.