This result has the form of " Snell's law ", except that we have not yet said that the ratio of velocities is constant, nor identified θ 1 and θ 2 with the angles of incidence and refraction (called θ i and θ t above). However, if we now suppose that the properties of the media are isotropic (independent of direction), two further conclusions follow: first, the two velocities, and hence their ratio, are independent of their directions; and second, the wave-normal directions coincide with the ray directions, so that θ 1 and θ 2 coincide with the angles of incidence and refraction as defined above.