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" 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.