When a wavefront is refracted from one medium to another, the incident (incoming) and refracted (outgoing) portions of the wavefront meet at a common line on the refracting surface (interface). Let this line, denoted by L, move at velocity u across the surface, where u is measured normal to L ‍ (Fig. 4). Let the incident and refracted wavefronts propagate with normal velocities v 1 and v 2 (respectively), and let them make the dihedral angles θ 1 and θ 2 (respectively) with the interface. From the geometry, ‍ v 1 is the component of u in the direction normal to the incident wave, so that ‍ v 1 = u sin ⁡ ⁡ θ θ 1 . Similarly, ‍ v 2 = u sin ⁡ ⁡ θ θ 2 . Solving each equation for 1/ u and equating the results, we obtain the general law of refraction for waves: