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 across the surface, where is measured normal to "L" (Fig.4). Let the incident and refracted wavefronts propagate with normal velocities formula_2 and formula_3 (respectively), and let them make the dihedral angles "θ"1 and "θ"2 (respectively) with the interface. From the geometry, formula_2 is the component of in the direction normal to the incident wave, so that formula_5. Similarly, formula_6. Solving each equation for and equating the results, we obtain the general law of refraction for waves: