Before revision: This conclusion remains valid if the secondary wavefronts are reflected or refracted by surfaces of discontinuity in the properties of the medium, provided that the comparison is restricted to the effect paths and the affected portions of the wavefronts.Moreover, using Huygens' construction to determine the law of reflection or refraction is a matter of seeking the path of stationary traversal time between two particular wavefronts; cf.
After revision: This conclusion remains valid if the secondary wavefronts are reflected or refracted by surfaces of discontinuity in the properties of the medium, provided that the comparison is restricted to the affected paths and the affected portions of the wavefronts.Moreover, using Huygens' construction to determine the law of reflection or refraction is a matter of seeking the path of stationary traversal time between two particular wavefronts; cf.