Translations:Snell's law/11/en
Building on this principle, you can consider a wavefront to be planar, as though its source is very far away (this approximation is useful, but is not strictly true in all circumstances). Given Huygens’ Principle and the plane wave approximation, you can derive Snell’s Law for refraction using the following two-layer model, in which the compressional wave velocity V in the upper layer is greater than in the lower layer (the results of the derivation are the same for the case where V in the lower layer is greater than in the upper layer). In this model the incident and refracted plane waves are color-coded in red and green, respectively, and sources of secondary wavefronts on the horizontal refracting boundary are represented by red dots. Secondary wavefronts propagating away from these sources into the lower layer are shown in gray and black.
