Snell's Law: optimum pathway analysis.
An analysis is made of the various possible paths traveling from one medium to another. It is  demonstrated that the most efficient possible path for traveling in the shortest possible time is the path determined by Snell's Law. Conversely, when light is refracted as it passes from one medium to another, it is completing the trip in the minimum possible time, less time than if it had passed through the various media in a straight line.
D006801:Humans; D007909:Lenses; D008027:Light; D008962:Models, Theoretical; D009799:Ocular Physiological Phenomena; D012029:Refraction, Ocular; D013997:Time Factors