Raytraced pictures are realistic because the process which generates them emulates the real world. The program (here Povray) bounces photons in the constructed scene. The pictures of a glass of water, (derived from Povray code by Jaime Vives Piqueres from 2004) illustrates this. As light travels slower in water than in air, the shortest paths are no longer straight lines. This Snell law can be explained in multi-variable calculus, where it appears as a Lagrange problem. Click on the picture to see it larger (3600 x 2700 pixel). You see then also details of the water surface or the caustics on the table. This relates to notions of curvature which appear in calculus.