A ray of light enters a half-cylinder through the curved side, perpendicular to the surface so that it does not refract. The ray does refract when leaving the prism. You can vary the index of refraction of the prism from 1 up to 2.5; the index of refraction of the surroundings is assumed equal to 1. You can also vary the angle of incidence from 0° up to 90°. The angle of refraction is determined by Snell's law: . If , then the ray undergoes total internal reflection.