A ray of light enters a half-cylinder through the flat side, where it is refracted. It exits perpendicular to the cylindrical surface so that it does not refract. 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 to be 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: .