Total internal reflection

Summary

Total internal reflection is the complete reflection of a wave at the boundary with a medium of lower refractive index. It happens when the angle of incidence exceeds a critical angle. Above that angle Snell's law has no real solution for the refracted ray, and no power crosses the boundary. A field still leaks a short distance into the second medium as an evanescent wave. It is the guiding principle of optical fibers and of many prisms.