Reflections on Total Internal Reflection

The phenomenon known as total internal reflection (TIR) is ubiquitous. It occurs when electromagnetic radiation that is propagating in one medium encounters a second medium with a lower refractive index at an inclination beyond a critical angle. Energy does not enter the second medium but instead entirely reflects; TIR can be observed anytime that the reflected energy lies in the visible spectrum. (This discussion primarily focuses on visible light, but the principles governing TIR are equally applicable to all wavelengths of electromagnetic radiation and most other wavelike phenomena in two or more dimensions.)