TIR (Total Internal Reflection)

TIR is an optical phenomenon in which a light ray striking the boundary between media at an angle greater than the so-called critical angle is completely reflected, with no transmission into the medium of lower refractive index.

How does total internal reflection work?

The phenomenon occurs when light passes from a medium with a higher refractive index (e.g. glass) into a medium with a lower index (e.g. air) at an angle exceeding the critical angle set by Snell's law. Above that angle, all of the ray's energy is reflected back into the optically denser medium, with no transmission losses.

Applications

TIR is the physical basis of optical waveguides and fibers, TIR lenses in LED luminaires (which concentrate light without reflection losses), prisms in binoculars, and fiber-optic sensors that detect changes in the surrounding refractive index.

Related terms

Optical waveguide

Numerical aperture

BSDF