Design Nomenclature / Light & Optics
Total internal reflection
Also called TIR, critical angle reflection
Beyond a critical angle, light travelling from a denser medium toward a less dense one stops refracting out and reflects entirely back inside. It is why the underside of a water surface reads as a mirror, why fibre optics work, and why a cut gem returns light instead of leaking it.
Not to be confused with
- RefractionThe bending of light as it crosses between media of differing index, because its phase velocity changes; Snell's law fixes the new angle from the ratio of the two indices. It is what makes a submerged rod look broken and what gives a lens any power at all.
- CausticA concentration of light formed where specular reflection or refraction folds a family of rays onto a diffuse surface — the bright cusp in a coffee cup, the shifting net on a pool floor. The brightness there is not a highlight but genuine energy focused by curvature.