---
title: "Total internal reflection: meaning in gemology"
description: "Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the…"
url: https://digitalgemology.com/learn/glossary/total-internal-reflection
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Optics

# Total internal reflection

Also called TIR

Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.

## In more detail

It is how a pavilion sends light back up through the crown. Total internal reflection loses no light. Facets cut too shallow for the critical angle let light out instead.

## How we measure it

All four of our light measures treat total internal reflection as lossless. Only the simulator also counts partial reflections at facets where light can escape.

## Related terms

- [Critical angle](https://digitalgemology.com/learn/glossary/critical-angle): The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).
- [Refractive index](https://digitalgemology.com/learn/glossary/refractive-index): Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- [Fresnel reflection](https://digitalgemology.com/learn/glossary/fresnel-reflection): Fresnel reflection is the share of light that a surface reflects instead of letting through. The Fresnel equations give that share from the two refractive indices, the angle and the polarization.
- [Leakage](https://digitalgemology.com/learn/glossary/leakage): Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Four light measures, side by side](https://digitalgemology.com/about/how-we-measure/light-measures)
- [What we do not model](https://digitalgemology.com/about/how-we-measure/limits)

[All 328 terms](https://digitalgemology.com/learn/glossary)