---
title: "Snell's law: meaning in gemology"
description: "Snell's law sets the direction of refracted light: n₁ sin θ₁ = n₂ sin θ₂. The angles are measured from the normal to the surface."
url: https://digitalgemology.com/learn/glossary/snells-law
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Snell's law

Also called law of refraction, refraction

Snell's law sets the direction of refracted light: n₁ sin θ₁ = n₂ sin θ₂. The angles are measured from the normal to the surface.

## In more detail

Light bends toward the normal when it enters a material of higher index, and away from it when it leaves. When no angle satisfies the law, the light cannot leave and reflects totally.

## How we measure it

Our light tracers refract rays by Snell's law. In doubly refractive stones, the simulator solves it for each of the two waves.

## Related terms

- [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.
- [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).
- [Total internal reflection](https://digitalgemology.com/learn/glossary/total-internal-reflection): 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.
- [Ray tracing](https://digitalgemology.com/learn/glossary/ray-tracing): Ray tracing follows straight lines of light through a model of a stone. At each facet it applies refraction and reflection, and it records where the light comes from or where it leaves.

## Learn more

- [Digital Gemology Cut Performance Index](https://digitalgemology.com/learn/cut-performance-index)
- [3D Gemstone Simulator](https://digitalgemology.com/about/how-we-measure/simulator)

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