---
title: "Critical Angle (TIR) calculator — AperiOptic"
description: "Total internal reflection threshold between two media. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/critical-angle-tir"
---

# Critical Angle (TIR) calculator

Total internal reflection threshold between two media.

Category: [Geometric Optics](https://aperioptic.com/calculators)

Formula: `sin θc = n₂ / n₁`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| n₁ (dense) | `n1` |  | 1.5 |
| n₂ (rare) | `n2` |  | 1 |

## Result with the default inputs

- Critical angle: 41.8103 °
- TIR exists: yes

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/critical-angle-tir?n1=1.5`](https://aperioptic.com/calculators/critical-angle-tir?n1=1.5). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Total internal reflection](https://en.wikipedia.org/wiki/Total_internal_reflection)

[All optics calculators](https://aperioptic.com/calculators)

---

The interactive page: https://aperioptic.com/calculators/critical-angle-tir (AperiOptic runs in the browser; free, no account). Every page of the site as Markdown: https://aperioptic.com/llms.txt