---
title: "Cauchy Equation calculator — AperiOptic"
description: "Visible-range index approximation n(λ) = A + B/λ² + C/λ⁴. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/cauchy-equation"
---

# Cauchy Equation calculator

Visible-range index approximation n(λ) = A + B/λ² + C/λ⁴.

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

Formula: `n = A + B/λ² + C/λ⁴`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Wavelength | `lam` | µm | 0.589 |
| A | `A` |  | 1.5046 |
| B (µm²) | `B` |  | 0.0042 |
| C (µm⁴) | `C` |  | 0 |

## Result with the default inputs

- n(λ): 1.516707

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/cauchy-equation?lam=0.589`](https://aperioptic.com/calculators/cauchy-equation?lam=0.589). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Cauchy's equation](https://en.wikipedia.org/wiki/Cauchy%27s_equation)

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

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