---
title: "Young's Double-Slit Fringe Spacing calculator — AperiOptic"
description: "Fringe period on a screen from two-slit interference. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/youngs-double-slit-fringe-spacing"
---

# Young's Double-Slit Fringe Spacing calculator

Fringe period on a screen from two-slit interference.

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

Formula: `Δy = λ·D / a`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Wavelength λ | `lam` | nm | 633 |
| Slit–screen D | `D` | m | 1 |
| Slit sep a | `a` | µm | 200 |

## Result with the default inputs

- Fringe spacing Δy: 3.165 mm
- Angular spacing: 3.165 mrad

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/youngs-double-slit-fringe-spacing?lam=633`](https://aperioptic.com/calculators/youngs-double-slit-fringe-spacing?lam=633). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Double-slit experiment](https://en.wikipedia.org/wiki/Double-slit_experiment)

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

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