---
title: "Coherence and Young's Double-Slit Fringes"
description: "Two coherent sources give evenly spaced fringes whose spacing follows w = lambda D over s. A bright fringe sits wherever the path difference is a whole number of wavelengths."
canonical: https://lightmysky.com/learn/science/coherence-and-youngs-double-slit-fringes-mt_mONvXhPmwq
source: https://lightmysky.com/learn/science/coherence-and-youngs-double-slit-fringes-mt_mONvXhPmwq.md
retrieved: 2026-09-12
---

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# Coherence and Young's Double-Slit Fringes

Two coherent sources give evenly spaced fringes whose spacing follows w = lambda D over s. A bright fringe sits wherever the path difference is a whole number of wavelengths.

Subject: Science · Area: Waves, Light & Sound · Ages 16 to 18
Page: https://lightmysky.com/learn/science/coherence-and-youngs-double-slit-fringes-mt_mONvXhPmwq

## Ready when they can

- Explains why the two slits must be lit from one source, and what coherent means here
- Uses the fringe spacing equation to get a wavelength from measurements on a screen
- Predicts what happens to the pattern when the slit separation, the screen distance or the colour changes

## Lesson: Two slits that paint with light

Coherent sources share one frequency with a fixed phase link over time. Young lit both slits from a single slit, since separate bulbs fire at random and never hold still.

**Example.** Light spreads from each slit and overlaps on the screen. Where crest meets crest a bright fringe grows, and where crest meets trough they cancel dark. Bright fringes sit where the path gap is a whole number of wavelengths.

Fringe spacing follows w equals lambda D over s, with D to the screen and s the slit gap. Measure w, D and s, then solve for the wavelength of the laser.

**Tip.** Wider slit gaps squeeze fringes together, while longer screens or redder light spread them out. Red spreads wider than blue, since longer wavelengths spread the pattern more.

**Recap.** One source keeps the slits coherent, and the fringe spacing reveals the wavelength.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Superposition and Stationary Waves](https://lightmysky.com/learn/science/superposition-and-stationary-waves-mt_AIxA2TYGJN)

## Opens up

- [Phasor Addition and the Mathematics of Interference](https://lightmysky.com/learn/science/phasor-addition-and-the-mathematics-of-interference-mt_dxh7HHbIUw)
- [Single-Slit Diffraction and the Limit of Resolution](https://lightmysky.com/learn/science/single-slit-diffraction-and-the-limit-of-resolution-mt_qiG0fGadBU)
- [The Michelson Interferometer](https://lightmysky.com/learn/science/the-michelson-interferometer-mt_Wlb1tZnJBi)
- [The Diffraction Grating and Measuring Wavelength](https://lightmysky.com/learn/science/the-diffraction-grating-and-measuring-wavelength-mt_yIjB-uJuFt)