---
title: "The Diffraction Grating and Measuring Wavelength"
description: "A grating with many slits throws light into sharp maxima at angles given by d sin theta = n lambda. The sharpness is what makes it a better wavelength measurement than two slits."
canonical: https://lightmysky.com/learn/science/the-diffraction-grating-and-measuring-wavelength-mt_yIjB-uJuFt
source: https://lightmysky.com/learn/science/the-diffraction-grating-and-measuring-wavelength-mt_yIjB-uJuFt.md
retrieved: 2026-09-12
---

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# The Diffraction Grating and Measuring Wavelength

A grating with many slits throws light into sharp maxima at angles given by d sin theta = n lambda. The sharpness is what makes it a better wavelength measurement than two slits.

Subject: Science · Area: Waves, Light & Sound · Ages 17 to 18
Page: https://lightmysky.com/learn/science/the-diffraction-grating-and-measuring-wavelength-mt_yIjB-uJuFt

## Ready when they can

- Converts lines per millimetre into a slit spacing before using the grating equation
- Finds a wavelength from a measured angle for a stated order
- Works out the highest order a given grating and wavelength can produce

## Lesson: Sharp lines measure light

A grating packs hundreds of slits per millimetre. Turn lines per millimetre into a spacing first: d equals one millimetre over N, then work in metres. With 500 lines per millimetre, d is 2 times ten to the minus 6 metres.

**Example.** Positions obey d sin theta equals n lambda for order n. Take d of 2 times ten to the minus 6 metres with light of 600 nm at order 2. Then sin theta equals 2 times 6 times ten to the minus 7 over d, which is 0.6. Many slits make these maxima sharp, which beats two slits for measurement.

Higher orders spread wider and die past 90 degrees, which caps what you can see. Push sin theta to one, solve for n, and round down. With this grating and light, d over lambda is 3.33, so order 3 is the highest you get.

**Recap.** Convert lines to spacing, aim each order with d sin theta, and cap orders where sine passes one.

## Practice

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

## Needs first

- [Coherence and Young's Double-Slit Fringes](https://lightmysky.com/learn/science/coherence-and-youngs-double-slit-fringes-mt_mONvXhPmwq)

## Opens up

- [Intensity in Single-Slit and Double-Slit Diffraction](https://lightmysky.com/learn/science/intensity-in-single-slit-and-double-slit-diffraction-mt_HURhzZdIkM)
- [Single-Slit Diffraction and the Limit of Resolution](https://lightmysky.com/learn/science/single-slit-diffraction-and-the-limit-of-resolution-mt_qiG0fGadBU)