{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "## Diffraction grating: $N$-slit interference\n", "\n", "One of the reasons that $N-$slit interference is so important is that in the large $N$ limit we obtain a **grating**.\n", "\n", "Diffraction gratings - based on ruling lines in metal - were invented by **Fraunhofer**. His motivation was to separate different\n", "wavelengths so that he could study **dispersion** in lenses, and then eliminate **chromatic abberation** in order to make better telescopes.\n", "\n", "This Notebook investigate $N$-slit diffraction for multicoloured light. \n", "\n", "We can specify the number of slits $N$, and also the spectrum of the input light.\n", "\n", "We shall focus on the difference between a double slit $N=2$ and many slits $N=10$ - not exactly the grating limit \n", "but large enough to resolve different wavelengths.\n", "\n", "What we find is that for $N=2$ interference pattern is *washed out*. We might say that this must mean that our source is\n", "**incoherent**. \n", "\n", "However we argue in Chapter 8 of Opticsf2f \n", "**coherence** is a property of the measurement as much as the source. \n", "\n", "This code demonstrates that simply by increasing\n", "the number of slits we resolve the fringes for each colour and hence would deduce that our source is coherent.\n", "\n", "If you scroll down to the bottom there is an interactive plot where you can vary $N$ using a slider.\n", "\n", "The Jupyter Notebook is Grating.ipynb see\n", "\n", "https://github.com/opticsf2f/Opticsf2f_CodeBook\n", "\n", "