= Resolving power of a diffraction grating
{title2=$\mathcal R=\lambda/\delta\lambda=|m|N$}
For $N$ coherently illuminated grating lines used in <diffraction order> $m$, the <Rayleigh criterion> yields the displayed ideal <resolving power of a diffraction grating>. The angular maximum-to-first-zero separation is $\lambda/(ND\cos\theta_m)$, while nearby wavelengths are separated by $|m|\delta\lambda/(D\cos\theta_m)$. The order must exist, carry appreciable <light intensity> and remain free from dominant instrumental broadening. Line count alone therefore does not determine whether a particular instrument resolves a specified doublet.
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