Angular dispersion
= Angular dispersion
{title2=$d\theta/d\lambda$}
The change of an optical propagation angle with <wavelength>. A <diffraction grating> at normal incidence has $d\sin\theta=m\lambda$, so differentiating gives $d\theta/d\lambda=m/(d\cos\theta)$. This converts a small spectral-line separation into <angular separation>, but resolving the two images also requires sufficient aperture and a narrow source image.