Angular dispersion ($d\theta /d\lambda$)

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.