\(\text{Dispersion of a material}=\frac{\text{d}n}{\text{d}\lambda }\) where \(n\) = @R05240@ and \(\lambda \) @W06659@; \(\text{angular dispersion}=\frac{\text{d}\mathit{\varPhi }}{\text{d}\lambda }\) where \(\mathit{\Phi}\) = @A00346@; and \(\text{linear dispersion}=\frac{\text{d}x}{\text{d}\lambda }\) where \(x\) = separation of spectral lines. The reciprocal of the last-named quantity is more frequently used (\(\frac{\text{d}\lambda }{\text{d}x}\)), and is commonly expressed in \(\unicode{x212B}\ \text{mm}^{-1}\).