Dispersion of a material \(= \frac{{\rm{d}}n}{{\rm{d}}\lambda}\) where \(n\) = refractive index and \(\lambda \) wavelength; angular dispersion \(= \frac{{\rm{d}}\mathit{\varPhi}}{{\rm{d}}\lambda}\) where \(\mathit{\Phi}\) = angle; and linear dispersion \(= \frac{{\rm{d}}x}{{\rm{d}}\lambda}\) where \(x\) = separation of spectral lines. The reciprocal of the last-named quantity is more frequently used \(\left(\frac{{\rm{d}}\lambda}{{\rm{d}}x}\right)\), and is commonly expressed in \(\pu{Å mm-1}\).