The ultimate resolving power λ/δλ of the spectrograph's trihedral prism is determined by diffraction of light at the prism edges (as in the case of a slit). When the prism is oriented to the least deviation angle in accordance with Rayleigh's criterion, λ/δλ = b | dn/dλ |, where b is the width of the prism's base (Fig. 5.28), and dn/dλ is the dispersion of its material. Derive this formula.

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