For visible wavelengths, the refractive index of the polycarbonate plastic widely used in eyeglasses is given approximately by n(λ) = b + c/ λ², where b = 1.55 and c = 11,500 nm² 
(a) Find an expression for the change in refractive index dn corresponding to a small wavelength change dλ.
(b) Use the results of part (a) and of Problem 78 to determine the variation df in focal length for a +2.25-diopter polycarbonate lens, over a wavelength range of 10.0 nm centered on 589 nm.