Problem 5.182

A crystalline plate cut parallel to its optical axis is $0.25 \mathrm{~mm}$ thick and serves as a quarter-wave plate for a wavelength $\lambda$ $=530$ nm. At what other wavelengths of visible spectrum will it also serve as a quarter-wave plate? The difference of refractive indices for extraordinary and ordinary rays is assumed to be constant and equal to $n_{e}-n_{0}=0.0090$ at all wavelengths of the visible spectrum.