A crystalline plate cut parallel to its optical axis is 0.25 mm thick and serves as a

Question:

A crystalline plate cut parallel to its optical axis is 0.25 mm thick and serves as a quarter-wave plate for a wavelength λ = 530 rim. 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 ne – no = 0.0090 at all wavelengths of the visible spectrum.