Problem 5.105

A plane light wave with wavelength \(\lambda=0.57 \mu \mathrm{m}\) falls normally on a surface of a glass \((n=1.60)\) disc which shuts one and a half Fresnel zones for the observation point \(P .\) What must the minimum thickness of that disc be for the intensity of light at the point \(P\) to be the highest? Take into account the interference of light on its passing through the disc.