A plane light wave with wavelength 0.57m falls normally on a surface of a glass (n

Question:

A plane light wave with wavelength λ – 0.57μm 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.