A plane light wave with wavelength and intensity Io falls normally on a large glass plate whose

Question:

A plane light wave with wavelength λ and intensity Io falls normally on a large glass plate whose opposite side serves as an opaque screen with a round aperture equal to the first Fresnel zone for the observation point P. In the middle of the aperture there is a round recess equal to half the Fresnel zone. What must the depth h of that recess be for the intensity of light at the point P to be the highest? What is this intensity equal to?