Problem 5.101

A plane monochromatic light wave with intensity $I_{0}$ falls normally on an opaque disc closing the first Fresnel zone for the observation point $P$. What did the intensity of light $I$ at the point $P$ become equal to after (a) half of the disc (along the diameter) was removed; (b) half of the external half of the first Fresnel zone was removed (along the diameter)?