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)?