a. The drawing shows a flat glass plate with blackened ring sections. This plate overlapped a parallel beam of monochromatic light with a wavelength λ. It turned out that the blackened rings of the plate coincided with even Fresnel zones for the axial point A. How did the light intensity change at this point?
b. A parallel beam of monochromatic light was blocked by a plate in which the blackened ring sections were replaced by layers of a dielectric that changes the phase of the passing wave by π. How did the light intensity change at point A in this case?