A point source of light with wavelength = 0.50m is located at a distance a = 100cm

Question:

A point source of light with wavelength λ = 0.50μm is located at a distance a = 100cm in front of a diaphragm with round aperture of radius r = 1.0 mm. Find the distance b between the diaphragm and the observation point for which the number of Fresnel zones in the aperture equals k = 3.