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It is well known that when a monochromatic plane wave of intensity Io is normally incident upon a circular aperture, the intensity at on-axis observation points behind the aperture oscillates between the values 4Io and zero as the distance from the aperture is increased. The reason for this is that the various Fresnel zones in the aperture contribute either constructively or destructively to the amplitude of the field at the observation point in question, causing the amplitude to oscillate between zero and twice the incident field value. For an incident wavelength λ, aperture radius a, and aperture-plane to observation-plane distance z, the number of zones that contribute is given by the Fresnel number, N = a2/λz. The maxima and minima occur at observation points where the Fresnel number is an odd or even integer, respectively.
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