Due to the wave nature of light, light passing through apertures is diffracted, and the diffraction increases with decreasing aperture size.
The resulting diffraction pattern of a uniformly illuminated circular aperture has a bright region in the centre, known as the Airy Disc, which is surrounded by concentric rings. The diameter of this disc is related to the wavelength of the illuminating light and the f-number of the circular aperture. The angle from the center at which the first minima occurs is
where λ is the wavelength of the light and d is the diameter of the aperture. The Rayleigh criterion for barely resolving two objects is that the center of the Airy disc for the first object occurs at the first minima of the Airy disc of the second.
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