Airy disc

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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.