By diffraction usually the phenomenon of deflection of light rays is meant, which occurs when light propagates in the vicinity of the edge of an obstacle, or when light passes through a small aperture in a screen. We have already mentioned (Part 1, Chapter 1) the appearance of fringes around the border of the geometrical shadow of an obstacle, which is caused by diffraction. The well-known Huygens principle, which states that secondary spherical waves are emitted by a wavefront, leads to the supposition that the propagation of secondary waves is responsible for this phenomenon. The existence of secondary waves originating from primary wavefronts enables light waves to penetrate the border line of the geometrical shadow.
KeywordsFocal Plane Geometrical Optic Angular Size Free Spectral Range Zone Plate
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