Diffraction by single and multiple apertures

The diffraction pattern resulting from a uniformly illuminated circular aperture actually consists of a central bright region, normally known as the Airy disc, which is surrounded by a number of much fainter rings. A circle of zero intensity separates each bright 

Fraunhofer diffraction at a circidar aperture dictates the fundamental limits of performance for circular lenses. It is important to note that the ultimate focal spot size d of a circular lens, produced from plane-wave (with wavelength X) illumination of the lens, is 

This limitation on the resolution of images by diffraction is quantified in terms of the so-caUed Rayleigh criterion the imaging process is said to be diffraction limited when the first diffraction minimum of the image of one source point coincides with the maximum of a neighbouring one. The numerical value for the Rayleigh diffraction angle A0r, is 

This is the best that can be done with the particular sizes of apertures encountered in the overall system the related value for a narrow slit (see just below) is provided for comparison. 

As just stated, the phenomenon of diffraction involves the spreading out of waves at obstacles (here a slit, or multiple slits) that are of similar order of dimension as the wavelength of the wave (for visible light a few micrometres to a few millimetres). 

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