Rayleigh Criterion
(a) This is a graph of intensity of the diffraction pattern for a circular aperture. Note that, similar to a single slit, the central maximum is wider and brighter than those to the sides. (b) Two point objects produce overlapping diffraction patterns. Shown here is the Rayleigh criterion for being just resolvable. The central maximum of one pattern lies on the first minimum of the other.
Source
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"OpenStax College, Limits of Resolution: The Rayleigh Criterion. January 12, 2013."
http://cnx.org/content/m42517/latest/
OpenStax CNX
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