Editing Fraunhofer diffraction (section)

===Focal plane of a positive lens as the far field plane===

[[File:Lens and wavefronts rotated.gif|right|thumb|Plane wave focused by a lens.]]In the far field, propagation paths for wavelets from every point on an aperture to a point of observation are approximately parallel, and a positive lens (focusing lens) focuses parallel rays toward the lens to a point on the focal plane (the focus point position on the focal plane depends on the angle of the parallel rays with respect to the optical axis). So, if a positive lens with a sufficiently long focal length (so that differences between electric field orientations for wavelets can be ignored at the focus) is placed after an aperture, then the lens practically makes the Fraunhofer diffraction pattern of the aperture on its focal plane as the parallel rays meet each other at the focus.<ref>{{harvnb|Hecht|2002|p=448}}</ref>