Editing Diffraction (section)

=== "Knife edge" <span class="anchor" id="Knife edge"></span> ===
The '''knife-edge effect''' or '''knife-edge diffraction''' is a truncation of a portion of the incident [[radiation]] that strikes a sharp well-defined obstacle, such as a mountain range or the wall of a building.
The knife-edge effect is explained by the [[Huygens–Fresnel principle]], which states that a well-defined obstruction to an electromagnetic wave acts as a secondary source, and creates a new [[wavefront]]. This new wavefront propagates into the geometric shadow area of the obstacle.

Knife-edge diffraction is an outgrowth of the "[[half-plane]] problem", originally solved by [[Arnold Sommerfeld]] using a plane wave spectrum formulation.  A generalization of the half-plane problem is the "wedge problem", solvable as a boundary value problem in cylindrical coordinates. The solution in cylindrical coordinates was then extended to the optical regime by [[Joseph B. Keller]], who introduced the notion of diffraction coefficients through his [[geometrical theory of diffraction]] (GTD). In 1974, Prabhakar Pathak and [[Robert Kouyoumjian]] extended the (singular) Keller coefficients via the [[uniform theory of diffraction]] (UTD).<ref>{{cite journal |last1=Rahmat-Samii |first1=Yahya |title=GTD, UTD, UAT, and STD: A Historical Revisit and Personal Observations |journal=IEEE Antennas and Propagation Magazine |date=June 2013 |volume=55 |issue=3 |pages=29–40 |doi=10.1109/MAP.2013.6586622 |bibcode=2013IAPM...55...29R |author-link=Yahya Rahmat-Samii}}</ref><ref>{{cite journal |last1=Kouyoumjian |first1=R. G. |last2=Pathak |first2=P. H. |title=A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface |journal=[[Proceedings of the IEEE]] |date=November 1974 |volume=62 |issue=11 |pages=1448–1461 |doi=10.1109/PROC.1974.9651}}</ref>

<gallery mode="nolines" widths="300" heights="238">
File:Diffraction sharp edge.gif|Diffraction on a sharp metallic edge
File:Diffraction softest edge.gif|Diffraction on a soft aperture, with a gradient of conductivity over the image width
</gallery>