Knifeedge effect
In electromagnetic wave propagation, the knifeedge effect or edge diffraction is a redirection by diffraction of a portion of the incident radiation that strikes a welldefined obstacle such as a mountain range or the edge of a building.
The knifeedge effect is explained by Huygens–Fresnel principle, which states that a welldefined 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.

Diffraction on a sharp metallic edge

Diffraction on a soft aperture, with a gradient of conductivity over the image width
The knifeedge effect is an outgrowth of the halfplane problem, originally solved by Arnold Sommerfeld using a plane wave spectrum formulation. A generalization of the halfplane 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). Pathak and Kouyoumjian extended the (singular) Keller coefficients via the uniform theory of diffraction (UTD).
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).