Jenkins, Francis A. Formerly, Department of Physics, University of California, Berkeley, California.
Watson, William W. Formerly, Yale University, New Haven, Connecticut.
Pau, Stanley College of Optical Sciences, The University of Arizona, Tucson, Arizona.
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A region of darkness caused by the presence of an opaque object interposed between the region and a source of light. A shadow can be totally dark only in the part called the umbra, in which all parts of the source are screened off. With a point source, the entire shadow consists of an umbra, since there can be no region in which only part of the source is eclipsed. If the source has an appreciable extent, however, there exists a transition area surrounding the umbra, called the penumbra, that is illuminated by only part of the source. Depending on what fraction of the source is exposed, the illumination in the penumbra varies from zero at the edge of the full shadow to the maximum where the entire source is exposed. The edge of the umbra is not perfectly sharp, even with an ideal point source, because of the wave character of light. See also: Diffraction
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