Darkness in a region, caused by an obstruction between the source of light and the region.

By extension, the term is applied to a similar condition when any form of radiant energy is cut off by an obstruction, as a radar shadow. The darkest part of a shadow in which light is completely cut off is called the umbra; a lighter part surrounding the umbra, in which the light is only partly cut off, is called the penumbra.

See also: penumbra, Umbra.

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Subjects: Optics Physics