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Photonics Dictionary

penumbra

A source of light will not cast a distinct shadow of an interfering, opaque object, but will cast a shadow having two parts: 1. the umbra, or the central, totally dark part of the shadow; and 2. the penumbra, or the partially darkened, outer area. When the source is larger than the object and at a suitable distance, the umbra does not exist.
 
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