Umbra,_penumbra,_and_antumbra
Umbra, penumbra and antumbra
Distinct parts of a shadow
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast.
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These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels, such as in sunspots.