UMBRA
The umbra, penumbra and antumbra are three distinct parts of a shadow, made by any light source after impinging on an opaque object of lesser size. In cases of equal or larger impinging objects, only an umbra and penumbra are generated. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast.
These phenomena are generally observed within solar systems, as the size of the stars within the system are larger than the orbiting satellites, hence these terms are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels of darkness, such as in sunspots.
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