Umbra, penumbra and antumbra

Distinct parts of a shadow

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.

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