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Reference.org
Umbra, penumbra and antumbra
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<p> The umbra, penumbra and antumbra are three distinct parts of a <a href="/facts/Shadow/f9zpBgFQ">shadow</a>, created by any light source after impinging on an <a href="/facts/Opacity_(optics)/l90q0sAV">opaque</a> object. Assuming no <a href="/facts/Diffraction/Mk80xXHT">diffraction</a>, for a <a href="/facts/Collimated_beam/lhd2hhPt">collimated beam</a> (such as a point source) of light, only the umbra is cast. </p><p>These names are most often used for the shadows cast by <a href="/facts/Celestial_body/BCHLa0Q2">celestial bodies</a>, though they are sometimes used to describe levels, such as in <a href="/facts/Sunspots/7E8cWSGl">sunspots</a>. </p>