= Penumbra
A <penumbra> is a region behind an opaque object from which only part of an extended source of <light> is visible. It contrasts with the <umbra>, where the entire source is blocked. During a <solar eclipse>, an observer in the lunar <penumbra> sees a partial <solar eclipse>.<Why Do Eclipses Happen? by NASA>[Shadow geometry: partial visibility of the solar disk in the penumbra]{ref}
An extended lamp, an opaque ball and a screen make the mechanism visible: different points of the lamp cast displaced shadows. Where only some of those shadows overlap, the screen receives reduced <irradiance>. Enlarging the lamp generally broadens the soft transition, whereas an ideal point source gives a sharp geometric boundary. The <penumbra> is a consequence of source extent even in <geometrical optics>; it does not require <diffraction>.
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