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.[ref]
Why Do Eclipses Happen? by NASA. Shadow geometry: partial visibility of the solar disk in the penumbra
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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