Shadows are formed when light cannot pass through an opaque body in its path. Illumination of the area behind the body is cut off. A small (pinpoint) or distant source of light casts a sharp shadow. A near, large, or diffuse light source produces a fuzzy shadow with a central dark area that receives no light (umbra) and a light outer area (penumbra) which receives some light from part of the source. [Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.
R E F E R E N C E S
Penumbra n [NL, fr. L paene almost + umbra shadow -more at Umbrage] [1666] 1a: a space of partial illumination [as in an eclipse] between the perfect shadow on all sides and the full light b: a shaded region surrounding the dark central portion of a sunspot 2: a surrounding or adjoining region in which something exists in a lesser degree: Fringe 3: a body of rights held to be guaranteed by implication in a civil constitution
[Merriam-Webster's Collegiate Dictionary, 10th Edition. Springfield, MA, USA: Merriam-Webster, Inc. 1995.]
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