A text-book of physics, largely experimental. On the Harvard college "Descriptive list of elementary physical experiments." (1891)
Hall, Edwin Herbert, 1855-1938, Bergen, Joseph Y. (Joseph Young), 1851-1917 · New York, H. Holt and Company · leaf 166
shadow, but a section of the entireshadow OSTV. A perfect shadow like this; equally darkat all points, is called an umbra. 120. Penumbra.—Suppose now that the source of light isof appreciable size—a candle-flame, for example—then theopaque object 0 cuts off all illumination from some portionsof the… … FIG. 90. shaded ring, PSSP, around the central area of totalshadow. This ring forms what is known as the penumbra(from two Latin words meaning almost and shadow). On account of the comparatively large size of mostsources of light most shadows are surrounded by a widemargin of penumbra. The student…
Physics
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