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UMBRA AND PENUMBRA

Hold the narrow end of a key between two stacks of books.
Use a shaded desk lamp to shine a bright light from about 1
foot high directly down onto the top of the key. Note
whether the shadow cast is sharp or fuzzy.

Now make the light source smaller by moving the light further away or by covering the shade of the lamp with a heavy piece of cardboard that has a small hole punched in the center. Shine the light on the key again. You will discover that when the light source is larger than the object upon which it shines, the shadow cast has two parts. The dark, inner part is called the umbra and the lighter, gray part is called the penumbra.

Use a diagram to make an analogy to the earth (the key) and
the sun (the light). The umbra is dark because all light is blocked from it by the object, while the penumbra is lighter because some light rays from the larger light source get past the edges of the object.

Draw the shadows cast if the sun were smaller that the
earth's diameter. Next, add the moon to the diagram, first
in a lunar eclipse, then in a solar eclipse position. Discuss the effects on the eclipses of the shadows cast.

Guess what umbrella is? It is the diminutive of the Latin
"umbra," which means "shadow." How about THAT? There's
more. The "pen" syllable in "penumbra" is from the Latin
"paene," which means "almost." Aren't words marvelous?

 

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