XV. PHENOMENA OF LIGHT 

 SHADOWS AND ECLIPSES 



1. Let the room be darkened, and a lighted candle so 

 arranged that a well-defined shadow of some opaque body 

 is made to fall on a white wall-surface or other screen. By 

 moving a sheet of paper back and forth between the screen 

 and the body determine how many dimensions a shadow 

 actually has, i.e., whether a shadow is a darkened region 

 (space) or a darkened surface. That which is seen on a 

 screen or wall bears what relation to the real shadow? If 

 there had been no screen across the path of the shadow, what 

 would have been true of its extent outward ? 



2. Note if there is any sharp line of division between the 

 umbra (dark part of the shadow) and the penumbra (the 

 lighter outer portion). Determine if the umbra extends 

 lengthwise throughout the shadow region. 



Support a sheet of paper so that a pinhole through the 

 paper may be jnoved in turn through the umbra, the pe- 

 numbra, and then outside the darkened area. By looking 

 through the pinhole towards the candle flame determine 

 (a) the reason for the existence of any shadow; (b) the 

 cause of the distinction between umbra and penumbra, and 

 why they merge without a sharp line of division. 



3. Make such arrangements that enough direct (or re- 

 flected) sunlight may come into a darkened room to strongly 

 illuminate a small ball that is to represent the earth. 



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