222 



GENERAL SCIENCE 



Experiment 62. Shadows. Place an opaque cylinder, two or 

 three inches in diameter, in a dark room. Place two lighted can- 

 dles about six inches 

 away from the cylin- 

 der, making the dis- 

 tance between the 

 candles equal to the 

 diameter of the cyl- 

 inder (Figure 194). 

 Is there any space 



behind the cylinder 



FIG. 194. 



that does not receive light from either candle? Why are there two 

 light shadows? Could we have used one large source of light in 

 place of -the two candles? Explain. 



The space that does not receive light from either candle 

 is called the umbra. The region that receives light from 

 one candle only is called the penumbra. 



Figure 195 illustrates a total eclipse of the moon. 

 When the moon passes into the umbra, the shadow of 

 the earth, a total 

 eclipse of the 

 moon occurs. If 

 the moon passes 

 so that it is part 

 in the umbra and 

 part in the penumbra, a partial eclipse occurs. 



Light as Energy. Light does work so quietly that 

 we may not think of it as a form of energy. Light has 

 much to do with our food supply. Plants containing 

 chlorophyll, the green coloring matter of our vegetation, 

 are able to produce starch, sugar, and other complex 

 organic substances, when stimulated by sunlight. The 

 change which takes place is a chemical change, and the 

 energy required is supplied by the sun as light. These 



FIG. 195. 



