CHAPTER XIII 

 LIGHT 



All day long light waves from the sun arrive in count- 

 less trillions. They make it possible for us to see our 

 way about, to avoid dangers and to do our work. They 

 bring color and beauty everywhere to delight our eyes. 

 Flashing through camera lenses, they even paint pictures 

 for us. They bathe and penetrate our bodies in health- 

 giving streams. They destroy disease germs. They purify 

 water. Beating on the countless green leaves of growing 

 plants, they do necessary work in making food for us to 

 eat. 



So many, indeed, are the benefits of light that for ages 

 man has sought to drive away the darkness of night by 

 artificial light. Even this light, however, is due to the 

 sun, for it is the sun that has stored in the materials man 

 uses the energy that is turned into light. 



To understand the way light acts is to understand many 

 of our daily experiences and how the wonderful mechan- 

 ism of our eyes enables us to use and to enjoy light. 



We know that light is a form of energy. It may be produced 

 from other kinds of energy, such as electricity or intense heat, 

 although the source of both light and heat is the sun. 



How Light Travels. There have been several theories ad- 

 vanced to explain how light comes to us, either from the sun or 

 reflected from some other object. The theory that has been most 

 widely accepted is that it travels as a wave motion in ether, a 

 medium which is supposed to fill all space, even between the 

 molecules of the densest material. Ether is very elastic, and so 

 allows waves to pass through it with very little loss of energy. 

 Heat and light waves pass through ether in straight lines in all 

 directions from the source. Taken together they are called radiant 

 energy. This theory was first proposed by Christian Huygens 

 in 1678, but was not generally accepted until within the last 

 century. Some scientists now question this theory. 



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