LIGHT 



3422 



LIGHT 



llGHT. In the first chapter of Gene- 

 sis there is told in the simple language of the 

 Scriptures the story of the creation of light: 



And God said, Let there be light : and there was 

 light. 



And God saw the light, that it was good: and 

 God divided the light from the darkness. 



And God called the light Day, and the dark- 

 ness He called Night. And the evening and the 

 morning were the first day. 



When this was written, and for many cen- 

 turies afterward, men knew little about light 

 except that it was "good," and that its princi- 

 pal source was the sun, which Shakespeare in 

 Twelfth Night calls 



That orbed continent, the fire 

 That severs day from night. 



Even such wise men as Plato and Euclid, who 

 lived about three centuries before Christ, 

 thought that light traveled from the eye of 

 the observer to an object, rath'jr than in the 

 reverse direction. The law of reflection (see 

 subhead, Reflection oj Light, below), however, 

 was known both to the ancient Greeks and to 

 the Arabs. 



What Is Light? Through the Middle Ages 

 and far into the modern period men were seek- 

 ing the scientific explanation of the nature of 

 light. Sir Isaac Newton (1642-1727), the dis- 

 coverer of the law of gravitation, advanced the 

 theory that light consists of minute particles 

 of matter emitted in straight lines from a 

 luminous body. Scientists who followed him 

 discovered that many of the known phenomena 

 connected with light disproved this explana- 

 tion, and in the opening years of the nineteenth 

 century the "wave-motion" theory, now gener- 

 ally accepted, was established by an English- 

 man named Thomas Young. To understand 

 this theory we must know something about 

 another belief that scientists accept almost uni- 

 versallythat all space is filled with a sub- 

 stance called ether. 



The ether theory came about in this way.: 

 It was known that energy in the form of light 



and heat is transmitted from the sun and other 

 glowing bodies, and that it can pass through 

 space containing no air (a vacuum) as readily 

 as through air. Clearly there must be some 

 medium by which it is transferred from one 

 place to another, and scientists have assumed 

 that all space not occupied by matter is filled 

 with such a medium. It is believed that this 

 substance, to which the name ether is applied, 

 is very elastic, that it cannot be compressed, 

 and that masses of ordinary matter can pass 

 readily through it. When a body is heated to 

 a certain degree its vibrating molecules com- 

 municate their motion to the ether, and the 

 resulting disturbances are sent through it in 

 the form of waves which travel in every direc- 

 tion. The energy thus transferred is called 

 radiant energy. We may now state the scien- 

 tist's definition of light: It is that portion of 

 radiant energy that is capable of producing the 

 effect of vision. That is, light waves, consist- 

 ing of vibrations in the ether sent out by a 

 luminous (light-giving) body, strike the retina 

 of the eye, cause the optic nerve to vibrate and 

 produce the sensation of sight. In light waves, 

 as in water waves, the vibration is perpendicu- 

 lar to the direction in which the waves travel. 

 Luminous bodies like the sun and stars are 

 natural sources of light; the carbon of an elec- 

 tric lamp, which the electric current makes 

 extremely hot, is an example of an artificial 

 luminous body. Objects not in themselves 

 luminous, such as the pieces of furniture in a 

 room, become visible because of the light that 

 they send to the eye of the observer, but this 

 light is received from some luminous body. 

 Light is itself invisible, though it has the power 

 of dispelling darkness. This is shown in the 

 well-known phenomenon of a sunbeam entering 

 a darkened room. We see its path because the 

 dust particles floating in the air reflect the sun's 

 light. Were the air in the room absolutely 

 free from dust the path of the sunbeam could 

 not be seen. Also, when one sails far up in 

 balloon, where the air is practically clear, it 

 grows dark and the stars can be seen. 



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