UNIT VI 

 HOW WE USE LIGHT 



PREVIEW 



Have you ever awakened from sleep on a dark night to 

 find yourself in utter darkness? How glad you were to 

 have an electric light close at hand! What a sense of 

 helplessness we get when we are without any light ! We 

 certainly enjoy light, for it gives us so much : our ability 

 to see views and pictures, to read, to see wonderful sun- 

 sets, the colors of birds and flowers, or the gray vastness 

 of the desert. It gives us our food, for we know green 

 plants depend upon it. 



Sir Isaac Newton, a great English scientist of the seven- 

 teenth century, believed that light consisted of very small 

 particles given off at great speed by all luminous bodies. 

 He thought that when these particles struck the eye, they 

 produced the sensation of light. This theory was accepted 

 by scientists for over a hundred years, and then discarded 

 in favor of a theory suggested by a Dutch physicist, 

 Huygens. 1 This theory stated that all luminous bodies 

 caused the ether, which was supposed to fill all space, to 

 vibrate. When these vibrations reach the eye, they give 

 the sensation of light. Recently a new discovery by a 

 German and an Indian professor give other ideas. This 

 is the so-called Quantum theory which is based on the 

 belief that light proceeds in little gusts or packets of energy 

 instead of continuous waves. Still another theory goes 

 back to that of Newton and says that light travels in the 



1 Christian Huygens (hl'genz), Dutch astronomer and physicist, 1629- 

 1695. 



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