CHAPTER XIII 

 LIGHT 



Light and its Properties. We have just learned that 

 sound waves are caused by the vibrations of the material 

 bodies about us, and we are now to learn that light is due 

 to waves produced in a similar way. Light waves are 

 not waves in the air, but in the ether, a medium which 

 fills all space but which possesses few of the properties 

 of ordinary matter. These waves in the ether are pro- 

 duced by the vibrations of the same little electrons which 

 produce the charges of electricity. '. 



While studying sound we learned that the rate of 

 vibration of the shorter and smaller strings was much 

 greater than the rate of vibration of the longer and larger 

 ones. The electron is very much smaller than the small- 

 est string, and its rate of vibration is very much greater. 



Heat and light are very closely associated. Not all 

 hot objects give off light ; but if the heating continues 

 the temperature is finally reached at which light is given 

 off, and we say the body is incandescent. Heat is due 

 to the motion of the molecules. As the temperature in- 

 creases the molecules strike each other harder and finally 

 cause the electrons to vibrate rapidly enough to produce 

 the sensation of light. 



The velocity of light is so great that for many years it 

 was supposed to pass instantly from its source to any dis- 

 tance. In 1676 a Danish astronomer by the name of 

 Roemer determined the speed to be 186,000 miles a 

 second. Other experiments have verified his results. 



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