NATURE OF LIGHT 



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and oiled paper are examples. Such bodies are called 

 translucent ; they let light through easily, but we can- 

 not see objects through them. 



Opaque bodies are those through which light waves 

 will not pass at all. Wood, granite, iron, and brick are 

 opaque. Since rays will not pass x through an opaque 

 substance, it is clear that those which fall upon it 

 must either be taken into the body and stopped there, 

 or be turned off from its surface. Waves taken in 

 by a body are said to be absorbed; when they are 

 turned off from its surface, they are said to be reflected 



( 



131. Shadows. When an opaque body is placed so 

 as to stop the waves that stream from a luminous source, 



FIG. 81 



it is said to cast a shadow. In other words, a shadow is 

 a space from which light waves are excluded by some 

 opaque mass. Whenever the luminous body is of suffi- 

 cient size, there will be a lighter edge about a shadow, 



