Radiant Heat and Light 169 



is no red in the light to reflect, obviously a thing cannot 

 look red in that light. 



When you look through a piece of colored glass, the 

 case is somewhat different. A piece of blue glass, for 

 instance, acts as a sort of strainer. The coloring matter 

 in it lets the blue light through it, but it holds back 

 (absorbs) the other kinds of light. So if you look 

 through a piece of blue glass you see everything blue; 

 that is, only the blue part of the light from different 

 objects can reach your eyes through this kind of glass. 

 Anything that is transparent and colored acts in a simi- 

 lar way. 



Why the sky is blue. And that is why the sky looks 

 blue. Air holds back all colors of light except blue ; 

 that is, it holds them back a little. A room full of air 

 holds the colors back hardly at all. A few miles of air 

 hold them back more ; mountains in the distance look 

 bluish because only the blue light from them can reach 

 you through the air. The hundred or more miles of 

 air above you hold back a considerable amount of the 

 other colors of light, letting through much more of blue 

 than of any other color. So the sky looks blue; that 

 is, when the air scatters the sunlight above you, it is 

 chiefly the blue parts of the sunlight that it allows to 

 reach your eyes. 



Why bodies of water look green in some places and 

 blue in others. Water acts in a similar way, but it 

 lets the green light through instead of the blue. A 

 little water holds back (absorbs) the other colors so 

 slightly that you cannot notice the effect in a glass of 

 water. But in a swimming tank full of water, or in a 



