Radiant Heat and Light 131 



the light that strikes it, so it looks dark, or black, just as a 

 keyhole looks black because it does not reflect any light 

 to your eyes. But without reflection, the book would be 

 entirely invisible. The only kind of print you could read 

 if there were no reflection would be the electric signs 

 made out of incandescent lamps arranged to form letters. 



What the ring around the moon is ; what sunbeams 

 are. The reason you sometimes see a ring around the 

 moon is that some of the moonlight reflects from tiny 

 droplets of water in the air, making them visible. In 

 the same way, the dust in the air of a room becomes 

 visible when the sun shines through it and is reflected 

 by each speck of dust ; we call it a sunbeam. But we 

 are not really looking directly at the sunlight ; we are 

 seeing the part of the sunlight that is reflected by the 

 dust specks. 



Have you ever noticed that when you stand a little 

 to one side of a mirror where you cannot see your own 

 image in it, you can sometimes see that of another per- 

 son clearly, while he cannot see his own image but can 

 see yours? It is easy to understand this by comparing 

 the reflection of the light from your face to his eye and 

 from his face to your eye, to the bouncing of a ball from 

 one person to another. Suppose you and a friend are 

 standing a little way apart on sandy ground where you 

 cannot bounce a ball, but that between you there is a 

 plank. If each of you is standing well away from the 

 plank, neither one of you can possibly bounce the ball 

 on it in such a way that he can catch it himself. Yet 

 you can easily bounce it to your friend and he can bounce 

 it to you. 



