Radiant Heat and Light 141 



cold, will be bent. And this is why you see what are 

 called " heat waves " above a stove or rising from a 

 hot beach or sidewalk. Really these are just waves 

 of hot air rising, and they bend the light that comes 

 through them so as to give everything behind them a 

 wavy appearance. 



Stars twinkle for much the same reason. As the 

 starlight comes down through the cold air and then 

 through the warm air it is bent, and the star seems to 

 be to one side of where it really is ; but the air does not 

 stand still, sometimes it bends the light more and 

 sometimes less. So the star seems to move a little back 

 and forth. And this is what we call " twinkling." 

 Really it is the bending of light. 



Application 35. Explain why an unevenness in your eye 

 will keep you from seeing clearly; how glasses can help 

 this; why good mirrors are made from plate glass, which 

 is very smooth, instead of from the cheaper and more un- 

 even window glass ; why fishes in a glass tank appear to be 

 where they are not. 



Inference Exercise 



Explain the following : 



201 . The fire in the open fireplace ventilates a room well by making 



air go up the chimney. 



202. A drop of water glistens in the sun. 



203. Dust goes up to the ceiling and clings there. 



204. When you look at a person under moving water, his face 



seems distorted. 



205. You sit in the sun to dry your hair. 



206. Paste becomes hard and unfit for use when left open to the 



air. 



207. In laundries clothes are partly dried by whirling them in per- 



forated cylinders. 



208. Circus balloons are filled by building a big fire under them. 



