106 Introduction to the Study of Science 



can deflect the heat from the ball to your hand held to one side. Does 

 the mirror feel as warm to the touch as your hand does when receiving 

 the reflected heat or when held in the same place as the mirror? 



Give reasons for the fact that the heat from the hot ball is not 

 transferred by conduction or convection. In what direction do con- 

 vection currents usually move? Is there any possible medium for 

 conduction ? 



In what sort of path do the heat rays move ? Can they be deflected 

 by a mirror or bright smooth metal surface? In what kind of path 

 do they move after being deflected? Can you determine the angle 

 of reflection of such deflected rays ? 



Do the sun's rays follow the same sort of path as that of heat from 

 the hot ball? With a mirror reflect the sun's rays to one side upon 

 your hand. Do you feel the heat thus reflected as strongly as when 

 the hand is directly exposed to the sun? 



Meaning of radiation. Evidently heat energy travels 

 from one place to another by a method that is different from 

 conduction and convection. It moves along straight lines 

 and does not appreciably heat the medium through which 

 it is transmitted. These two facts distinguish sharply this 

 method of transfer from the other two. This third way is 

 generally known as radiation. 



Exercise. Test your knowledge of radiation as a method of 

 transfer of heat energy by the following questions. How could you 

 locate without touching it, a hot but utterly black stove which has no 

 light or other qualities to reveal it to hearing, smell, or sight ? What 

 fact enables you to define its location? Has the path of the heat 

 rays anything to do with your locating the stove? Has the heated 

 part of the air anything to do with it? Suppose the heat is deflected 

 by a mirror or other reflector, can you then locate the stove without 

 approaching it closely? If you know the angle of the reflected heat 

 rays, can you locate the stove? 



Place a thermometer eighteen inches from the center of the flame of 

 an alcohol burner and on the level with it ; and another an equal dis- 

 tance directly above the flame. Which heats up more quickly? 

 Which records the greater temperature? How is heat energy trans- 

 ferred in each case? 



Radiant energy. The facts of observation emphasized by 

 the experiments enable one to understand the theory of radia- 



