Radiant Heat and Light 125 



pass through empty space with lightning-like quickness. 

 And when this radiant heat strikes things, it is partly 

 absorbed and changed to the usual kind of heat. 



This radiant heat is closely related to light. As a 

 matter of fact, light is only the special kind of ether 

 waves that affect our eyes. Radiant heat is invisible. 

 The ether waves that are visible we call light. In terms 

 of ether waves, the only difference between light and 

 radiant heat is that the ripples in light are shorter. 

 So it is no wonder that when we get a piece of iron hot 

 enough, it begins to give off light ; and we say it is red 

 hot. What happens to the ether is this : As the mole- 

 cules of iron go faster and faster (that is, as the iron gets 

 hotter and hotter), they make the ripples in the ether 

 move more frequently until they get short enough to be 

 light instead of radiant heat. Objects give off radiant 

 heat without showing it at all; the warmth that you 

 feel just below a hot flatiron is mainly radiant heat. 



When anything becomes hot enough to glow, we say 

 it is incandescent. That is why electric lamps are called 

 incandescent lamps. The fine wires called the fila- 

 ment in the lamp get so hot when the electricity 

 flows through them that they glow or become incandes- 

 cent, throwing off light and radiant heat. 



It is the absorbing of the radiant heat by your hand 

 that makes you feel the heat the instant you turn an 

 electric lamp on. Try this experiment : 



Experiment 42. Turn on an incandescent lamp that is 

 cold. Feel it with your hand a second, then turn it off at 

 once. Is the glass hot? (The lamp you use should be an 

 ordinary 25, 40, or 60 watt vacuum lamp.) 



