Radiant Heat and Light 



149 



out toward the middle instead of bulging, and so on. 

 But the only lens that most people make much use of 



A 



V 



7 \~7 



A Li 



FIG. 78. Lenses of different kinds. 



(except opticians) is the convex lens that bulges out 

 toward the center. The convex lens makes a clear 

 image and it is the only kind of lens that will do this. 



Why you can set fire to paper with a magnifying glass. 

 A convex lens brings light to a focus, and it also brings 

 radiant heat to a focus. And that is why you can set 

 fire to things by holding a convex lens in the sunlight 

 so that the light and heat are focused on something that 

 will burn. All the sun's radiant heat that strikes the 

 lens is brought practically to one point, and all the light 

 which is absorbed at this point is changed to heat. 

 When so much heat is concentrated at one point, that 

 point becomes hot enough to catch fire. 



Application 36. Explain why there is a lens in a moving- 

 picture machine ; why a convex lens will burn your hand if 

 you hold it between your hand and the sun ; why the front 

 of a good camera is made so that it can be moved closer to 

 the plate or farther away from it, according to the distance 

 of the object you are photographing ; why there is a lens in 

 your eye. 



