120 



LIGHT 



FIG. 92 



139. Lenses. A lens is generally a piece of glass hav- 

 ing one or both of its faces curved ;. its use is to refract 



light waves for different 

 purposes. When the faces 

 curve inward the lens is 

 called concave; when they 

 curve outward the lens is 

 convex. Fig. 92 shows both 

 shapes (in dark lines), and 

 it also shows how each is like two prisms taken together. 

 Look at each carefully until it is plain that parallel rays 



would spread apart after /\ 



leaving a concave lens, 

 and would come together 

 after passing through 

 a convex lens (Fig. 93). 



Convex lenses are the more widely used. The general 

 effect of convex lenses is to converge (bring together) 



the rays that pass 

 through them. The 

 point where such rays 

 meet is called tine focus, 

 /(Fig. 93). When the 

 rays entering the lens 

 areparallel they all meet 

 at one point, which is 

 commonly called the 

 principal focus. 



FIG. 94 ^ 



Experiment 94. Using 



a convex lens, hold it so as to converge the sun's rays on a piece 

 of paper (Fig. 94). Move the paper nearer or farther from the 



