LENSES 



255 



Whenever light passes through a prism, it is bent toward the 

 base of the prism, or toward the thick portion of the prism, and 

 emerges from the prism in quite a 

 different direction from that in 

 which it entered (Fig. 125). 

 Hence, when an object is looked 

 at through a prism, it is seen quite 

 out of place. In Figure 125 the 

 candle seems to be at S, while in 

 reality it is at A. 



Lenses. If two prisms are ar- 



FIG. 125. When looked at through 

 the prism, A seems to be at S. 



FIG. 126. Rays of light are con- 

 verged and focused at F. 



ranged as in Figure 126, and two parallel rays of light fall upon 

 the prisms, the beam A will be bent downward and the beam B 



will be bent upward toward the 

 thick portion of the prism, and 

 after passing through the prism 

 the two rays will intersect at some 

 point /", called a focus. 



If two prisms are arranged as in 

 Figure 127, the ray A will be refracted upward toward the 

 thick end, and the ray B will be re- 

 fracted downward toward the thick 

 end. The two rays, on emerging, are 

 widely separated and do not intersect. 

 Lenses ar,e very similar to prisms; 

 indeed, two prisms placed as in Figure 

 126 and rounded off would make a 



FIG. 127. Rays of light are 

 diverged and do not come 

 to any real focus. 





23 456 



FIG. 128. The different types of lenses. 



very good convex lens. 

 A lens is any transparent 

 material, but usually 

 glass, with one or both 

 sides curved. 



The various types of 

 lenses are shown in Fig' 



