CH. IX] 



IMAGES FORMED BY LENSES 



281 



These spheres are of like radius 

 if the surfaces are similarly 

 curved, and of unlike radius if 

 the surfaces are unlike. While 

 a lens with one plane face may 

 be considered a segment of a 

 single sphere, optically it is 

 better to consider two spheres, 

 the curved surface from a 



FIG. 161-162. A CONCAVE LENS Snow- 



aim 



sphere of finite, and the plane ING TH E PRINCIPAL, VIRTUAL Focus; AND 

 e e e ^ , CONVEX LENS SHOWING THE REAL PRIN- 



face from a sphere of infinite CIPAL Focus ( F F ) 



radius (fig. 167, 3, 6). 



455. Images formed by lenses. As light entering a dense trans- 

 parent body obliquely is bent toward 

 the normal at the point of entrance, it 

 follows that if the lens has convex faces 

 the light rays will be made more con- 

 vergent; if it has concave faces the 

 light rays will be rendered more diverg- 

 ent (fig. 161-162). From the change 

 in the direction of the rays on entering 

 and on leaving a lens it is possible to 

 form images of objects by means of 

 lenses (fig. 163-166). 



456. Forms and principal features 

 of spherical lenses. As shown in fig. 

 167 lenses may be convex on both faces, 

 or convex on one face and plane or con- 

 cave on the other. Lenses may also be 

 concave on both faces or concave on 

 one face and plane or convex on the 

 other. 



If lenses are thick in the middle and 

 thin on the edge, they make the rays 

 of light entering them more converg- 

 ent. On the other hand, if they are 



FIG. 163-164. To SHOW THE 

 FORMATION or A REAL AND or 

 A VIRTUAL IMAGE BY A CONVEX 

 LENS. (COMPARE FIG. 11-12). 



The size of the image depends 

 upon its relative distance from 

 the center of the lens. If it is 

 farther from the center than the 

 object it will be larger than the 

 object, but if nearer it will be 

 smaller (fig. 84). 



