LENSES. 



15 



There are any number of such related points, and they are 

 called conjugate foci. If the source of light S be moved away 

 from the lens the focus S' will approach the lens until it reaches 

 the principal focus C, which happens when S is at a very great 

 distance. On the other hand, if the rays diverge from a point 



Fig. 11. 



between the principal focus and the lens, they will still 

 diverge after emerging from the lens, but less so than before, 

 and hence will have no real focal point, but will seem to come 

 from a point S' behind the lens, as shown in Fig. 11. This 

 apparent focus is known as a virtual or imaginary focus. 



BICONCAVE LENS. This is just the reverse of the biconvex 

 lens, the spherical surfaces being 



turned inward, and rays of light _^ 



diverged or scattered. It has two - 



radii of curvature and a principal //_'. 



axis. Rays of light parallel to the c^''^ ~-~ 



axis are diverged, but if the rays > =*- 



were prolonged backward they ~~ 



would meet in a point or focus, as 



shown in Fig. 12. The lens has 



never a real focus, but only an imaginary one. The focus C 



corresponding to rays parallel to the axis is called the principal 



imaginary focus. 



If rays diverging from any point on the axis fall upon the 

 lens, after emerging they will be still more divergent, and will 

 seem to emanate from a point between the principal focus and 



Fig. 13. 



the lens. As in the case of a biconvex lens, there are any num- 

 ber of such reciprocally related conjugate foci. Fig. 13 repre- 

 resents two such points, S and S'. 



The focus S' approaches the lens more and more as the source 

 of light S is brought nearer to the lens, and vice versa. 



