REFRACTION 



123 456 



FIG. 71. The different types of lenses. 



joined as in Figure 69, and rounded off, would make a 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 Figure 71. 



The first three types 

 focus parallel rays at 

 some common point F t as 



in Figure 70. Such lenses are called convex or converging 



lenses. The last three types, called concave lenses, scatter 



parallel rays so that they do not come to a focus, but di- 

 verge widely after passage through the lens. 



113. The Shape and Material of a Lens. The main or 



principal focus of a lens, that is, the point at which rays 



parallel to the base line AB 



meet (Fig. 71), depends 



upon the shape of the lens. 



For example, a thick lens, 



such as A (Fig. 72), focuses 



the rays very near to the 



lens; B, which is not so 



thick, focuses the rays at a 



greater distance from the 



lens ; and C, which is a very 



thin lens, focuses the rays 



at a considerable distance 



from the lens. The distance 



Of the principal foCUS from FIG . 72 ._ The ^ re curved the lens, the 



the lens is called the focal 



length of the lens, and from 



the diagrams we see that the more convex the lens, the shorter 



the focal length. 



shorter the focal length, and the nearer the 

 focus is to the lens. 



