256 



REFRACTION 



ure 128. The first three types focus parallel rays at some conv 

 mon point F, as in Figure 126. 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 diverge widely after 

 passage through the lens. 



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 

 meet, depends upon the 

 shape of the lens. For ex- 

 ample, a thick lens, such as 

 A (Fig. 129), focuses the 

 rays very near to the lens ; 

 By which is not so thick, 

 focuses the rays at a greater 

 distance from the lens ; and 

 C 9 which is a very thin lens, 

 focuses the rays at a considerable distance from the lens. The 

 distance of the principal focus from 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. 



The position of the principal focus depends not only on the 

 shape of the lens, but also on the refractive power of the mate- 

 rial composing it. A lens made of ice would not deviate the 

 rays of light so much as a lens of similar shape composed of 

 glass. The greater the refractive power of the lens, the greater 

 the bending, and the nearer the principal focus to the lens. 



There are many different kinds of glass, and each kind of 

 glass refracts the light differently. Flint glass contains lead; 

 the lead gives the glass great refractive power, enabling it to 



129. The more curved the lens, the 

 shorter the focal length, and the nearer the 

 focus is to the lens. 



