HOW LENSES FORM IMAGES 119 



The position of the principal focus depends not only on the 

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

 material composing the lens. 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 makes the glass dense, and gives it great refractive 

 power, enabling it to bend and scatter light in all directions. 

 Cut glass and toilet articles are made of flint glass because 

 of the brilliant effects caused by its great refractive power, 

 and imitation gems are commonly nothing more than polished 

 flint glass. 



114. How Lenses Form Images. Suppose we place an arrow, 

 A, in front of a convex lens (Fig. 73). The ray AC, parallel 

 to the principal axis, will 

 pass through the lens 

 and emerge as DE. The 

 ray is always bent 

 toward the thick portion 

 of the lens, both at its 



FIG. 73. The image is larger than the object. 

 entrance into the lens By means of a lens, a watchmaker gets an en- 



and its emergence from 

 the lens. 



In Section 105, we saw that two rays determine the posi- 

 tion of any point of our image; hence in order to locate the 

 image of the top of the arrow, we need to consider but one 

 more ray from the top of the object. The most convenient 

 ray to choose would be one passing through O, the optical 

 center of the lens, because such a ray passes through the lens 

 unchanged in direction, as is clear from Figure 74. The point 



