FIG. 83. Diagram showing the effect of the convex 

 lens on light-waves. 



226 ELEMENTARY SCIENCE 



trum is useful to remember; you can do so easily by re- 

 calling the initials of the colors in the proper order, namely, 

 roygbiv. 



Lenses are bits of glass very carefully manufactured so 

 as to be perfectly uniform in texture and then very care- 

 fully shaped by 

 grinding them so 

 that their refrac- 

 tion of light is 

 geometrically per- 

 fect. A convex 



lens (see Fig. 83) refracts light so that it converges to a 

 point (the focus) at the side of the lens opposite to the 

 source of light. A concave lens refracts light so that it does 

 not converge, but spreads (see Fig. 84). Now suppose 

 that your eye is at the point marked F in Fig. 83, and that 

 you are trying to look through the lens at an object on the 

 other side of it. First you will so adjust 

 the distance between your eye and the 

 lens that the image of the object comes to 

 a focus upon that part of your eye (the 

 retina) that is sensitive to images, for 

 otherwise the image will appear blurred 

 or you cannot see it at all. Do you not 

 see that the effect produced by thus look- 

 ing at an object through a convex lens is to make it look 

 larger; that is, to magnify it? If you remove the lens and 

 look at the object with the naked eye, it at once looks smaller. 

 A concave lens has the reverse effect. Thus you can see 

 why convex lenses are used in the glasses for near-sighted 

 eyes, while concave lenses are used for far-sighted eyes. 



FIG. 84. Diagram 

 showing the effect of 

 the concave lens on 

 light-waves. 



