34 



OUR PHYSICAL WORLD 



that, as the glass is ground down, the surface formed will be 

 a spherical surface. The amount of curvature of the surface 

 will depend upon the length of the rod used. 



If you look through a glass prism at some object such as the 

 window sill, you will demonstrate first that the prism must be 

 so placed as to allow the ray of light coming from the window 

 sill to enter your eye after its refraction. If you will think how the 

 ray of light is refracted (see Fig. 151) on entering and leaving 



an optically denser medium 

 than the air, you will have 

 no difficulty in placing it in 

 approximately the correct 

 position at your first trial. 

 You will note, secondly, that 

 the window sill seems sur- 

 rounded with a halo of 

 color. A convex lens may 

 be thought of as a series of 

 prisms, and you will observe 

 as you look through your 

 large convex lens that the 

 image of an object seen 

 does have a fringe of color about it. This defect of the lens 

 is known as chromatic aberration. 



This defect is remedied in large measure by making the 

 lens of several elements. This power of glass, or similar refractive 

 media, to spread the component color rays of white light so that 

 they form a color band as in the rainbow is known as its dispersive 

 power. Fortunately, the refractive power and the dispersive 

 power of lenses are largely independent of each other, so that 

 one kind of glass may have high refractive power but low dis- 

 persive power, while another sort has low refractive power but 

 high dispersive power. 



Suppose then we were to put behind a plano-convex lens 

 (see Fig. 152) of high refractive but low dispersive power a 



FIG. 151. Diagram showing refraction 

 of light by a prism. The beam entering the 

 prism is not only refracted but also dispersed 

 into its component colors, only the extremes 

 of which are shown, the red (r) and the 

 violet (v). 



