THE SENSES 



8 95 



issues parallel to itself ; in other words, it undergoes no refraction 

 (Fig. 374). 



Refraction and Dispersion by a Prism. The beam of light is bent 

 towards the normal N as it passes across BA and away from the 

 normal N' as it passes across BC (Fig. 375) ; at both surfaces it is 

 bent towards the base of the prism AC. At the same time the light 

 suffers dispersion that is, the rays of shorter wave-length are more 

 refracted than 

 those of greater 

 wave-length. The 

 deviation of any 

 given ray is 

 measured by the 

 angle which the 

 refracted ray 

 makes with its 

 original direction. 

 The amount of 

 dispersion pro- 

 duced by a prism 

 is measured by 

 the difference in 

 the deviation of 

 the extreme rays 

 of the spectrum. 



The dispersion produced by a given substance is proportional to 

 the difference of its refractive indices for the extreme rays. 



Refraction by a Biconvex Lens. A straight line ACB passing 

 through the. centres of curvature of the two surfaces of the lens is 

 called the principal axis. A point C lying on the principal axis 

 between the two centres of curvature, and possessing the property 



FIG. 375- REFRACTION AND DISPERSION BY A PRISM. 



FIG. 376. REFRACTION BY A 

 BICONVEX LENS. 



FIG. 377. FORMATION OF IMAGE BY 

 BICONVEX LENS. 



that rays passing through it do not suffer refraction, is called the 

 optical centre of the lens. Any straight line, DCE, passing through 

 the optical centre is a secondary axis. Rays of light proceeding from 

 a point in the principal axis are focussed in a point on that axis. 

 When the rays proceed from an infinitely distant point in the 

 principal axis i.e., when they are parallel to it they are focussed 

 in F, the principal focus. Similarly, rays parallel to, or proceeding 

 from, a point in a secondary axis are focussed in a point on that axis ; 



