PHYSIOLOGICAL OPTICS. 143 



in which it travels is altered and it is bent toward the perpendicular 

 to the surface between the two media (Fig. 39). The angle between 

 the incident ray and the perpendicular is the angle of incidence 

 (i and i'), that between perpendicular and refracted ray, the angle 

 of refraction (r and r'). The extent to which this bending or 

 REFRACTION occurs depends on two things. One is the angle of 

 incidence. The greater this angle is, the more the ray is refracted, 

 provided of course that the medium is the same. The other factor 

 is the nature of the medium; the denser the medium the more it 

 refracts. It is found that for a given medium there is a constant 

 relationship between the direction of the incident ray and that of the 

 refracted one ; the sine of the angle of incidence divided by the sine of 

 the angle of refraction is always the same for the same medium, 

 greater when the medium is dense and less when it is rare. It is 

 usual to express the refracting power of a medium in this way and 

 to call it the REFRACTIVE INDEX, designated by ju. The example in 

 Fig. 39 will make this clear. AC and AI are incident rays, 

 CD and IJ, the rays after refraction. The angles of incidence are 

 i and i', the angles of refraction, r and r'. 



Sine of angle of incidence 



c^ r i r r 7- = M f r wa ter = 1.3; 



Sine of angle of refraction 



BE 



BC BE GH 



that is or (since BC = CD) =1.3. Similarly =1.3. 

 L>r Dr J rv. 



CD 



The ray ALM, being perpendicular to the surface, is not refracted 

 but passes through unchanged in direction. 



If the direction of the light is reversed and if it passes from the 

 dense to the rare medium the direction of refraction is also reversed, 

 the rays being bent away from the perpendicular instead of towards 

 it. 



Refraction at a Convex Surface. It is through a convex re- 

 fracting surface that the light first passes on entering the eye, as it 

 goes from the air into the layer of tears and the cornea. 



The central point on a curved refracting surface is called its 

 PRINCIPAL point (P.P. Fig. 40) ; a line joining this with the centre 

 of curvature (N.P.) is the principal axis of the refracting surface 



