LAW OF REFRACTION. 295 



along the line be ; then the relative positions of the source of light 

 d, the refracted ray be, and the point of incidence, b, are deter- 

 mined by the following considerations : 



1. As in the case of reflection, the incident ray, db, the 

 refracted ray, be, and the normal at the point of incidence, fbg, 

 lie in the same plane, the two former being respectively situated 

 on opposite sides of the last. 



2. The incident ray and the refracted ray are so situated with 

 reference to the normal, that the sine * of the angle of incidence, 

 dbf, bears to that of the angle of refraction, ebg, a constant 

 ratio, no matter whether the angle of incidence be acute or 

 obtuse. 



The ratio of the sine of the angle of incidence to that of the 

 angle of refraction is called the refractive index relatively to the 

 two media ; thus, when light passes from air into water the ratio 



4 3 



is nearly -o = l*33; when from air into glass, about -^ = 1'5. 



Conversely, when light passes in the opposite direction, the values 

 are the reciprocals of these, viz., from water into air about -: = '75 



2 



from glass into air about ~ = '67. In general, when light passes 

 o 



from a rarer into a denser medium the refractive index is greater 

 than unity, so that the refracted ray is bent towards the normal; 

 whilst, if the light pass from a denser into a rarer medium, the 

 refractive index is less than 1, and consequently the ray is bent 

 away from the normal. 



One result of this alteration of direction of light, on passing from 

 one medium to another, is that any object under water viewed from 



* By the sine of an angle is meant a trigonometrical value, depending only 

 on the magnitude of the angle, and thus 

 determined : Let the angle be enclosed 

 by the lines, AB, CB, fig. 138. .From 

 some point, D, on one of these lines 

 let fall a perpendicular, DE, on the 

 other line ; then the ratio of the length, 

 DE, to the length, DB, is called the sine 

 of the angle, DBE. Calling this angle 

 a, ^relationship is usually written Fig> m gine of an Ang]e> 



Sino= BT5 



In similar fashion, the ratio of the length BE to the length BD is called 

 the cosine of the angle DBE ; written thus : 



