THE LAW OF SINES 3 



The angle I C makes with the normal N N r , viz. I C N, is called the 

 angle of incidence ; and the angle R C makes with the normal N' N, 

 viz. R C N', is called the angle of refraction. 



Conversely, if a my R C, travelling in water, meet the surface of 

 air A B in the point C, it will not continue in a straight line, but 

 will be bent to the point I farther away from N. Thus, when a 

 ray passes from a rarer to a denser medium it is bent or refracted 

 towards the normal, and when it passes out of a dense medium into 

 a rarer one it is bent or refracted away from the normal. 



Further, if the shaded portion of the figure were glass instead of 

 water, the refracted ray R C would be bent still nearer N', and, 

 conversely, if the ray passed out of glass into air, it would be more 



FIG. 1. The refraction of light. The law of sines. 



bent away from the normal than if it had passed out of water into 

 air. 



The angle of incidence I C N is connected with the angle of re^ 

 fraction RON' (as stated above) by what is known as Snell's Law of 

 Sines. The constant relation between the two sines for two specific 

 media is called the refractive index of the medium, and is usually 

 indicated in problems by the symbol p. 



This law, stated with reference to the figure, would be : 



1- = u = the refractive index of water, 

 sine R C .N 



In I C take any point, P, and from P draw P T perpendicular 

 to NX'. Similarly in RC take any point, F, and draw FH per- 

 pendicular to N N 7 . 



B2 



