CH. IX] 



REFRACTION OF LIGHT 



273 



relationship in this and in all other cases is not the relative size of the 



sin i or 0.86603 



two angles, but the sines of the angles, thus: - - = 1.33. 



smr or 0.65115 



That is, the sine of the angle in air is 1.33 times the sine of the angle 



in water; and this would hold 



true for any other pair of sines, 



so that the law is universal for 



the wave length of light giving 



this index of refraction. 

 The sine and corresponding 



angle are always greater in the 



rarer medium and consequently 



less in the denser medium. It 



follows from this that when the 



ray passes from a rarer to a 



denser medium and the angle 



is made less, the ray must bend 



toward the normal. Conversely 



in passing from a denser to a 



rarer medium where the angle 



is greater, the ray must bend from the normal. This is a general 



law (see fig. 155, 157). 



447. Absolute index of re- 

 fraction. This is the index of 

 refraction obtained when the in- 

 cident ray passes from a vacuum 

 into a given medium. As the 

 index of the vacuum is taken as 

 unity, the absolute index of any 

 substance is always greater than 

 unity. For many purposes, as 

 for the object of this book, air 

 is treated as if it were a vacuum, 

 and its index is called unity, but 

 in reality the index of refraction 

 of air is about 3 ten-thousandths 



FIG. 154. REFRACTION OF LIGHT IN 

 PASSING FROM AIR TO WATER. 



N Normal at the point of refraction. 

 sin i In this example sin 60 or 0.86603 

 sin r In this case sin 40 38' or 0.65115 

 = 1.33, average index of refraction for air 

 and water. 



FIG. 155. REFRACTION OF LIGHT IN 



PASSING FROM AIR TO GLASS. 



N Normal at the point of refraction. 

 sin i In this example sin 60 or 0.86603 

 sin r In this example sin 34 45' or 0.56975 

 = 1.52, average index of refraction for air 

 and glass. 



