274 



INDEX OF REFRACTION 



[Ch. IX 



greater than unity. Whenever the refractive index of a substance is 

 given, the absolute index is meant unless otherwise stated. For ex- 

 ample, when the index of refraction of water is said to be 1.33, and 

 of crown glass 1.52, etc., these figures represent the absolute index, 

 and the incident ray is supposed to be in a vacuum. 



§ 448. Relative index of re- 

 fraction. — This is the index of 

 refraction between two contigu- 

 ous media, as for example be- 

 tween glass and diamond, water 

 and glass, etc. It is obtained 

 by dividing the absolute index 

 of refraction of the substance 

 containing the refracted ray, by 

 the absolute index of the sub- 

 stance transmitting the incident 

 ray. For example, the relative 

 index from water to glass is 1.52 

 divided by 1.33. If the light 

 passed from glass to water it 

 would be, 1.33 divided by 1.52. 

 By a study of the figures 

 showing refraction, it will be 

 seen that the greater the re- 

 fraction the less the angle and consequently the less the sine of 

 the angle, and as the refraction between two media is the ratio 



Fig. 156. Refraction of Light in 

 Passing from Glass to Air. 



N Normal to the refracting surface. 

 sin i In this case sin 34 45' or 0.56975 



sin r In this case sin 6o° or 0.86603 



1 



i-52 



If fig. 155 and 156 are compared it will 

 be seen that the ray of light follows exactly 

 the same path in leaving the denser me- 

 dium that it took on entering it. 



of the sines of the angles of incidence and refraction 



sin / 



sin r 



, it will 



be seen that whenever the sine of the angle of refraction is increased 

 by being in a less refractive medium, the index of refraction will show 

 a corresponding decrease and vice versa. That is, the ratio of the sines 

 of the angles of incidence and refraction of any two contiguous substances 

 is inversely as the refractive indices of those substances. The formula is: 



( 



Sine of angle of incident ray\ /Index of refraction of refr»cting mediutrA 

 Sine of angle of refracted ray/ \Index of refraction of incident medium/ 



