412 THE OUTGO OF ENERGY 



showed that (1) the refracted ray lies in the 

 same plane with the incident ray and the per- 

 pendicular, and (2) the sine of the angle of in- 

 cidence stands in an unalterable relation to the 

 sine pf the angle of refraction. 



The sine of the angle of incidence is to the 

 sine of the angle of refraction as the velocity of 

 the light ray in the first medium is to its velocity 

 in the second, or refracting medium. The ratio 

 of the velocity of light in a vacuum to its velocity 

 in any medium is termed the index of refraction, 

 or refractive power of that medium. If the 

 velocity of light in a vacuum be taken as 1, that 

 of light in air at temperature and 760 mm. 

 pressure, will be 0.9997, a difference so slight 

 that the velocity in air is usually taken as the 

 unit. The law of refraction is commonly ex- 

 pressed as follows : Let n represent the index of 

 refraction, a the angle of incidence, and b the 

 angle of refraction ; then 



sin a 

 sin a = n sin 0. or n = r . 



sin b 



As a rule, the physically denser medium is 

 also optically denser. Thus the refractive index 

 for the Frauenhofer line 1 D, on passing from air 



1 White light is composed of rays of different refrangibility ; 

 hence the use in such measurements of pure spectral rays. 



