176 



MINERALOGY 



the refracted ray is modified and the vibrations of the modified 

 portion are restricted to one plane. The amount of this modified 

 light will depend upon the angle of incidence, the character of 

 the surface, and the substance. Light in which the vibrations take 

 place in one plane only is termed polarized light, or plane polarized 

 light ; when the vibrations are in circular orbits, circular polarized ; 

 and when they are in elliptical orbits, elliptically polarized. 



Both the reflected and refracted ray are completely polarized 

 when the angle between them is 90, or, as Brewster's law expresses 

 it, tan (angle of polarization) = n (the index of refraction). In 

 case of rock salt, n = 1.544, or the angle of polarization would be 

 57 5' ; when, in Fig. 322, the angle noR = 57 5' in case of rock salt 

 and air, the angle R'oRx would be 90 and both the reflected ray 

 oR' and the reflected ray oRi are completely plane polarized. The 

 vibrations in the reflected ray take place at right angles to the 

 plane of the paper and the ray is said to be polarized in the plane 

 of the paper, parallel to the plane of incidence RoR'. In the re- 

 fracted ray, the vibrations take place parallel to the plane of the 

 paper, and it is said to be polarized in the plane perpendicular to the 

 plane of the paper, and at right angles to the plane of incidence. 

 The two rays after polarization are vibrating in planes at right 



angles. This is the condition in 

 all isotropic substances. 



In anisotropic substances, in 

 case of refracted light, both the 

 ordinary and extraordinary rays 

 are completely polarized, their vi- 

 bration planes are at right angles 

 and rigidly fixed by the molecular 

 arrangement of the crystal. 



In a cleavage piece of calcite, 

 Fig. 323, when the four sides of the 

 rhombic faces are approximately equal, the ordinary ray o upon 

 emerging will be vibrating parallel to aa', its plane of polarization 

 will be parallel to the short diagonal c'c' ; the extraordinary ray e 

 will vibrate parallel to cc', its plane of polarization will be parallel to 

 the long diagonal aa', and, furthermore, it is impossible for light to 

 emerge from the calcite the vibrations of which do not conform to 

 either of these two directions. The two vibration planes and planes 

 of polarization are rigidly fixed by the crystalline structure of the 

 calcite. 



a 



