OPTICAL i'i;nn.i!Tii:s <>F CRYSTALS hi:{ 



pa ing from one medium into anothrr. Sultaiice> differ greatly 

 in thr \v:iy they transmit light ; one class, known as isotropic sub- 

 stances, transmits light equally or \vit h t he same velocity in all direc- 

 tion-. If n point within such a transparent isotropic substance be 

 imagined as the source of light, the light wa.vcs will travel in all 

 directions from this point with the same velocity, and if it were 

 pos-iMe to stop the wave at any instant, say after an inch had been 

 traversed from the point of emission, the extreme wave front would 

 phere of an inch radius. Every point of the surface would l>e 

 one inch from the source of light, and each-ray would have traveled 

 exactly the same distance, whatever the direction. 



Isotropic substances include all gases, most liquids, amorphous 

 solids, as glass, and crystals of the isometric system. Solids, how- 

 ever, when under stress or strain and which under normal conditions 

 are isotropic may show anomalies and apparently belong to the 

 second class, or anisotropic substances ; in which the wave front is 

 not a sphere and the velocity of the transmitted ray will vary with 

 the direction. 



In anisotropic substances the velocity of light will vary with the 

 direction in which the light is traveling, but in parallel directions 

 within the same medium the velocity will be the same. 



The anisotropic class includes the tetragonal, hexagonal, ortho- 

 rhombic, monoclinic, and triclinic systems of crystals, and also iso- 

 metric and isomorphous solids when under stress or strain, as well 

 as those liquid crystals which show double refraction. 



The wave front in anisotropic substances is not a sphere, but 

 its form will depend upon the substance. 



When light strikes the surface of a transparent substance, as 

 glass, it is modified in several ways : (1) some is reflected ; (2) 

 some is refracted ; (3) some is polarized ; (4) some is absorbed or 

 lost as light, as it is transformed to energy of another kind. All 

 four effects will depend upon and will vary with the nature of the 

 surface, the angle of inclination of the ray, and the substance. 



Reflection. If from the head of the arrow, in Fig. 310, a ray of 

 light is traveling in the direction ao, it will strike the surface ss' 

 at the point o. The reflected portion will travel along and in the 

 direction of oa' with a velocity which is unchanged. The two direc- 

 tions ao and a'o will be symmetrical in regard to the normal no 

 at t he point of incidence, and will lie in the same plane. The angle 

 aon is the angle of incidence = i = the angle noa', the angle of re- 

 flection. The ray from b, traveling in the same direction and with 



