182 



MINERALOGY 



In Fig. 332, the ray R is reflected at the point o in the direction 

 of oR'. Some of the light, however, enters the medium P and is 

 refracted in the direction oRi and at R 3 is reflected in the direction 

 of RIO' and passes out of the medium P, in the direction o'R". 

 The light, or ray, o'R" is made up of two rays, one which has trav- 

 eled the path oRio' in the 

 medium P, and a reflected 

 ray i'o', which will prob- 

 ably be vibrating in a 

 different phase and there- . 

 fore in position to inter- 

 fere. If the refracted ray 

 is retarded 1/2 X due to 

 the differences of paths 

 and velocities of the two 

 rays while the refracted 

 ray is passing through the 

 medium P, then the eye 

 at R" will perceive no 



light, or darkness will result when monochromatic light is being 

 used. 



When the medium P is of uniform thickness the relative paths 

 for each ray will be the same for all points and the surface will be 

 uniformly lighted. If the paths within the medium P can be pro- 

 gressively varied, or if the section P is wedge-shaped, as indicated 

 by the dotted line, 



then the difference 

 of phase between 

 the reflected and 

 refracted rays will 

 increase with the 

 thickness of the 

 wedge, as the in- 

 ternal path fol- 

 lowed by the ray 



3 



FIG. 333. Diagram of the Quartz Wedge. 



R will be much shorter than the path followed by the ray 

 2, and on emerging at o' and o'" will be retarded proportionally. 

 At the edge of the wedge, Fig. 333, where the thickness is zero, 

 there will be no interference or diminution in the illumination. 

 As the wedge thickens, the refracted ray will be retarded more 

 and more behind the reflected ray, with a decrease of the illumi- 



