SPECIFYING QUARTZ CRYSTAL ORIENTATION 239 



the other sheet, which is an ovaloid of revolution, touches the sphere at the 

 two points where the double sheet is pierced by the optic axis. Of the two 

 rays traveling along one line, the one with a velocity corresponding to the 

 sphere is called the ordinary ray; the one with a velocity corresponding 

 to the ovaloid is called the extraordinary ray. For quartz the ovaloid is 

 prolate and lies inside the sphere. For tourmaline the ovaloid is oblate and 

 lies outside the sphere. The small arrows show the direction of vibration. 

 Each of the components is said to be polarized since for each all the vibra- 

 tion is in one direction. 



Since both sheets are surfaces of revolution with the optic axis as the 

 axis of revolution, we can never tell the x axis from the y axis by optical 

 means.* Only the z axis is a unique direction and can be determined op- 

 tically. If this figure is taken to represent the case for blue light there will 

 be a slightly larger but similar figure for red light since, in the crystal, red 

 light travels faster than blue light. 



Some kinds of crystals have velocity plots for which the double sheet 

 surfaces touch at four points. Hence they have two optic axes and are 

 called biaxial. All hexagonal, rhombohedral and tetragonal crystals are 

 uniaxial, all others except the isometric ones are biaxial. Rochelle Salt is 

 biaxial. 



2.7 Polarizers and Analyzers 



In the Nicol prism means were found to eliminate the ordinary ray; the 

 other is transmitted as polarized light. That is, ordinary light of any or 

 all colors upon passing through a Nicol prism emerges as plane polarized 

 light with no change in color. 



Transparent colored media appear colored because they absorb some 

 colors of light more than other colors. In colored crystals the two rays 

 themselves often differ in their color absorption so that the crystal as viewed 

 by means of the ordinary ray seems to be of a different color than as viewed 

 by the extraordinary ray. Quinine iodo-disulfate, or Herapathite, absorbs 

 most visible colors of one ray almost completely; transmits about 60% of the 

 visible colors in the other ray. Hence light emerging from this crystal is 

 almost completely polarized. In the commercial product called "polaroid", 

 myriads of such crystals, microscopic in size, are contained in a celluloid- 

 like sheet and oriented by stretching the sheet. This material now re- 

 places Nicol prisms for all but the most exacting uses. 



If we put two identical polaroid sheets together with their transmission 

 vibration directions parallel as in Fig. 2.20 we can see through them but if 

 their transmission vibration directions are at right angles we cannot see 



* Methods depending on etch pits are excluded. They are optical only in the sense 

 that observing natural faces is optical. 



