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BELL SYSTEM TECHNICAL JOURNAL 



lar etch-pits, in bending a light beam. By the latter methods the indi- 

 vidual etch-pits are never seen, nor does their nature need to be known. 

 Nevertheless, the resultant optical effect of hundreds of similar etch pits 

 is as characteristic of structure orientation as the individual pits themselves. 



5.4 Optical Effect of Etch-Pits 



The gross optical effect of hundreds of similar etch-pits results from the 

 fact that each of the pits has minute facets which are similarly inclined to 

 those of all the other pits. Though the pits of Figs. 5.5 and 5.6 may not 

 appear to be formed from groups of flat facets they are generally so regarded. 

 ''Curved-facets" are theoretically considered to be made up of individual 

 flat-facets which are parallel to possible atomic planes (and hence may be 

 given index numbers as in Chap. III). This view is the same as that taken 



Fig. 5.7 — Reflection of light from a single set of similarly oriented etch-pit facets, A, 

 is like that from a single mirror, B. Reflection from all three sets of facets of a Z-cut 

 section will give a three-fold etch-figure on a screen, as in C. 



with regard to natural faces, which are of course produced by essentially 

 opposite effects, i.e., acid corrosion m the case of etch-pits, and growth from 

 solution in the case of natural faces. Actually, many "curved-facets" 

 give optical effects showing no discernible evidence of individual flat facets. 

 However, the question is academic, so far as use of the pits for orientation 

 purposes is concerned, for such facets are still definitety related to the cr\-stal 

 structure. 



Etch-pit facets may be used to reflect a light beam into specific patterns or 

 to refract the beam on transmission through the material into similar (but 

 not identical) patterns. The different basic optical means of using etch-pit 

 facets are shown in Figs. 5.7, 5.8, 5.9. Included in each figure is a diagram 

 of the effects obtained by illuminating an idealized Z-cut section. This 

 idealized section is assumed to have only simple, equilateral, three-sided 



