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HANDBOOK OF PHOTOGRAPHY 



points like Q, unequally distant fi'om A and B, will be unlighted due to the arrival 

 there of canceling, out-of-phase waves. 



Such reinforcements and cancellations due to single-wavelength light traveling 

 different distances can also be caused by reflecting a beam from surfaces at varying 

 distances. Thus in a system of parallel mirrors a, b, and c, as in Fig. 7, which reflect 

 light of wavelength X, all waves starting from line A A' A" and ending at X will not have 



Fig. 7. — Diffraction reinforcements (right) and cancellations (left) of X rays caused by 



crystal structure. 



covered the same distance. If the angle at which the light strikes a mirror is 6 and d 

 is the distance between planes, then route A'B'X is longer than route ABX by twice 

 the distance times the sine of angle 6. Therefore, although all waves starting from 

 A A' A" are in phase, they will be in phase on arrival at X only when the distance-differ- 

 ence between them {2d sine d) = X, or 2X, or 3X, or . . . nX. A graphic means of 

 illustrating X-ray diffraction bj^ a single crystal is the combination of Figs. 6 and 7 into 



the crystal atom of Fig. 8, a three-dimen- 

 sional affair with many planes at different 

 angles. A monochromatic X-ray beam, S 

 is reflected from the atom sheets a, b, and c 

 (which compose the crystal) on surface X. 

 As in Fig. 7, those sheets so spaced that X 

 rays are reflected at some whole wave- 

 length distance ahead or behind other rays 

 make a diffraction pattern characteristic of 

 the crystalline. The result is as though 

 the energy beam had come from L (see also 

 Fig. 6) and had been diffracted from pin- 

 holes at A, B, and C. 



Various diffraction methods are used 

 widely in industrial processes to reveal facts 

 about the internal structure of material 

 which could be disclosed in no other way. 

 Analysis of crystal structure in all different 

 types of metals and alloys is a particularly 

 fertile field. This application is also bringing to Hght fundamental facts in biology, 

 in medicine, and in research into the structure of the atom and its energy relations. 

 Although widespread apphcation of diffraction methods is relatively new, it is 

 proving a remarkable instrument wherever investigation takes place into the essential 

 processes of nature. 



Radiographic Darkroom. — The principles involved in processing X-ray films are 

 the same as in any other photographic procedure, and the ordinary photographic 

 darkroom may be used for X-ray film work. There are, however, certain differences 



L" 



Fig. 8. — Reflections of diffracted X-ray 

 beam coming from S behave as though 

 they came from L and passed through 

 apertures A, B, and C. 



