. Elementary biophysics: selected topics. Biophysics. Figure 44 Figure 45 farther than the first ray by the amount indicated by the heavy line. A detector (such as a photographic film) into which these rays go will yield one of three possible results: (a) If the extra distance is just equal to a whole wave, then the two rays arrive at the detector in the same phase of their motion (that is, crests are with crests, and troughs with troughs). They add up to give an intensity greater than that from a single ray. (b) If the extra distance is just equal to half a wave, then the two rays arrive at t
. Elementary biophysics: selected topics. Biophysics. Figure 44 Figure 45 farther than the first ray by the amount indicated by the heavy line. A detector (such as a photographic film) into which these rays go will yield one of three possible results: (a) If the extra distance is just equal to a whole wave, then the two rays arrive at the detector in the same phase of their motion (that is, crests are with crests, and troughs with troughs). They add up to give an intensity greater than that from a single ray. (b) If the extra distance is just equal to half a wave, then the two rays arrive at the detector exactly out of phase (the crest of ray I would be superposed on the trough from ray II) and the rays cancel, giving zero intensity. (c) If the extra distance is neither of these two alternatives there will also be a cancellation, because in actuality the detector receives many rays, not just two, and except for the two cases presented the rays will be at all phases of the motion with respect to each other and there will be an over-all cancellation. For any spacing between planes, there will be precisely one angle 6 which causes the rays to be exactly one wave apart, so that they reinforce each other; at any other angle general cancellation will occur. Bragg was the first to point out that arrays of atoms in a crystal could be thought of as forming planes in the way shown. This is clearly an approximation, but the exact treatment of the problem by advanced methods yields precisely the same criterion for reinforcement as Bragg obtained from his simple model. The relation for the condition of reinforcement is X = 2d sin 0, where X is the wavelength of the x-rays used and d is the spacing between planes. As will be indicated below, X and 0 are usually measured, which permits the computation of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations m
Size: 1585px × 1576px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversi, booksubjectbiophysics
