27.9

X-Ray Diffraction

The physics of X-ray diffraction. Not all diffraction gratings are commercially made. Nature also creates diffraction gratings, although these gratings do not look like an array of closely spaced slits. Instead, nature’s gratings are the arrays of regularly spaced atoms that exist in crystalline solids. For example, Figure 27-39 shows the structure of a crystal of ordinary salt (NaCl). Typically, the atoms in a crystalline solid are separated by distances of about , so we might expect a crystalline array of atoms to act like a grating with roughly this “slit” spacing for electromagnetic waves of the appropriate wavelength. Assuming that and that in Equation 27.7, then . A value of in this equation gives a wavelength of . This wavelength is much shorter than that of visible light and falls in the X-ray region of the electromagnetic spectrum. (See Figure 24-10.)

Figure 27-39 In this drawing of the crystalline structure of sodium chloride, the small red spheres represent positive sodium ions, and the large blue spheres represent negative chloride ions.

A diffraction pattern does indeed result when X-rays are directed onto a crystalline material, as Figure 27-40a illustrates for a crystal of NaCl. The pattern consists of a complicated arrangement of spots because a crystal has a complex three-dimensional structure. It is from patterns such as this that the spacing between atoms and the nature of the crystal structure can be determined. X-ray diffraction has also been applied with great success toward understanding the structure of biologically important molecules, such as proteins and nucleic acids. One of the most famous results was the discovery in 1953 by Watson and Crick that the structure of the nucleic acid DNA is a double helix. X-ray diffrent patterns such as that in Figure 27-40b played the pivotal role in their research.

Figure 27-40 The X-ray diffraction patterns from (a) crystalline NaCl and (b) DNA. The image of DNA was obtained by Rosalind Franklin in 1953, the year in which Watson and Crick discovered DNA’s structure. (a, Courtesy Edwin Jones, University of South Carolina; b, © Omikron/Photo Researchers)

Test your understanding of the material in Sections 27.5, 27.6, 27.7, 27.8 and 27.9:

· | Diffraction
· | Resolving Power
· | The Diffraction Grating
· | Compact Discs, Digital Video Discs, and the Use of Interference
· | X-Ray Diffraction

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.