Diffraction of X rays by atoms in crystals

Now we consider diffraction in terms of the path differences of waves on scattering. This will lead to an understanding of the effect on the diffraction pattern of the periodicity of the crystal from unit cell to unit cell. There are two methods that have been used to consider this, one by Max von Laue and the other by W. L. Bragg. 

Laue realized in 1912 that the path length differences PD, PD2, PD3 for waves diffracted by atoms separated by one crystal lattice translation must be an integral number of wavelengths for diffraction (i.e., reinforcement) to occur further, he showed that this condition must be true 

FIGURE 3.7. Examples of diffraction patterns from a variety of masks. The mask with the holes in it is to the left and its diffraction pattern is to the right. The diffraction pattern of (a) a single round hole in a mask, (b) two round holes, and (c) two round holes further apart. Note that in (b) and (c) the diffraction pattern is sampled (in lines) with a spacing that is inversely proportional to the distance between 

FIGURE 3.7. (cont d). Caption starts on opposite page. 

FIGURE 3,8. Diagram illustrating that the phase shift on scattering of X rays is 180° 

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