OVERVIEW

It was "as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you." Thus Ernest Rutherford described his astonishment at the result of his undergraduate student's experiment in 1911. The experiment was a simple one: expose thin foils of gold to ? particles and watch for appreciable scattering. The then-current model of matter was that it was a "bunch of electrons and some nondescript smeared out jelly of positive charge." The ? particle weighs some 8000 times more than an electron, yet was unexpectedly deflected by the jelly. This observation led to a radical change in our conception of matter, resulting in the Rutherford atom, a tight nucleus surrounded by orbiting electrons.

The scattering of particles and waves such as X rays provides much information on the inner structure of matter. Diffraction patterns are stunningly beautiful in their regularity, reflecting in a highly indirect fashion the ordering of atoms of the crystalline solids (such as TaSe ₂, shown in Figure 1.1) exposed to the beam. These patterns can be analyzed to understand this geometric structure and other properties. Indeed, much of what is known about the structure of solids is due to analysis of diffraction patterns. Such a study was critical, for example, in understanding the spiral staircase of DNA's double helix.

The diffraction patterns are emphatically not magnifications of the crystalline structure; rather, the various distances and angles of the blips can be translated...