Maureen M. Julian Hollins College Holllns College, VA 24020
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Crystals and X-Rays A demonstration
Because of the subtleties of X-ray photographs, X-ray diffraction is a difficult topic to introduce in an undergraduate general chemistry course. Its importance should not he underestimated because of the contrihutions of structural analysis to hiochernistry (structure of DNA), metallurgy, chemistrv. medicine. and manv other fields. Since manv of our science students take only one year of chemistry I hive developed a dramatic lecture hall demonstration ou crystals and x-rays using a mirror ball typically found in roller skating rinks (see figure). In a darkenedlecture hall, asingle spotlight is focused on the stationary mirror hall and the walls of the hall are immediately covered with a display of light spots.
Historical Introduction In 1912 Max Laue' theorized that the wavelengths of X-rays mieht be iust about the same as the distances between atoms in crystafs. If this were the case then X-rays would be diffracted or bent by crystals. Max Laue lacked X-ray expertise so he interested two young experimentalists, Walter Friedrick and Paul Knipping to undertake the crystal and X-ray experiment. They had done doctoral studies under W. C. Rontgen, the discoverer of X-rays. After a few tries the beautiful blue crystals of copper sulfate pentahydrate gave the hoped for diffraction pattern. Scientists were strivine to interoret the relations hi^ hetween the pattern of spots-which appeared on the film and the atoms which make up the crystal. Many leading scientists, including William H. Bragg, a t the University of Leeds in England, visualized corpuscular X-rays tunneling down the many open avenues in the crystal. On July 21,1912, Bragg wrote to the mathematical physicist Sir Arthur Schuster: "I wonder whether the rays producing the side [diffracted] spots are really "rays3' ~roceedingin straight lines from Some point in the crystal (say where the X-ray impinges or emerges), or are they sections of some loci by the photographic plate. I t all seems most my~terious!"~ During that summer atLee&, Bragg had many discussions the puzzles of diffraction with his sou, william L~~~~~~~B ~who ~was home ~ on ~ vacation , from cambridge University. When young Bragg returned to his studies he set a "ysta' of mica in the path of the X-ray beam. First, he no-
ticed that the shape of the spot became flattened as the angle of the crystal about its axis increased. This increase contradicted the corpuscular theory and made him suspect that the crystal was behaving in a way analogous to the optical reflecting of .rays. Furthermore, as the crystal was rotated t throueh twice throueh an auele. the diffracted s ~ o moved that angle. He ;hen postulated thatthe layers of atoms within the cwstal were actina as mirror-like reflectine planes and so applied the optical diffraction equation for gratings to X-rays. Thus, the famous Bragg equation nX = 2d sin6 was bortl. I n 1915, only three years after Max Laue's outstanding experiperiment, 24-year-old William Lawrence Bragg became the youngest recipient of the Nobel Prize which he received jointly with his father. This demonstration facilitates an historical as well as dramatic introduction to X-ray diffraction.
Demonstration Single mirror First in a darkened room a beam of light is directed at a small hand mirror to demonstrate that the shape of the reflected bean! changes as the mirror is rotated. Then the mirror is held horizontally so the light spot is directly overhead. While the students follow the light spot, the mirror is slowly rotated so that the light spot moves toa position ninety degrees from the ceiling, for example on the wall directly ahead of them. Then their attention isdirected to the mirror which is at an angle of 45' to the horizon. This procedure is repeated several times so that the demonstrationis clear that the reflection rotates twiceas far as the s i d e mirror causine the reflehtion.
Mirror
demonstration,
Again with the room darkened,the cloth covering the mirror hall which has been carefully positioned is removed and immediately a dramatic pattern appears on the ceilingand walls. I,, this
optical analogue of crystals and X-rays, the beam of light is the "monochromatic" X-ray heam, themirror ball represents thecrystal, the light spats on the ceiling and walls are the diffractedX-ray beams, the wall becomes the film, and the room itself becomes the giant camera. The spotlight is "monochromatic" because light is considered to he reflected at only a single angle. The "crystal"can be rotated, showing that the pattern rotates with the crystal and the two effects mentioned under "single mirror" can he demonstrated.Although it is more with the multiplicity of spots, a single spot be selected and the change in shape can he seen as well as the fact that the spot will travel twice as far as the corresponding rotation.of the mirror hall. An important differencebetween the model and the prototype is that in the real crystal the Bragg planes interpenetrate while in the model the little mirrors are glued to the outside of the sphere. However, the mirror hall and the crystal being X-rayed reflect at only one position. In the crystal, this position is called the Bragg angle. Another difference between the model and the prototype is that in the mirror hall crystal all planes reflectequally well resulting in light spots of equal brightness. In the real crystel all planes are not equally oooulated with atoms. The intensitv of the reflected X-rav beam . . ~, dt,pends
