Lawrence Bragg and William Henry Bragg founded the science of X-ray crystallography, verified the very short wavelength nature of X radiation, developed spectrometers for measuring X-ray wavelengths, and deduced crystal structures of many substances.
The development of X-ray crystallography helped to answer two major questions of science: What are X rays? and What are crystals? It gave birth to a new technology for the identification and classification of crystalline substances. Shortly after the discovery of X rays by Wilhelm Conrad Röntgen, X rays were put to a variety of uses, particularly in medicine, but the true nature of X rays eluded researchers. Some proclaimed X rays as a form of energy that traveled in waves. The wave theory that portrayed radiation
William Henry Bragg.
Diffraction occurred when a light ray encountered a well-polished surface that had been etched precisely with a series of closely, evenly spaced lines, usually several thousand lines per centimeter. A surface so prepared was called a grating. After the light struck the grating, some waves left the surface “in phase” and continued onward as reflected light. Because of the fine etches, some adjacent waves had to follow slightly different path distances and thus, on reflection, emerged “out of phase” and were terminated immediately. The selective reflection of only those waves that emerged in phase was termed “diffraction.” If one knew the spacing between the etch lines and the angle of reflection of the light, then it was possible to calculate the wavelength of the diffracted light. The phenomenon of diffraction was used to measure the wavelengths that corresponded to the various colors within visible light. Nevertheless, attempts to diffract X rays from gratings failed. This led others to proclaim that X rays were not wave forms but instead were composed of tiny “corpuscles” of high-energy matter that were able to penetrate opaque materials.
From studies of large, natural crystals, chemists and geologists had established the elements of symmetry through which one could classify, describe, and distinguish various crystal shapes. A century earlier, René-Just Haüy had demonstrated that diverse shapes of crystals could be produced by repetitive stacking of tiny solid cubes. Auguste Bravais later showed, through mathematics, that all crystal forms could be built from a repetitive stacking of merely a few (fourteen) three-dimensional arrangements of points (lattice points) into “space lattices,” but no one had ever proved that actual matter was arranged in space lattices. Indeed, at the times of Haüy and Bravais, the model of the atom had not been derived and, hence, even the nature of the substance contended to be present at the points that defined a space lattice was conjectural. They did not know if the tiny building blocks modeled by space lattices actually were solid matter throughout, like Haüy’s cubes, or if they were the basic building blocks of crystals (mostly empty space with the only solid matter located at the lattice points described by Bravais). With the disclosure of the atomic model of Danish physicist Niels Bohr in 1913,
In 1912, Max von Laue first used X rays to study crystalline matter. German physicist Wilhelm Wien had estimated, based on the new quantum theory, that X rays could have wavelengths between 10
–10
and 10
–9
centimeters. Laue recognized that failed attempts to achieve diffraction of X rays might have resulted because no synthetic grating had lines spaced finely enough to cause diffraction of such short wavelengths. Based on his knowledge of the contemporary theory about atoms and crystals, Laue recognized that the interatomic distances in crystals should be small enough to allow the crystal to function like a diffraction grating for a spectrum of appropriately short wavelengths. When Laue and coworkers Walter Friedrich
Laue received the 1914 Nobel Prize in Physics for discovery of the diffraction of X rays in crystals, but the conclusion that X rays actually were being diffracted was accepted as a near certainty rather than proved.
Laue documented the general nature of crystal structures, but he was unable to deduce specific details. This resulted from the fact that he transmitted his X-ray beam through the crystal, and his resulting photographic spot images could be interpreted in detail for only the simplest of crystals and then only through very cumbersome mathematics. Sir Lawrence Bragg circumvented the complexities of the interpretation by “reflecting” beams from crystals. He postulated that if the lattice models of Bravais applied to actual crystals, then a crystal structure could be thought of simply as constructed of atoms arranged in a pattern consisting of a few sets of regularly spaced parallel, flat planes. Lawrence Bragg knew from Laue’s work that X rays, like visible light, behave in accordance with wave theory. He reasoned, then, that when X rays enter a crystal, some adjacent waves must be reflected from the regularly spaced atoms lying along the surface plane and some will travel farther to be reflected from atoms in underlying parallel planes. In order to achieve diffraction, the adjacent waves—even though they traversed two different but parallel paths—would have to leave the surface plane in phase. The distance between the planes and the angle of incidence of the X-ray beam determined the extra distance (path difference) that a beam reflected from a lower plane would have to traverse in comparison to a beam reflected from an upper plane. In order for both reflected X-ray beams to emerge from the crystal in phase, this path difference had to equal exactly one incident X-ray wavelength or a whole-number (integral) multiple of that wavelength. If it did not, then waves and troughs of adjacent beams would interfere when they rejoined above the upper plane, and no reflected beam composed of out-of-phase waves could persist.
The “Bragg equation,”
The Braggs founded an entirely new discipline, X-ray crystallography, that continues to grow in scope and application. Of particular importance was the early discovery that atoms, rather than molecules, determine the nature of crystals. X-ray spectrometers of the type developed by the Braggs were used by other scientists to gain insights into the nature of the atom, particularly the innermost electron shells. The tool allowed timely validation of some of Bohr’s major concepts about the atom.
X-ray diffraction became a cornerstone of the science of mineralogy. The Braggs, chemists such as Linus Pauling, and a number of mineralogists used the tool and did the pioneering work in deducing the structures of all major mineral groups. Huge reference indexes of X-ray “powder patterns” were compiled by organizations such as the American Society for Testing Materials (ASTM), and X-ray diffraction became the primary definitive method for identification of crystalline materials, including about two thousand naturally occurring minerals and many times that number of synthetic inorganic and organic compounds. Mineralogists who relied primarily on optical methods prior to the Braggs’ work were able to go beyond physical properties and laborious reflected-light microscopy to identify and characterize opaque minerals. Clay mineral species, because of their occurrence in small particles, defied discovery and identification under the polarizing microscope. The contribution by the Braggs permitted development of the discipline of clay mineralogy through which many properties of soils and sedimentary rocks could be explained on the basis of the clay minerals contained within them.
Metallurgy progressed from a technology to a science as metallurgists became able, for the first time, to deduce the structural order of various alloys at the atomic level. Diffracted X rays were applied in the field of biology, particularly under the direction of Lawrence Bragg at the Cavendish Laboratory. The tool proved essential for deducing the structures of hemoglobin, proteins, viruses, and eventually the double-helix structure of DNA (deoxyribonucleic acid).
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Beam Line: A Periodical of Particle Physics 25 (Summer, 1995). http://www.slac.stanford.edu/pubs/beamline/pdf/95ii.pdf. Special issue titled “One Hundred Years of X Rays” is devoted to discussion of the history and uses of X rays. Articles include “Early History of X Rays” and “Medical Applications of X Rays.” Features photographs, diagrams, and reproduced newspaper articles from the late nineteenth and early twentieth centuries. (Beam Line is published by the Stanford Linear Accelerator Center.) -
Bragg, William H. X-Rays and Crystal Structure. Vol. 7 in Physical Sciences, edited by W. L. Bragg and G. Porter. Amsterdam: Elsevier, 1970. This volume is a compilation of papers given from 1909 through 1915. The one cited here by William Henry Bragg was given in 1914. It is a very readable, nontechnical report that shows clearly the progress at the time the Braggs were doing the work that would lead to their Nobel Prize. -
Bragg, William L. Atomic Structure of Minerals. Ithaca, N.Y.: Cornell University Press, 1937. This book was written when Lawrence Bragg was a visiting professor at Cornell University in 1934. It is a reference book on minerals arranged according to chemical groups. It is well illustrated and has many citations that show the extensive contributions to mineralogy from the Braggs and scientists such as Linus Pauling, W. H. Taylor, and R. W. Wyckoff. -
Crowther, I. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. Lawrence Bragg accepted the directorship of the National Physical Laboratory in 1937 when it was vacated because of the unexpected death of Ernest Rutherford; he remained at Cavendish until 1953. Provides a wealth of information about Bragg’s use of X-ray diffraction and its subsequent use by other researchers. -
Michette, Alan, and Sawka Pfauntsch, eds. X-Rays: The First Hundred Years. New York: John Wiley & Sons, 1996. Collection of essays published in commemoration of the one hundredth anniversary of Röntgen’s discovery of X rays. Reviews the history of scientific work related to X rays as well as modern applications. Includes an extensive glossary. -
Nobel Foundation. Physics. Vol. 1. New York: Elsevier, 1964. The compilations in this volume are all in English, and these lectures delivered by Nobel laureates are more readable than their works in technical journals. The presentation speech to Röntgen and the lectures by von Laue, Lawrence Bragg, and Barkla in this volume are particularly pertinent to the topic of X-ray diffraction. Although the Braggs shared the Nobel Prize in 1915, only Lawrence Bragg provided a Nobel lecture. -
Zoltai, Tibor, and James H. Stout. Mineralogy: Concepts and Principles. Minneapolis: Burgess, 1984. This textbook, intended for undergraduate students of mineralogy, provides a resource for the reader wishing rigorous technical explanations of the X-ray study of crystals. It contains all necessary background in crystallography and crystal chemistry in chapters 2 through 8. Chapter 10 shows derivation of the Bragg equation and provides a very solid and very thorough introduction to X-ray crystallography.
Röntgen Wins the Nobel Prize for the Discovery of X Rays
Barkla Discovers the Characteristic X Rays of the Elements
Salomon Develops Mammography
Discovery of the Compton Effect