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In order to begin to grasp the fundamental concepts of x-ray crystallography, one must be familiar with the major accomplishments of two scientists.  They are Max von Laue and William Lawrence Bragg.  Many of the techniques and ideas that came out of their experimentation are still relevant and applicable to modern experimental techniques.

Max von Laue



When von Laue started his work, the true nature of x-rays was still unknown.  Many experiments had been completed related to their energy and polarization.  Based on their energy, experimental evidence had suggested that if x-rays had a wavelength, it would have to be on the order of 0.1-1 Angstrom.  Diffraction patterns, similar to those made by visible light had never been detected in x-rays.  This is because the width of the slit, or diffraction grating, required to make x-rays interfere with one another was much too small to be created by humans.


von Laue hypothesized that if x-rays were similar to other forms of wave-like radiation, they could be made to produce diffraction patterns when passed through a crystal.  He predicted that the crystal could serve as the diffraction grating, because he knew that the distance between molecules in a crystal was thought to be around the same order of magnitude as the proposed wavelength of an x-ray.  He focused his effort on developing the theoretical basis for how this type of interference would occur.  In order to confirm his hypothesis in lab, von Laue would first need to be able to develop a mathematical model that could predict the relative location and intensity of the spots that would be produced as part of the diffraction pattern.  


von Laue was in frequent contact with Arnold Sommerfeld, a medical doctor turned x-ray theorist.  Sommerfeld was very skeptical of using a crystal as a diffraction grating.  He theorized that the intrinsic motion within any crystal would prevent von Laue from observing any regular diffraction pattern.  He reluctantly offered his own assistant, Walter Friedrich, to aid von Laue in his inquiry.  Friedrich was a tremendous asset, as he had developed and used an apparatus to study x-rays while working under Sommerfeld.  Paul Knipping, another researcher in the field, also agreed to contribute.  As von Laue was fine tuning the mathematics that he would need to corroborate his hypothesis, his assistants started the experimentation.

The apparatus consisted of an x-ray tube that focused a beam of x-rays through a small hole in a lead box that contained a crystal of copper sulfate.   Below is a image from The Cathode Ray Tube Site of an x-ray tube that is similar to the one that von Laue's research team used in their experiment.  I've labeled some of the most important parts.

The cathode is used to heat a filament until it is "white hot" and starts to release electrons.  These electrons travel toward an anode made of tungsten.  The extremely high temperatures produced require that the anode be made of a material with a high melting point.  When conditions within the glass tube are right, the beam of electrons causes the anode to release x-rays in the path indicated with the blue line.  A regulator is used to adjust the pressure of the gas within the tube.

Photographic plates were used to monitor the x-rays that exited through another small hole in the back side of the box.  Originally, the photographic plate was placed between the source of the beam and the crystal.  The prediction was that a diffraction pattern would result from x-rays that were reflecting off of the crystal.  When this failed to produce a pattern, Knipping placed plates on all sides of the crystal.  A second attempt yielded a diffraction pattern on the plate behind the crystal.  Here is an image that depicts the final experimental setup from the International Union of Crystallography (IUCr):

These images, also from IUCr, show the first successful diffraction photograph, as well as other images that were recorded in subsequent attempts:

These images confirmed von Laue's hypothesis that x-rays were wave-like in nature and that they possessed extremely small wavelengths that could be diffracted by a crystal.   It also meant that scientists could now study these types of diffraction patterns to gain information about the arrangement of atoms in a crystal.  However, von Laue still had to improve the mathematical models that he used to connect the spots made on the photographic plate with the actual positions of the atoms within the crystal.

William Lawrence Bragg


  • Australian-born, English physicist
  • shared Nobel Prize in Physics in 1915 with his father "for their services in the analysis of crystal structure by means of x-rays"


Even at a young age, Bragg was no stranger to the field of x-ray research.  His father, William Henry Bragg, was a professor of mathematics and physics that eventually worked under JJ Thomson and went on to invent the x-ray spectrometer.  It is widely reported that the first documented surgical use of the x-ray in Australia was when W.H. Bragg used it to examine his son's arm after he injured it while riding a tricycle.  The younger Bragg excelled in academics at a young age and finished his undergraduate studies by the age of 18.  He eventually shifted his focus from mathematics to physics and began researching the diffraction of x-rays by crystals at the University of Cambridge.


The scientific community was still buzzing over the results of Max von Laue's experiment.  However, von Laue's mathematical model was somewhat inconsistent and inaccurate.  He attributed the spots on the photographic plate to the areas where multiple x-rays had diffracted and then combined constructively.  However, nothing appeared in some of the areas where von Laue expected to see spots.  He assumed that this meant that x-rays only possessed certain wavelengths and not others.  Bragg was not satisfied with that explanation.  He envisioned a continuous spectrum in the x-ray region and hypothesized that the crystal must have had a different structure than the one envisioned by von Laue and others.  Bragg hypothesized that crystals must contain an atomic lattice.  This view set him apart from other scientists of his day who thought that crystals were made up of a lattice of molecules.  Bragg theorized that the path of x-rays shot through a crystal depended on planes and lattice points within the crystal.  Lattice points occur when planes of atoms in the crystal intersect.   


In order to test his hypotheses, Bragg would need a different apparatus from the one used by von Laue’s research team.  He needed a source that produced x-rays of a definite wavelength.  In addition, he needed to be able to easily change the position of the source to allow the x-rays to enter the crystal at various, controlled angles.  The optimal instrument for conducting his experiments was the x-ray spectrometer that was invented by his father.  For his role in the experimentation, the elder Bragg shared the Nobel Prize with his son.  A picture of the instrument, from the Science Museum in London, is pictured below.

 The Braggs experimented with several types of x-ray tubes, each containing an anode made of a particular type of metal that produced x-rays of a specific wavelength.  The most successful results came from x-rays generated from platinum, palladium, and rhodium.  The wavelengths of these x-rays were optimal for determining the lattice structure within crystals of a variety of substances.  Although both Braggs received the prize in 1915, the theoretical basis for the work is attributed almost entirely to the younger Bragg.  His contributions to science were far reaching, impacting a variety of disciplines.  However, Bragg is known as the father of modern x-ray crystallography for two main reasons- the advancements he made in understanding crystal structure and his mathematical model for x-ray reflection which is referred to as Bragg's law.


Carroll, P. J., & Carroll, M. H. (n.d.). Structure Determination by X-ray Crystallography . In University of Pennsylvania X-Ray Crystallography Facility [Online Course]. Retrieved January 7, 2009, from‌course/‌index.html

Dijkstra, H. (2009). Early villard x-ray tube [Data file]. Retrieved April 1, 2009, from‌h.dijkstra19/‌big/‌x-ray/‌villard-big.jpg

Granqvist, G. (2009). The Nobel Prize in physics 1915 [Presentation speech]. Retrieved April 3, 2009, from‌nobel_prizes/‌physics/‌laureates/‌1915/‌present.html

Granqvist, G. (2009). The Nobel Prize in Physics 1914 [Presentation speech]. Retrieved March 30, 2009, from Nobel Prize Foundation Web site:‌nobel_prizes/‌physics/‌laureates/‌1914/‌present.html

Kapecki, J. A. (1972). An introduction to x-ray structure determination. Journal of Chemical Education, 49(4), 231-236.

Science Museum, London. (n.d.). X-ray spectrometer [Data file]. Retrieved April 9, 2009, from‌hommedia.ashx?id=11200&size=Small

University of Cambridge. (2002). X-ray diffraction. In Cavendish Laboratory educational outreach. Retrieved March 30, 2009, from‌camphy/‌xraydiffraction/‌xraydiffraction_index.htm

Wikipedia. (2006). Max von Laue [Data file]. Retrieved April 1, 2009, from‌wiki/‌File:Max_von_Laue.jpg#file

Wikipedia. (2008). William Lawrence Bragg on TIME magazine- October 3, 1938 [Data file]. Retrieved April 1, 2009, from‌wiki/‌File:William_Lawrence_Bragg_on_TIME_Magazine,_October_3,_1938.jpg