Day13: Sir William Henry Bragg and William Lawrence Bragg

The reason: They found the fundamental equation in scattering techniques.

Sir William Henry Bragg and William Lawrence Bragg were awarded the Nobel Prize in Physics “for their services in the analysis of crystal structure by means of X-rays” in 1915. They are more famous about so called “Bragg’s law”. And William Lawrence Bragg is the youngest novel prize laureate. And they are from Australia, unfortunately I don’t know so much about Australian physicist, there is Bragg institute in Sydney which should be named after Bragg. Here is Bragg’s law,

n λ = 2 d sin θ

Where n is an integer, λ is the wavelength of incident wave, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes.

The Bragg’s law is satisfied with the crystals scattered by every quantum beam such as electron, X-ray, and neutron. This equation is fundamental for every diffraction techniques. As an experimental physicist, I am still using this equation.  We physicist have 3 quantum beams, every beam have advantage and disadvantage. As I described before, I personally think, the technique in neutron diffraction is most mature, X-ray diffraction is now hottest and expanding, and Electron diffraction is expanding and most promising.

• Keenness on experiment

No known theory can be distorted so as to provide even an approximate explanation of wave-particle duality. There must be some fact of which we are entirely ignorant and whose discovery may revolutionize our views of the relations between waves and ether and matter. For the present we have to work on both theories. On Mondays, Wednesdays, and Fridays we use the wave theory; on Tuesdays, Thursdays, and Saturdays we think in streams of flying energy quanta or corpuscles. — Sir William Bragg ‘Electrons and Ether Waves’, The Robert Boyle Lecture 1921, Scientific Monthly, 1922, 14, 158.

 As we can see on this quote, scientists were struggling to understand wave-particle duality almost 100 years ago. Sometimes I feel I just can’t have more the experiment skill and keenness for observations than these genius experimentalists in science history did because they didn’t have computer, knowledge nor textbook, they seriously thought and interrupted the data in front of them honestly. Although now we have knowledge, computer, textbook, and techniques, I don’t just think I can have the critical thinking and deep thought experiment like they did. Also, I could easily forget how peculiar what we are observing at our experiment. The diffraction can only occur because it should be diffracted (waves) by crystals and counted at detector (particle).

• The future is still based on this equation.

As a neutron scattering experimentalist Bragg scattering is the strongest when beam scattered by perfect crystal. If there were defect on the crystal, still we could study well. One of my skills on neutron scattering is pair distribution function analysis, on which we don’t need to assume crystal. Originally, the method was developed for studying glass and amorphous material. That experiment method is useful when we are looking at small deviation from crystal. For this analysis method we don’t use Fourier transform of background instead of using Bragg scattering. This method is so powerful because we could use Bragg scattering and Background simultaneously from just one data set.  The Bragg scattering still the central issue among scattering community, even though we could develop new scattering techniques to take out more information from the sample in the future. The Bragg scattering have to be the thing we care most during experiment.

You can have free access these historical articles appeared on Nature,

http://www.nature.com/milestones/milecrystal/library/pdf/090219a0.pdf

http://www.nature.com/milestones/milecrystal/library/pdf/090360d0.pdf