Bragg's Law a Cornerstone of Crystallography

Bragg's Law: a cornerstone of the science of crystallography

In physics parlance, Bragg's Law gives the angles for coherent and incoherent scattering from a crystal lattice. The law was derived by physicist Sir William Lawrence Bragg in 1912 and first presented on 11 November of the same year to the Cambridge Philosophical Society. In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid.

The Bragg's Law Equation:

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. Note that moving particles, including electrons, protons and neutrons, have an associated De Broglie wavelength.


[Image: Sodium chloride (table salt) 3D crystal structure]

Another way of describing it is that X-rays scattered from a crystal will show constructive interference provided their wavelength fits the Bragg's equation. The law confirmed the existence of real particles at the atomic scale; it also provided a powerful tool for studying crystals in the form of X-ray and neutron diffraction. It may sound complicated but significantly, Bragg's Law is the cornerstone of the science of crystallography.


The Braggs: William Henry Bragg (father) and William Lawrence Bragg (son)


[Image:  The Braggs: Sir William Henry and Sir William Lawrence]

The Braggs, Sir William Lawrence Bragg and his father, Sir William Henry Bragg, were awarded the Nobel Prize in physics in 1915 for their work in determining crystal structures beginning with NaCl (sodium chloride or table salt), ZnS (zinc sulphide), and diamond. They are the only father-son team to jointly win. William Lawrence Bragg was 25 years old.

Sir William Henry was born in England. In 1855, he came to Australia as Professor of Mathematiacs and Physics at the University of Adelaide in south Australia. The Braggs returned to England in 1909 where the father and son team worked together in pioneering crystal structures by using X-rays, therefore, founding the science of X-ray crystallography.  They shared the 1915 Nobel Prize in Physics "for their services in the analysis of crystal structure by means of X-rays."


Resources:

• IUCR.org/Dictionary/Bragg's_Law.  Accessed April 1, 2013.


• Britannica.com.  Accessed April 1, 2013.


• The Nobel Prize in Physics 1915.  Nobel Foundation. Accessed April 1 2013.

Images Credits:

The Braggs: Sir William H. and Sir William L.  Adelaide.edu.au.  Accessed April 1, 2013.

Bragg Diffraction Planes and Sodium Chloride Crystal Structure, Wiki Commons Public Domain. Accessed April 1, 2013.