Bragg law


Bragg law
Relation between the spacing of atomic planes in crystals and the angles of incidence at which the planes produce the most intense reflections of electromagnetic radiation and particle waves.

The law, first formulated by Lawrence Bragg, is useful for measuring wavelengths and for determining the lattice spacings of crystals (see crystal lattice), and is the principal way to make precise energy measurements of X rays and low-energy gamma rays. See also William Bragg.

* * *

      in physics, the relation between the spacing of atomic planes in crystals and the angles of incidence at which these planes produce the most intense reflections of electromagnetic radiations, such as X rays (X-ray) and gamma rays, and particle waves, such as those associated with electrons and neutrons. For maximum intensity of reflected wave trains, they must stay in phase to produce constructive interference, in which corresponding points of a wave (e.g., its crests or troughs) arrive at a point simultaneously. The Bragg law was first formulated by Lawrence Bragg (Bragg, Sir Lawrence), an English physicist.

 The diagram shows waves 1 and 2, in phase with each other, glancing off atoms A and B of a crystal that has a separation distance d between its atomic, or lattice, planes. The reflected (glancing) angle θ, as shown by experiment, is equal to the incident angle θ. The condition for the two waves to stay in phase after both are reflected is that the path length CBD be a whole number (n) of wavelengths (λ), or nλ. But, from geometry, CB and BD are equal to each other and to the distance d times the sine of the reflected angle θ, or d sin θ. Thus, = 2d sin θ, which is the Bragg law. As may be seen from the diagram, when n = 2 there is only one wavelength along path CB; also, the reflected angle will be smaller than that for, say, n = 3. Waves reflected through an angle corresponding to n = 1 are said to be in the first order of reflection; the angle corresponding to n = 2 is the second order, and so on. For any other angle (corresponding to fractional n) the reflected waves will be out of phase and destructive interference will occur, annihilating them.

      The Bragg law is useful for measuring wavelengths (wavelength) and for determining the lattice spacings of crystals. To measure a particular wavelength, the radiation beam and the detector are both set at some arbitrary angle θ. The angle is then modified until a strong signal is received. The Bragg angle, as it is called, then gives the wavelength directly from the Bragg law. This is the principal way to make precise energy measurements of X rays and low-energy gamma rays. The energies of neutrons, which by quantum theory have wave attributes, are frequently determined by Bragg reflection.

* * *


Universalium. 2010.

Look at other dictionaries:

  • Law — /law/, n. 1. Andrew Bonar /bon euhr/, 1858 1923, English statesman, born in Canada: prime minister 1922 23. 2. John, 1671 1729, Scottish financier. 3. William, 1686 1761, English clergyman and devotional writer. * * * I Discipline and profession… …   Universalium

  • Bragg, Sir Lawrence — ▪ British physicist in full  Sir William Lawrence Bragg  born March 31, 1890, Adelaide, S. Aus., Australia died July 1, 1971, Ipswich, Suffolk, Eng.  Australian born British physicist and X ray crystallographer, discoverer (1912) of the Bragg law …   Universalium

  • Bragg , Sir William Lawrence — (1890–1971) British physicist William Lawrence Bragg was the son of William Henry Bragg. Born in Adelaide, Australia, he was educated at the university there and at Cambridge University, where he became a fellow and lecturer. After the war, in… …   Scientists

  • Bragg — /brag/, n. 1. Braxton /brak steuhn/, 1817 76, Confederate general in the U.S. Civil War. 2. Sir William Henry, 1862 1942, and his son, Sir William Lawrence, 1890 1971, English physicists: Nobel prize winners 1915. * * * (as used in expressions)… …   Universalium

  • Bragg, Sir William (Henry) — (b. July 2, 1862, Wigton, Cumberland, Eng. d. March 12, 1942, London) British scientist, a pioneer in solid state physics. With his son (William) Lawrence Bragg (1890–1971), he shared a 1915 Nobel Prize for research on the determination of… …   Universalium

  • law — law1 lawlike, adj. /law/, n. 1. the principles and regulations established in a community by some authority and applicable to its people, whether in the form of legislation or of custom and policies recognized and enforced by judicial decision. 2 …   Universalium

  • Bragg diffraction — (also referred to as the Bragg formulation of X ray diffraction) was first proposed by William Lawrence Bragg and William Henry Bragg in 1913 in response to their discovery that crystalline solids produced surprising patterns of reflected X rays… …   Wikipedia

  • Bragg — may refer to:*Bragg, Texas, a US ghost town *Electoral district of Bragg, a state electoral district in South Australia *Bragg (crater), a crater on the Moon *Bragg Communications, a Canadian cable television provider *Bragg (surname), people… …   Wikipedia

  • Bragg'slaw — Bragg s law (brăgz) n. The fundamental law of x ray crystallography, nλ = 2dsinθ, where n is an integer, λis the wavelength of a beam of x rays incident on a crystal with lattice planes separated by distance d, and θis the Bragg angle.   [After… …   Universalium

  • Bragg's law — In physics, Bragg s law is the result of experiments into the diffraction of X rays or neutrons off crystal surfaces at certain angles, derived by physicist Sir William Lawrence Bragg [ There are some sources, like the Academic American… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.