Blaze -  The flat side of a groove in a diffraction grating.
Bloch Functions - Solutions of the Schrodinger equation with a periodic potential (e. g. for an electron moving in a crystal lattice).
Bloch Wall - The transition layer, which may be as great as 1.2 J..1m, which separates two domains magnetized in different directions and in which the change in spin direction between the two domains is assumed to take place gradually. Also known as the domain boundary.
Bohnenberger Eyepiece - An eyepiece, containing an unsilvered 45° flat glass plate, used in the measurement of the level error of a meridian circle.
Bohr Frequency Condition - Gives the frequency, v, assoicated with the quantum of radiation emitted by an atom making a transition from energy E to a lower energy Eo as E-Eo = hv, h where h is Planck's constant. The reverse condition holds for absorption.
Bonr Magneton - The unit of atomic magnetic moment, denoted by β  or μB. It is equal to ehl41tmc, where e is the electronic charge. h is Planck's constant, m is  e electron rest mass and c is the speed of light. It is the moment of a single electron spin and its value is 9.27 X 10-21 erg/gauss or 9.27 x 1O-24JT-1.. 
Bohr Magneton Number - The magnetic moment per atom expressed in Bohr magnetons, usually denoted by no.
Bohr Orbit - One of the electron  orbits permitted by Bohr's theory of the atom, in which the electron has an orbital angular moentum which is an integral multiple of hl2Π, where h is Planck's constant.
Bohr Radius - The radius of the smallest electron orbit in Bohr's model of the hydrogen atom. It is equal to 'h2 / 4,Пwhere h is Planck's constant, m is the rest mass of the electron and e the electronic charge. It has a value of 0.53 x 1 a-8 cm.
Bohr Theory - A combination of Rutherford's concept of the atom as a central. positively charged nucleus surrounded by planetary electrons, with the ideas of the quantum theory. The possible electron orbits are restricted to those whose orbital angular momentum is an integral multiple of h/2Π where h is Planck's constant. The jump of an electron from its orbit to one of a smaller radius i3 accompanied by the emission of electromagnetic radiation, one quantum of which (i.e. hv where v is the frequency) is equal to the difference in energy between the two,orbits concerned,
Bolometer - A very sensitive instrument for the measurement of small amounts of radiant heat.
Bolometric Scale - A scale of stellar magnitude based on the measurement of radiant energy.
Boltzmann Constant - The gas constant per atom (or per molecule for a molecular gas). It is equal to the gas constant per mole divided by Avogadro's constant. and may also be considered as the ratio of the  mean total energy per atom (or molecule) in a gas to the absolute temperature. Its value is 1.380 x 10-16 erg/deg, or 1.380 x 10-23 J/deg,
Boltzmann Distribution Law - For a physical system consisting of a large number of independent particles in statistical equilbrium: a law giving the average number of particles within the system having positions and speeds within well-defined limits. The distribution funciton, f, i.e. the number of particles which at time t have such specified positions and speeds, is given by f = A exp(-E/kT), where A is a normalization constant, E is the energy of the. particle as a function of position and speed, k is Boltzmann's constant and T is the absolute temperature.
Boltzmann Factor - A name sometimes given to the Stefan-Boltzmann constant.
Born-oppenheimer Approximation - An approximation used in considering the electronic behaviour of molecules or solids, according to which, under certain conditions, nuclear motion may be neglected, the interacting particles being treated as point charges and masses which interact electrostatically.
Born-Oppenheimer Method - A method for' calculating tile force constants between atoms in a molecule or solid, on the assumption that the electrons follow the motions of the nuclei adiabatically.
Bose-Einstein Condensation - For a vapour to the molecules of which Bose-Einstein statistics apply: the. condensation of the vapour to a state in which some of the molecules have a momentum of nearly zero instead of having their momentum spread over a large range of values.
Bose-Einstein Statistics - The form of quantum statistics, having to do with the distribution of particles among various allowed energy values, applicable to an assembly of particles in which many particles are allowed in a given state.
Boson - A particle to which Bose-Einstien statistics apply. Such partiled do not follow the Pauli exclusion principle. Photons, 1t-meson, a-particles, and all nuclei of even mass number are bosons. Their spin, in common with that of all bosons, is zero or integral.
Boundary Wave - A concept due to Young according to which the light diffracted by a hole in an opaque screen is expressed as the sum of the geometrical wave (i.e. one transmitted according to geometrical optics) F and an edge or boundary wave, which orginates at the edge of the hole and is propagated as. a cylindrical wave with non-uniform amplitude.
Boyle Law - States that, for a given mass of gas at constant temperature, the product of pressure and volume is constant, i.e. the volume of the gas varies inversely as. the pressure. The law holds only for ideal or perfect gases and fails for all real gases to a greater or lesser extent. 
Boyle Temperature - The temperature at or near which Boyle's law  provides a good approximation to the true equation of state for a  given gas.
Brackett Series - A series in the line spectrum of the hydrogen atom.
Bragg Curve - A curve in which the specific ionization along the path of a beam of α-particles in a gas (usually air) is plotted against the distance from the a-particle source, and from which the range of the particles may be determined.
Bragg Cut-off Wavelength - The wavelength above which Bragg reflection cannot occur.
Bragg Equation: Bragg Law - The equation setting out the condition for the diffraction "reflection" of a parallel beam of monochromatic (monoenergetic).X-rays from a crystal, which may be stated: nλ. = 2d sin θ, where n (an integer) is the order of diffraction ("reflection"), A. is the wavelength of the X-rays. and d is the distance between parallel atomic planes ill the crystal which give diffraction maxima when the incident beam is inclined at an angle θ to these planes.
Bragg Reflection - The diffracted beam produced by reinforcement from successive members of a set of crystal planes when the Bragg equation is satisfied.
Bragg Spectrometer - An instrument for the X-ray analysis of crystal structure in which a monochromatic beam of X-rays was diffracted by a crystal and the diffracted beam detected and measured by an ionization chamber.
Branching Decay - Radioactive decay which can. proceed in two or more different ways.
Branching Raction - In branching decay: the fraction of nuclei which disintegrate in a specified way. It is usually expressed as a percentage.
Branching Ratio - In branching decay: the ratio of the branching fractions for two specified modes of disintegration.
Bravais Biplate - A biplate used as a sensitive detector of traces of optical polarization.
Bravais Lattice: Space Lattice - An infinite three-dimensional array of points in space, such that each point h,15 the same environment.
Only fourteen distinct arrays or lattices of this kind are possible and these may be referred to one or other of the seven crystal systems.
Breakdown Voltage - The voltage ,at which a given insulator or  insulating material fails to withstand the voltage and becomes conducting.
Breeding - Of fissile material: the transformation of a fertile substance into a fissile substance in a nuclear reactor when the number of fissile nuclei produced is greater than the number of fissile nuclei consumed, Le. when the conversion ratio is greater than unity.
Breeding Gain - Of a nuclear reacter :the ratio of the excess of fissile nuclei produced over fissile nuclei consumed to the fissile nuclei consumed, i.e. the breeding ratio minus one.
 Breeding Ratio - Of a nuclear reactor: the ratio of the number of fissile nuclei produced to the number of fissile nuclei consumed when it is greater than unity, i.e. the conversion ratio when it is greater than unity.
Bremsstrahlung - The electromagnetic radiation associated with the deceleration of charged particles.
Brewster Bands: Brewster Fringes - Interference fringes which are seen when white light is viewed through two plane parallel. plates whose thicknesses are nearly equal.
Brewster Law - States that, when light is reflected by a refracting medium, the tangent of the polarizing angle is equal to the refractive index; or, in other words, when light is reflected at the polarizing angle the reflected and refracted rays are 900 apart.
Brewster Stereoscope- A prismatic device for viewing a pair of stereoscopic pictures of which the separation is greater than the interocular distance.
Brewster Electrical - A term referring to anyone of a variety of electrical networks employing a bridge circuit, which is essentially a four-terminal network to the input terminals of which is applied an e.m.f., the output terminals being "bridged" by a detector. By adjusting one or more components in the network the voltage across the "bridge" can be brought to zero and. in this unique condition, specific relations exist between the values of the components in the system, which may then be used for measuring one component in terms of another