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Babinet’s Compensator

It is a device used for producing circular & elliptically polarized light and for their detection.

Back EMF

It is the electromagnetic force in an inductive circuit which acts in such a direction so as to oppose any change of current in the circuit.


Term generally used in nuclear physics. The background radiation refers to the energetic particles reaching earth surface mainly due to cosmic rays comprising neutrons, muons, neutrinos, gamma etc.


Process in which materials meant for vacuum application are subjected to heat condition to reduce outgassing rate.

Ballistic Galvanometer

A moving coil galvanometer, in which coil has high inertia that indicates presence of an electric charge by single impulse imparted to coil by small instantaneous current, the quantity of electricity that passes being proportional to deflection of coil.

Ballistic Pendulum

A physical pendulum consisting of a large mass suspended from a rod; when it is stuck by a projectile, its displacement is used to measure the projection’s velocity.


Science of mechanics that deals with behavior and effects of projectiles, especially bullets, rockets etc.

Balmer Series

The spectrum of wavelength falling in visible region due to transition of electrons from higher orbits to second orbit is called Balmer series.

Band Spectrum

This spectrum is due to transition of electrons combined with rotatory, translatory and vibration effects of molecules. Hot gases in molecular state produce band spectrum.  It is also called molecular spectra. It consists of bright bands of different colors over dark background. Each band consists of closely spaced lines. The spacing between two bands and also width of the band depends on nature of compound. At very high temperature, the band spectrum changes to line spectrum as the molecules split in to atoms.     


Band Theory

Theory which aims at classifying materials as conductors, insulators, semiconductors based on the distribution of electron energy states. In solids, due to proximity of atoms, each distinct atomic state splits into series of closely packed electron states called as electron energy band. There are three types of electron band structures possible at 0 K as per this theory.

Band Width

Term used in amplifier. It is the band of frequencies over which the amplification gain remains constant.


It is a unit of pressure.


Unit used for nuclear scattering interactions. It is used to represent the measure of probability of interaction between small particles. The value of one barn is 10-28 m2 and is approximate crossection area of Uranium nucleus.   


Instrument invented by Evangelista Torricelli to measure atmospheric pressure and hence for assisting in forecasting weather. 

Bartlett Force

It is type of nuclear force in which there is exchange of spin coordinates but not position coordinates between nucleons.


They are a type of Fermions which are heavier than mesons.  They constitute the two nucleons with anti particles & Hyperons. 


Fermions whose mass is at least as great as mass of Proton and which can interact strongly are called Baryons.


A battery is an electrochemical cell which consists of an anode, a cathode and an electrolyte. It is used to provide a static potential for power or release electrical charge when needed.


It is a crystal structure equivalent to two interpenetrating simple cubic cells. The total number of atoms in unit cell is two. The coordination number is eight.

Beat Frequency

Phenomenon which can be heard when two sound waves of different frequency approach human ear; constructive and destructive interference leads to alternation of soft and loud sound. "The beat frequency equals absolute value of the difference in frequency of the two waves."