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### Physics Dictionary (Contd....)

Aurora
It is a luminous atmospheric phenomenon appearing as streamers or bands of light sometimes visible in the night sky in northern or southern regions of the earth. It is thought to be caused by charged particles from the sun entering the earth’s magnetic field and stimulating molecules in atmosphere.

Avalanche Multiplication
Phenomenon observed in gas filled detectors as per which when high electric field is applied to anode wire, there occurs rapid multiplication of charges created in primary ionization event near the wire where electric field intensity is high. This process is called avalanche multiplication.

Average Binding Energy
It is obtained by dividing total binding energy of a nucleus by the number of nucleons in nucleus. This energy is a measure of strength of nuclear bonding. The more tightly bound the nucleons; the more stable the nucleus becomes and larger is its binding energy.

The number of atoms or molecules in a mole of a substance is called Avogadro’s number.

Equal volumes of ideal gases existing under same conditions of temperature and pressure contain equal number of molecules.

Whenever a body is fully or partially submerged in a fluid, a buoyant force acts on the body from the surrounding fluid. The force is directed upward and has a magnitude equal to weight of fluid that has been displaced by body.

Axial Chromatic Aberration
For an object at infinity, the difference in focal length of a lens for the red and violet light is called the axial chromatic aberration.

Axis of Precession
The axis about which the direction of rotation of body precesses is called as axis of precession.

Axis of Rotation
When a rigid body undergoes a combined translation and rotational motion, there is at least a point inside or outside the body whose velocity is zero. The body is said to have pure rotation about that point. Then axis passing through that point and perpendicular to plane of rotation is known as axis of rotation.

Axis
Vertical and horizontal lines that make up quadrants of coordinate plane are referred to as axis.

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.

Background
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.

Baking
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.

Ballistics
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.