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.
Avogadro Number
The
number of atoms or molecules in a mole of a substance is called Avogadro’s
number.
Avogadro’s Law
Equal
volumes of ideal gases existing under same conditions of temperature and
pressure contain equal number of molecules.
Avogadro’s Principle
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.Physics Dictionary (Contd...)
Atmospheric Electricity
The
conductivity of atmosphere arises from ions created by cosmic rays. The
conductivity is variable; as we go to higher altitudes, conductivity gradually
increases and at about 50km, atmosphere is pretty good conductor. This
electrical phenomenon takes place between two conducting spherical surfaces,
earth and top of stratosphere. Each of these surfaces is equipotential.
Atmospheric Pressure
The
atmospheric air exerts normal pressure upon all surfaces with which it is in
contact, and is known as atmospheric pressure.
Atom Bomb
Nuclear
weapon which harnesses the forces that hold the nucleus of an atom together, by
using the energy released when the nucleus splits into nucleons. The atom bomb
works on principle of nuclear fission. Nuclear reactions occur when neutrons
are fired at closely packed atoms with heavy fissile nuclei like uranium-235 / Plutonium
isotopes. This leads to nuclear fission which further converts to chain
reaction.
Atom Percent (at %)
It
is defined as concentration specification on basis of number of moles (or
atoms) of a particular element relative to the total number of moles (or atoms)
of all elements within an alloy.
Atom
It
is tiny basic unit of matter which is neutral and from which any substance gets
constructed. It has positively charged nucleus constituting nucleons (Protons
& Neutrons) surrounded by electrons revolving round the nucleus.
Atomic Clock
It is
accurate type of time device in the world, which is controlled by vibrations of
an atomic or molecular system such as Cesium or Ammonia. The precision of an
atomic clock depends on fact that an atom, caused to oscillate, will always
vibrate at same frequency. NIST-F1, the United States standard atomic clock, is
said to be so accurate that it would neither gain nor lose a second in over 30
yrs. They are used as primary standard in global navigation satellite system
such as GPS.
Atomic Mass Unit (amu)
It is
defined as 1/12 of mass of Carbon-12 atom. It is average of Proton & Neutron
rest mass i.e. 1.67377 x 10-24 gm. The unit is used to express
relative masses of various isotopes of element.
Atomic Number
The
number of protons found inside the nucleus of a particular atom. The modern
periodic table is structured based on this number.
Atomic Orbital
A
quantum state attributed to electron around atom in quantum mechanics to
describe wave characteristic of electron.
It is expressed by mathematical function which is used to calculate
probability of finding any electron in any specific region around atom’s
nucleus.
Atomic Packing Fraction
It
is the ratio of volume of atoms per unit cell to volume of unit cell in a
crystal structure.
Atomic Physics
It
is study of structure of atom, arrangement of electrons around nucleus i.e.
energy states, interaction of atoms with other particles and with electric and
magnetic fields.
Atomic Radius
It
is defined as half the distance between neighboring atoms in a crystal of a pure
element.
Atomic Structure
It
is study of configuration of atom which is essential for knowing many
phenomenon involving electrons. Bohr’s atomic model is early model which has
explained structure of atom to a great extent. Later the limitations of that model
overcome by attributing dual nature to particle. The atomic structure
represents the manner in which electron states are occupied.
Atomic Weight
It
is the weighted average of the atomic masses of an atom’s naturally occurring
isotopes. It is usually expressed in amu.
Atomicity
Total
no. of atoms present in one molecule of an element of a substance is called as
atomicity.
Atto
Prefix
in metric system denoting a factor of 10-18.
Audio Frequency
It
is the frequency range of periodic vibrations audible to human ear. The range
is 20Hz to 20 KHz.
Audio Meter
It
is an instrument for measuring hearing activity for pure tones of normally
audible frequencies.
Aufbau’s Principle
Principle
which states that sub-shell of lower energy in an atom is filled up first while
that of higher energy is filled up later.
Auger Effect
See
Auger electron.
Auger Electron
In
photoelectric effect, following expulsion of photoelectron, another electron
from high energy state may occupy its location emitting X-ray photon. This
photon may in turn eject electron from loosely bound outer electron shell, called
auger electron and such photoelectric effect is called “Auger Effect”.
Physics Dictionary (Contd...)
Atmospheric Electricity
The
conductivity of atmosphere arises from ions created by cosmic rays. The
conductivity is variable; as we go to higher altitudes, conductivity gradually
increases and at about 50km, atmosphere is pretty good conductor. This
electrical phenomenon takes place between two conducting spherical surfaces,
earth and top of stratosphere. Each of these surfaces is equipotential.
Atmospheric Pressure
The
atmospheric air exerts normal pressure upon all surfaces with which it is in
contact, and is known as atmospheric pressure.
Atom Bomb
Nuclear
weapon which harnesses the forces that hold the nucleus of an atom together, by
using the energy released when the nucleus splits into nucleons. The atom bomb
works on principle of nuclear fission. Nuclear reactions occur when neutrons
are fired at closely packed atoms with heavy fissile nuclei like uranium-235 / Plutonium
isotopes. This leads to nuclear fission which further converts to chain
reaction.
Atom Percent (at %)
It
is defined as concentration specification on basis of number of moles (or
atoms) of a particular element relative to the total number of moles (or atoms)
of all elements within an alloy.
Atom
It
is tiny basic unit of matter which is neutral and from which any substance gets
constructed. It has positively charged nucleus constituting nucleons (Protons
& Neutrons) surrounded by electrons revolving round the nucleus.
Atomic Clock
It
is accurate type of time device in the world, which is controlled by vibrations
of an atomic or molecular system such as Cesium or Ammonia. The precision of an
atomic clock depends on fact that an atom, caused to oscillate, will always
vibrate at same frequency. NIST-F1, the United States standard atomic clock, is
said to be so accurate that it would neither gain nor lose a second in over 30
yrs. They are used as primary standard in global navigation satellite system
such as GPS.
Atomic Mass Unit (amu)
It
is defined as 1/12 of mass of Carbon-12 atom. It is average of Proton & Neutron
rest mass i.e. 1.67377 x 10-24 gm. The unit is used to express
relative masses of various isotopes of element.
Atomic Number
The
number of protons found inside the nucleus of a particular atom. The modern
periodic table is structured based on this number.
Atomic Orbital
A
quantum state attributed to electron around atom in quantum mechanics to
describe wave characteristic of electron.
It is expressed by mathematical function which is used to calculate
probability of finding any electron in any specific region around atom’s
nucleus.
Atomic Packing Fraction
It
is the ratio of volume of atoms per unit cell to volume of unit cell in a
crystal structure.
Atomic Physics
It
is study of structure of atom, arrangement of electrons around nucleus i.e.
energy states, interaction of atoms with other particles and with electric and
magnetic fields.
Atomic Radius
It
is defined as half the distance between neighboring atoms in a crystal of a
pure element.
Atomic Structure
It
is study of configuration of atom which is essential for knowing many
phenomenon involving electrons. Bohr’s atomic model is early model which has
explained structure of atom to a great extent. Later the limitations of that
model overcome by attributing dual nature to particle. The atomic structure
represents the manner in which electron states are occupied.
Atomic Weight
It
is the weighted average of the atomic masses of an atom’s naturally occurring
isotopes. It is usually expressed in amu.
Atomicity
Total
no. of atoms present in one molecule of an element of a substance is called as
atomicity.
Atto
Prefix
in metric system denoting a factor of 10-18.
Audio Frequency
It
is the frequency range of periodic vibrations audible to human ear. The range
is 20Hz to 20 KHz.
Audio Meter
It
is an instrument for measuring hearing activity for pure tones of normally
audible frequencies.
Aufbau’s Principle
Principle
which states that sub-shell of lower energy in an atom is filled up first while
that of higher energy is filled up later.
Auger Effect
See
Auger electron.
Auger Electron
Physics Dictionary (Contd...)
Anti Logarithm
If ‘y’ is result of logarithm of a given
number ‘x’ then ‘x’ is antilog of ‘y’.
Anti Matter
Matter
which comprises of antiparticles.
Anti Particle
Every
fundamental particle is presumed to have an antiparticle which has same mass
but opposite charge.
Anti-Node
Position
of particles on a wave which vibrate simple harmonically with maximum amplitude
is called anti node.
Antistokes Lines
The
smaller frequency lines in Raman spectra are called Anti-Stokes lines.
Aphelion
The
point in the orbit of planet, at which it is farthest from sun, is termed as
Aphelion.
Aperture
Word
attributed to small opening in an optical system or arrangement or instrument,
through which light is made to pass for focusing it on to any plane.
Aplanatic Points
See
Aplanatism
Aplanatism
The
property of the surface by virtue of which all rays starting from a particular
point object on its axis, after reflection or refraction at the surface,
converge or appear to diverge from a single point image, is called aplanatism
and the particular object and image points are called aplanatic points while
the surface is said to be aplanatic surface with respect to those two
points.
Apochromat
A
term applied to photographic & microscope objectives indicating highest
degree of color correction.
Apogee
Point
at which a satellite or any object orbiting earth is farthest from center of
earth. At its apogee, object travels slower than at any other point in orbit.
Arc
The arc refers to the set of points on a circle that
lie in the interior of circle. The arc can be any part of circumference.
or
The arc is a closed segment of a differentiable curve
in the 2D plane.
Archimedes Principle
As per this principle when a body is fully or
partially submerged in a fluid, a buoyant force from the surrounding fluid acts
on the body. The force is directed upward and has a magnitude equal to weight
on fluid that has been displaced by body.
Area
It
is mathematical quantity that describes the amount of space extended by two
dimensional surface area in the plane.
Artificial Transmutation
An
artificially induced nuclear reaction caused by the bombardment of a nucleus
with subatomic particles or small nuclei. The phenomenon was first observed and
proposed by Ernest Rutherford.
Asteroids
They
are small, rocky objects revolving around the sun that are too small to be
called planets. They are also called Planetoids.
Astigmatism
Defect
in optical system where rays that propagate in two perpendicular planes have
different foci. It is a vision condition that causes blurred vision either due
to the irregular shape of cornea, the clear front cover of the eye, or
sometimes the curvature of the lens inside the eye. An irregular shaped cornea
or lens prevents light from focusing properly on the retina, the light sensitive
surface at the back of the eye. As a result, vision becomes blurred at any distance.
Astronaut
A
person who is trained to pilot, navigate, or otherwise participate as a crew
member of a space craft is known as Astronaut.
Astronomical Unit (au)
An astronomical unit is approximately mean distance
between Earth and Sun. It is a derived constant and used to indicate distances
with in solar system. Symbol au is recommended by international astronomical
union.
1a.u. = 149,597,870.691 KM
Astronomy
It
is the Branch of science which deals with study of universe comprising
celestial objects such as planets, stars etc. outside earth atmosphere. The
study covers evolution, composition, motion, etc of universe.
Astrophysics
The
branch of astronomy that deals with the physical and chemical processes that
occur in stars, galaxies and interstellar space is called Astrophysics.
Astrophysics deals with structure & evolution of stars, properties of
interstellar space and its interaction with system of stars, and with structure
and dynamics of cluster of stars such as galaxies.
Athermanous
Substances
opaque to the thermal radiation are called athermanous.
Atmosphere
The
mixture of gases surrounding the earth or other celestial body, held in place
by gravity is referred to as atmosphere. It forms five distinct layers at
different heights. The earth’s atmosphere consists in ascending order, the
Troposphere (containing 90% of the atmosphere mass), the Stratosphere, the Mesosphere,
the Thermosphere and Exosphere. Atmosphere is composed primarily of Nitrogen
(78%), Oxygen (21%) and plays major role in water cycle, Nitrogen cycle &
Carbon cycle.
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