PHYSICS DICTIONARY

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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.  

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.

 

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Angstrom

It is the unit of length usually used to express sizes at atomic scale. It is equal to 10^-10 meter.

Angular Acceleration

It is defined as the rate of change of angular velocity.

Angular Aperture

Angular aperture of an objective is largest angular extent of wave surface which it can transmit.

Angular Displacement

When a particle describes circular path, the angle described by the radius vector in a given time interval is called angular displacement. Its direction is perpendicular to plane of rotation along the axis.

Angular Momentum

It is the ability of a body by virtue of which it imparts rotatory motion to other bodies. Angular momentum is product of linear momentum and perpendicular distance of direction of linear momentum from center of rotation.

Angular velocity

It is defined as rate of change of angular displacement.

Anisotropy

It is the phenomenon of exhibiting different values of a physical property in different crystallographic directions.

Annealing

It is the process of heat treatment of metal by which microstructure and consequently properties of a material are altered. Annealing is generally heating of metal to specific temperature, for certain duration and then allowed to cool slowly thus softening by re-crystallizing. Cold worked metals usually undergo this for relieving strain.

Anode Rays

Rays of positively charged particles formed inside gas discharge tubes. These rays were discovered by Goldstein in 1886.

Anode

It is the electrode in an electrochemical cell that experiences oxidation, or gives up electron.

Anodizing

It is an electrochemical metal finishing process used for increasing thickness and toughness of the naturally occurring protective oxides on surface of metals. The process is called anodizing, as the part to be chemically treated forms the anode electrode of electric circuit. The process increases non-corrosiveness & non-conductivity. 

Anomalous Dielectric Dispersion

The fall in permittivity of a dielectric material with increasing frequency of external electric field is a phenomenon called anomalous dielectric dispersion. 

Anomalous Expansion of Water

When temperature of water is raised from 0 ^oC, its volume decreases up to 4 ^oC and above  4^oC its volume increases. This is called anomalous expansion. It is due to formation of more number of Hydrogen bonds.

Anomalous Zeeman Effect

When spectral lines of an atom split into three or more unequally spaced lines in the presence of magnetic field then the phenomenon is called as anomalous Zeeman effect. This usually happens for the systems where net spin i.e. spin quantum number is not zero.

Antenna

A type of transducer that converts RF fields into alternating current or vice versa. A receiving antenna intercepts RF energy and delivers oscillating electric current to electronic equipment, and the transmitting antenna is fed with electric current to generate RF field. They consist of metallic conductors, electrically connected to transistor or receiver.

Anti Ferromagnetism

Phenomenon of magnetic moment (spin moments) of neighboring atoms or ions aligned in exactly opposite directions.

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.

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Alternating Current

Current in which charge periodically reverses and average value is zero. It usually implies a sinusoidal variation of current and voltage.

Altitude

The height of an object with reference to sea level or ground level is called as altitude.

Ammeter

Device used to measure current. It is designed with a small resistance put in parallel to the galvanometer. It is generally connected in series with in a circuit, in which electric resistance is to be measured.

Amorphous

Materials that do not crystallize, i.e. in which, long range atomic order is absent, are referred to as amorphous.         

Ampere

It is the constant current which if maintained in two straight parallel conductors of infinite length and of negligible cross-section and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 N/M of length.  

Amperes Law

A law of electromagnetism which expresses contribution of a current element of length dl to the magnetic induction ‘B’ at a point near the current carrying element. The law was derived by A M Ampere. Whenever an electric charge is in motion, there is magnetic field associated with that motion. The flow of charges through a conductor sets up magnetic field in the surrounding region. Any current may be considered to be broken up into infitesimal elements of length dl and each such element contributes to magnetic induction at every point in neighborhood. The contribution dB of the element is found to depend upon current I, the length dl of element, distance ‘r’ of point ‘p’ from current element, and angle ‘Ѳ’ between current element and line joining point ‘p’.

              dB=  (KldIsinѲ/r²);  K = µ_o/4Π = 10^-7 Weber/Ampere-meter  

Amplifier

Device used for increasing the strength of a weak signal.

Amplitude Modulation

The process of changing the amplitude of high frequency carrier wave in accordance with intensity of signal without changing frequency, is called amplitude modulation.

Amplitude

The maximum displacement of a vibrating particle in simple harmonic motion is called as amplitude. 

Analog

Analog is defined as a mechanism in which data is represented by continuously variable physical quantities.

Anelasticity

In some engineering materials, there also exists time dependent elastic strain component. That is elastic deformation will continue even after stress application and upon load release, some finite time is required for complete recovery. This time dependant elastic behavior is known as anelasticity.    ther particles and with electric and magnetic fi

Anemometer

The term is derived from Greek word “anemos”, meaning wind. It is device used for measuring speed of air flow in wind tunnels, and in other gas flow applications.

Angle

The amount of space between two straight lines having a common end point usually measured in degrees.

Angle of Incidence

Angle between incidence ray and normal to the surface is called angle of incidence.

Angle of Minimum Deviation

When the angle of incidence in a prism is increased, the value of angle of deviation first decreases and then increases. The minimum value of deviation is called the angle of deviation.

Angle of Reflection

It is the angle between reflected ray and normal to the surface, from which light is reflected.

Angle of Refraction

It is the angle between refracted ray and the surface from which light is refracted.

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Adiabatic Process

It is a thermodynamic process in which there is change in temperature, no heat exchange with surroundings & Boyle’s law doesn’t hold good. Adiabatic relation between P, V & T are as follows: 

TV^γ-1 = const; PV^γ = const; P ^1-γT ^γ= const; γ = C_p/C_v

Admittance

It is the term coined by Oliver Heaviside. It is inverse of impedance. In fact, it is vector sum of Conductance and Susceptance. It determines with how much ease an alternating current flows through a complex circuit.

Adsorbate

The gas or liquid that is accumulated over the surface of a liquid or solid is referred to as adsorbate.

Adsorbent

The solid or liquid on whose surface, molecules of other substance are adsorbed.  

Adsorption

It is the process of attracting foreign atoms or molecules on to the host liquid or solid substance. Any surface of a solid or liquid exhibit residual forces of attraction normal to surface; hence molecules or atoms impinging on surface are adsorbed.

Aerodynamics

It is branch of fluid dynamics which deals with study of forces and the resulting motion of objects, when air interacts with them.

Aerosol

It is a colloidal suspension of solid or liquid particles in a gas; smoke or fog. Usually aerosol is sealed in a metal container under pressure with an inert gas or other activating agent and released as spray or through nozzle.

After Glow

See Phosphorescence. 

Air Break Down

It is dielectric breakdown of air when intense electric field is applied between two electrodes. For instance, dielectric breakdown strength of dry air at STP between spherical electrodes is 33KV/cm.

Air

Mixture of gases with traces of water vapor, etc which makes up earth’s atmosphere. Composition is 78% Nitrogen, 21% Oxygen, 0.03% CO₂, 0.00005% Hydrogen, 0.933% Argon, 0.0018% Neon, 0.0005 Helium, 0.0001 Krypton etc.  

Albedo

The fraction of the total light incident on a reflecting surface, especially a celestial body, which is reflected back in all directions, is called as Albedo.

Algorithm

A finite set of unambiguous instructions performed in a prescribed sequence to achieve a goal, especially a mathematical rule or procedure used to compute a desired result. Algorithms are basis for most computer programming.

Allobar

It is a form of an element differing in isotopic composition from naturally occurring form. 

Allotropy

It is the property of substance to exist in two or more different crystal structures.

Alloy

A metallic substance that is composed of two or more elements is called as alloy.

Alpha Iron

Name used in metallurgy for iron or solid solution with iron as main continent with a BCC (Body centered cubic) structure.

Alpha Particle Spectrum

The line spectrum formed due to emission of alpha rays from isotopes due to nuclear states transition.

Alpha Particle

Positively charged particle identical to Helium nucleus (i.e. two protons and two neutrons) emitted from radioactive substance having heavy nuclei. They are typically emitted due to transition in energy states of parent nucleus to form daughter nucleus.