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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.