A term applied to photographic & microscope objectives indicating highest degree of color correction.


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.


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.                                                                                         


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.


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.


They are small, rocky objects revolving around the sun that are too small to be called planets. They are also called Planetoids.


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.


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


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.


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.


Substances opaque to the thermal radiation are called athermanous.


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.

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.