# Letter A

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Aberration (in optics)
A defect in the image formed by a lens or curved mirror

Aberration (in astronomy)
It is the apparent displacement in the position of a star as a result of the earth’s motion round the sun.

Absolute Zero
Absolute zero is the lowest possible temperature at which gas would have a zero volume and is obtained by extrapolation of the temperature-volume relation to the volume V=0. At absolute zero, motion of all atoms and molecules ceases. The value of the temperature at absolute zero is $T=0K=273.15^0C$. Practically Absolute zero cannot be attained because it is impossible to establish a system with exactly T = 0 K.

Absolute temperature scale
Absolute temperature scale has absolute zero as the lowest temperature limit. Absolute temperature scales only have positive numbers. Kelvin and Rankin scales are absolute temperature scales.

Absorption spectrum
Absorption spectrum is formed when the light or electromagnetic radiation from source of continuous spectrum is passed through a medium that absorbed radiation of certain wavelengths. These radiations absorbed by the substance under investigation constitute the absorption spectrum.

Acceleration
Acceleration is the rate of change of velocity with time. Mathematically a=v/t. Like velocity acceleration is also a vector quantity having both magnitude and direction. Unit of acceleration is $m/s^2$.

Accelerator
An accelerator is the apparatus used for increasing the kinetic energies of the charged particles and this apparatus is used for research purposes in nuclear and particle physics.

Accuracy
Accuracy as the word suggests is the closeness of a experimental measurement to the standard value of that quantity.

Achromatic lens
Achromatic lenses are those lenses for which are designed to limit the effects of chromatic and spherical aberration. Chromatic aberration (a defect in some optical lens systems that adds false color to images) is corrected by the achromatic lens as it brings two colors to the same focus

Acoustic
Acoustic is a study of sound or sound waves.

Action-reaction forces
pair of forces involved in an interaction that are equal in magnitude and opposition in direction.

Activity
Activity is the number of decays per second of any radioactive substance.

It is the molecular force of attraction in the area of contact between two surfaces of two bodies and this force acts to hold these two bodies together.

Process in which no heat enters or leaves a system is called an adiabatic process

Air resistance
Force of resistance which air exerts on objects moving through it.

Albert Einstein
Albert Einstein was a German born physicist who formulated Special Theory of Relativity (1905) and General Theory of Relativity (1916) which revolutionized the modern thought of the nature of space and time. He also formed a theoretical base for the exploitation of atomic energy $(E=mc^2)$. In 1921 he won the Nobel prize in physics for explaining the photoelectric effect.

Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms (or ‘decays’) into an atom with a mass number 4 less and atomic number 2 less.

Alpha particle
Alpha particles are positively charged particle that consists of two protons and two neutrons bound together. They are emitted by an atomic nucleus undergoing radioactive decay and are identical to the nucleus of a helium atom. Since they have relatively large mass, alpha particles are the slowest and least penetrating forms of nuclear radiation. They can even be stopped by a piece of paper.

Alternating Current
Alternating current or A.C. is an electric current that reverses its direction with a constant frequency (f).

Altimeter
An altimeter is an instrument that is used to measure and indicate the height above sea level at which an object, such as an airplane, is located.

Ammeter
Ammeter is an instrument that measures electric current in Amperes.