Acceptor

Terminology used in semiconductor physics which refers to the doping atom of group-III element of periodic table which are deficient of electrons and hence leads to holes when doped appropriately. The semiconductor having holes as majority charge carriers is called P-Type semi conductor formed due to addition of acceptors.

Accuracy

The degree of closeness of measurement value of a quantity to the expected value is called accuracy. In other words, the extent to which a given measurement agrees with the standard value for that measurement. 

Achromatic Combination of Lenses

A lens combination, for which the image of an object illuminated with white light is distinctly obtained at the same place for all colors of light, is said to be an achromatic combination of lenses.

Achromatic Lens

A lens combination, for which the image of an object illuminated with white light is distinctly obtained at same place for all colors of light, is said to be an achromatic combination of lenses.

Achromatic Prism

A Prism composed usually of two prisms which have unequal dispersive powers. They are combined in such a fashion that a ray of light passed through them is refracted or bent into a new position, but it is free from color.

Acoustic Impedance

It is the ratio of acoustic pressure to acoustic volume flow. Acoustic impedance varies strongly when the frequency of sound is changed. The acoustic impedance at a particular frequency indicates how much sound pressure is generated by air molecules vibration at that frequency.

Acoustics

The branch of physics that deals with the process of generation, reception and propagation of sound.

Actinides

Any of the series of 15 metallic elements from Actinium (atomic number=89) to Lawrencium (atomic no. 103) in the periodic table. They are all radioactive. The heavier elements are extremely unstable and not of naturally occurring.

Actinium Series

Set of unstable heavy nuclei which decay radioactively beginning with Actino-Uranium, isotope of Uranium U-235 and end with Actinium D, constituting non radioactive isotope Lead, of mass number 207. 

 Actinometer

It is radiometric instrument used for measuring heating capability of radiation. They are used chiefly for meteorological measurement of terrestrial and solar radiation. 

Activator

The name assigned to the impurity atoms occurring in relatively small concentrations in host material or a small stoichometric excess of host material, which leads host material to exhibit phenomenon of Luminescence.

Active Device

It is a device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Active devices are components that are capable of controlling voltages or currents and create switching action in the circuit.

Activity

The number of nuclei of a given radioactive sample disintegrating per second is referred to as activity of radioactive substance.

Acute Angle

Angle with a measure between 0o & 90o.

Adhesion

It is the force of attraction between molecules of different substances. 

Adiabatic Demagnetization:

It is the process of cooling certain materials to extremely low temperatures down to milli Kelvin. The process was first proposed by chemists Peter Debye (1926) and William Francis Giauque. The process capitalizes on paramagnetic properties of some materials (usually in gaseous form) to reduce their temperatures to very low range. The process involves reducing temperature of sample (usually gas) by keeping in contact with cold reservoir; with magnetic field induced in it till thermal equilibrium is attained. After thermal equilibrium is attained, magnetic field intensity is increased thus reducing entropy due to orderly alignment of particles. After the isolation of sample from reservoir, the magnetic field is reduced which results in still further reduction of temperature by many fold.

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: 

TVγ-1 = const; PVγ = const; P 1-γT γ= const; γ = Cp/Cv

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.