## Pages

### PHYSICS DICTIONARY

Aberration
Defect in images formed by optical system arrangement.

Aberration of Starlight
The phenomenon of apparent displacement of star in the sky due to finite speed of light and motion of earth in its orbit about the sun is known as aberration of starlight.

Abrasive
A hard and wear resistant material that is used to wear, grind or cut away other material.

Abscissa
The horizontal coordinate of a point in a plane Cartesian coordinate system obtained by measuring parallel to the X-axis is called as abscissa.

Absolute Error
The difference between true value and measured value is called as absolute error.

Absolute Humidity
Absolute humidity denotes the amount of humidity in air regardless of the saturation level, expressed as the total mass of water molecules per air volume.

Absolute Permeability
Constant of proportionality between magnetic density and magnetic field strength of a material put in uniform magnetic field.

Absolute permittivity
Permittivity of vacuum is called absolute permittivity and its value is 8.85 x 10-12 F/m.

Absolute pressure
When pressure is measured above absolute zero (or complete vacuum), it is called as absolute pressure.

Absolute Temperature
Temperature measured using Kelvin scale when zero is absolute zero.

Absolute Zero
The temperature at which entropy of a system reaches minimum is referred as absolute zero.

Absorbed Fraction
A term used in internal dosimetry. It is the fraction of photon energy (emitted within a specified volume of material) that is absorbed by volume. The absorbed fraction depends on source distribution, photon energy, size, shape and composition of volume.

Absorbing Power
The ratio of amount of radiations absorbed by the body in a certain time to the amount of radiations incident on it in the same time is called absorbing power of body.

Absorptance
Ratio of amount of radiation absorbed by a surface to the amount of radiation incident upon it is called as absorptance. It is measure of ability of an object to absorb radiation.

Absorption Spectrum
Absorption spectrum is the characteristic property of absorbing material. Using this spectrum, one can identify what are the elements present in absorbing material. It is due to absorption of radiation by matter. Absorption is based on Kirchhoff’s law, which states that a substance which emits particular wavelength of radiation when excited also possess the property of absorbing the same wavelength from incident radiation when unexcited. Absorption spectra consist of dark lines over a bright background. When the white light is passed through the gas in atomic state (say sodium vapor), line absorption spectrum is formed. When white light is passed through molecular gas (say iodine vapor), band absorption spectrum is formed.

Absorption
The optical phenomenon where by the energy of a photon of light is assimilated with in a substance, normally by electronic polarization or by an electron excitation event.

Absorptivity
It is fraction of radiant energy falling upon the body which is absorbed or transformed into heat. This ratio varies with character of the surface and the wave length of incident energy.

Abundance
The ratio of the number of atoms of a specific isotope in a mixture of isotopes of an element to the total number of atoms present is called abundance. It is expressed in percentage.

Acceleration
It is a physical quantity which is defined as rate at which velocity of an object change with time.

Acceleration due to Gravity
The acceleration acquired by body due to gravitational pull is known as acceleration due to gravity.

Accelerator
Device used to accelerate charged particles to gain high energies. They are used in medical applications, making of radio isotopes etc.