PHYSICS DICTIONARY

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Dimension

Dimension of a space or object is informally defined as minimum number of coordinates required to specify any point with in it. Dimension of physical quantity is combination of basic physical dimensions (usually mass, length, time, temperature etc) which describe it.

Dimensional Analysis

Analysis to find relation among physical quantities by using their dimensions.

Dimensional Equation

When the dimensional formula of physical quantity is expressed in the form of an equation, then such equation is known as dimensional equation. 

Dimensional Formula

An expression showing the powers to which the fundamental units are to be raised to obtain one unit of derived quantity is called as dimensional formula.

Diode

It is an electronic device that rectifies electric current i.e. current flow in one direction only.

Dipole Moment

The product of either charge in an electric dipole with the distance separating them.

Dipole Relaxation

When external alternating field is applied to some materials it leads to formation of permanent dipole moment, the phenomenon termed as dipole relaxation. 

Dipole(Electric)

A dipole is an entity in which equal positive and negative charges are separated by a small distance.

Dirac Constant

Constant used in quantum mechanics equal to Planck’s constant divided by 2π. It has a value of  1.05 x 10^-34 joules seconds.

Direct Band Gap Semiconductor

The semi conductor in which an electron in conduction band can fall to any empty state in valence band giving off the energy difference (energy gap) as a photon of light. 

Direct Current

Continuous electric current flowing in one direction without substantial variation in magnitude is called as direct current.

Directly Ionizing Radiation

Comprises charged particles (electrons, protons, charged particles, heavy ions) that deposit energy in the absorber through a direct one-step process involving Coulomb interactions between the directly ionizing charged particle and orbital electrons of the atoms in the absorber.

Discriminator

Discrimination in radiation detection circuits refers to the process of distinguishing between different types of radiation on the basis of pulse height. A discriminator circuit selects the minimum or maximum pulse height that is to be counted.

Disintegration Constant

See decay constant.

Disintegration

Physical process by which atomic nucleus emits particles or energetic rays, which make composition of nucleus to change.

Dislocation

A linear crystalline defect around which there is atomic misalignment.

Dispersion of Light

The phenomenon of splitting of light into different colors is called as dispersion. It arises due to variation of refractive index with wavelength.

Dispersion

Whenever the speed of wave depends on its frequency, supporting medium is called dispersive. The phenomenon of changing of shape by wave having range of frequencies in a medium is called as dispersion.

Dispersive Power of Grating

The rate of variation of angle of diffraction with the wavelength of light is called dispersive power of grating.

Dispersive Power of Prism

The rate of variation of the angle of deviation with the wavelength of light is called as dispersive power of prism.

Dispersive Power

The ratio of angular dispersion between two colors to deviation of mean ray produced by the prism is called dispersion power of material of prism for those colors. 

Displacement

The shortest distance between the initial and final positions of a body.

                                                  or

Distance travelled by an object in a specified direction.

Displacement Current

Changing electric field leads to a current which flows as long as electric field is changing and produces magnetic field. The current is called as displacement current.   

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Diamagnetism

It is a very weak form of magnetism that is non permanent and persists only while an external field is being applied. It is induced by a change in orbital motion of electrons due to an applied magnetic field. The magnitude of induced magnetic moment is extremely small, and in a direction opposite to that of applied field. Thus relative permeability is less than unity and magnetic susceptibility is negative. When placed between poles of strong electromagnet, diamagnetic materials are attracted towards regions where field is weak.        

Diamond

A pure extremely hard, high refractive crystalline form of carbon naturally crystallized in isometric system, which is usually colorless and is used as a gemstone and in abrasives, cutting tools etc.

Diatomic Molecule

Molecule containing two atoms.

Diatonic Scale

It is a musical scale in which 8 tones of some definite frequencies are arranged such that the frequency of the 8^th tone is twice the frequency of 1^st tone.

Dielectric Constant

It is macroscopic quantity that measures how effectively an electric field polarizes the material. It is property of a dielectric material which determines the amount of electrostatic energy that can be stored by material when a given voltage is applied to it.  

Dielectric Materials

They are materials which are electrical insulators or in which an electric field can be sustained with a minimal dissipation of power. There are two types of dielectrics: i) Polar ii) Non-polar. The dielectrics are used for separation of conductors in capacitors to store energy.

Dielectric Strength

The maximum electric field, a dielectric can withstand before reaching breakdown condition is called as dielectric strength of dielectric.

Differential Equation

A differential equation is the mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders. 

Differentiation

Derivative of continuous function at a given value will give rate of change of function near that value.

Diffraction

The bending of waves at the corners of an object is called diffraction.

                                            (or)

Deviation of light from rectilinear path is called as diffraction. The phenomenon was discovered by Grimald in 1665.

Diffusion Coefficient

It is the constant of proportionality between the diffusion flux and the concentration gradient in the Fick’s first law. Its magnitude is indicative of rate of atomic diffusion.

Diffusion Flux

The quantity of mass diffusing through and perpendicular to a unit crossectional area of material per unit time is called as diffusion flux. 

Diffusion Pump

Pump used to attain high vacuum of order 10^-6 mbar.  The pump consists of leak tight enclosure in which there is bath of fluid having low boiling point, surrounded by heating coil. Water is circulated on the side walls of container to prevent thermal run away. The fluid which gets heated turns to vapor and rises above. Upon cooling, the fluid falls again to bath with high speed. While falling, the fluid takes the air molecules and thus creates low pressure region at the top. Due to this low pressure region, there is transport of molecules from the system to be evacuated. The molecules diffuse into vapor jet in a diffusion manner.  

Diffusion

Physical phenomenon of transport of mass by atomic motion due to difference in concentration levels is called as diffusion.

Diffusivity (Mass ) or Diffusion Coefficient

The rate, at which quantity of a substance diffuses from one region to another region through each unit of crossection per unit time when the volume concentration gradient is unity, is called as diffusivity.

Diffusivity (Thermal)

It is measure of ability of a substance to transmit heat for a difference in temperature. It is expressed as thermal conductivity divided by the product of specific heat capacity and density. It is measure of thermal inertia.

Digit

Symbol used to make numerical.

Digital

Technology of representing data using discrete values.

Dilatometer

Scientific instrument that measures volume changes caused by a physical or chemical process.

Dilute Magnetic Semiconductor

Alloy formed by replacing few original atoms in semiconductor by magnetic elements. The most common dilute magnetic semiconductors are II-VI compounds (like CdTe, ZnSe, CdSe, CdS, etc.), with transition metal ions (e.g. Mn, Fe or Co) substituting their original cations. The information is not only carried by charge but also spin of electrons, an additional degree of freedom. The compounds are useful for novel electronic devices. 

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Decay

It is process of disintegration of nucleus.

Decay Constant

Decay constant of radioactive nuclide describes the rate of decay i.e., its decay probability in unit time.

or

The probability that fraction of number of atoms of a radioactive nuclide decay in unit time.

Decay Energy or Q value

Decay energy is a term used to quantify the energy released during the decay process. It can be used to determine whether a certain decay mode for a nucleus is possible or not.

Deceleration

 Decrease in acceleration.

Deci

It is a prefix in the metric system denoting a factor of one tenth.

Decibel

A unit used to express relative difference in power or intensity, usually between two acoustic or electric signals, equal to ten times the common logarithm of ratio of two levels.

Deewar Flask

Flask invented by Scottish physicist, Sir James Deewar in 1892. It is an arrangement of double walled vessel separated by vacuum. Vacuum greatly reduces the transfer of heat preventing temperature change. Glass is used, as it is bad conductor of heat; its surface is usually lined with a reflective material like silver to reduce transfer of heat by radiation. This is encapsulated in plastic so that it will further reduce loss of heat.

Degeneracy

It is a concept in quantum mechanics as per which physical state having same energy can have different arrangements.

Degenerate States

The states of a system with different Eigen functions but all corresponding to same Eigen values are known as degenerate states.

Degree of Degeneracy

Number of linearly independent Eigen functions corresponding to same Eigen value.

Degrees of Freedom

The number of independent coordinates required to express the position and motion of a system comprising one or more particles is called degrees of freedom of that system. 

Demagnetization

Process by which magnetization is made zero for a material.

Demodulation

The process of recovering the audio signal from modulated wave is known as demodulation. Demodulation involves rectification of modulated wave and removing carrier frequency to allow audio signal reach load i.e. speaker.

Densitometer

It is a device that measures degree of darkness (optical density) of a photographic or semi-transparent material or a reflecting surface.

Density Operator

The quantum analogue of classical density function is known as density operator.

Density

Mass per unit volume of the substance.

Depleted Uranium

Uranium having a percentage of U²³⁵ smaller than 0.7% found in natural uranium is called as depleted Uranium. It is generally obtained as by product from nuclear reactor or as by product from uranium.

Depletion Layer

When a P-N junction forms and combines with holes to form negative ions on P-side and electrons leave from N-type region there is formation of positive ions on N-type side. This combined region on either side is also called space-charge region. This region is an insulating region.

Derivative

The essence of calculus is the derivative. The derivative is the instantaneous rate of change of a function with respect to one of its variables. This is equivalent to finding the slope of tangent line to the function at a point.

Derived Air Concentration (DAC)

The Derived Air Concentration (DAC) for any radionuclide, is that radioactive concentration in air, such that if a worker works in such environment, for a year (50 weeks of 40 hour each for a total of 2000 hours per year), would result in the Annual Limit Intake (ALI) for inhalation. The normal breathing rate, based on Reference Man, is 2,400 m³ per year at work. The Derived Air Concentration is the ALI divided by the volume breathed in a year at work.

Destructive Interference

Interference at the points, where intensity of light is least is called as destructive interference.

Deuteron

It is nucleus of Hydrogen-2 atom, consisting of one Proton & one neutron bound together with a total binding energy of 2.25MeV. The Hydrogen-2 atom, a stable isotope of Hydrogen with a natural abundance of 0.015% is called Deuterium, which consists of Deuteron (nucleus) and electron.

Deviation

The difference between the measured value and the arithmetic mean is called as deviation.

Devitrification

It is the process in which a glass (non-crystalline or vitreous solid) transforms into a crystalline solid.

Dew Point

It is the temperature at which the present amount of humidity in air starts to condensate.

Dextrorotatory

Those substances which rotate the plane of polarization of the incident plane polarized light in a clock wise direction when seen in the direction of light are called dextrorotatory substances.

Diaphragm

It is a thin disk which vibrates when receiving or producing sound waves as in microphone or loudspeaker.

Diagnostic Physics

It is the branch of physics associated with diagnostic procedures that use X-rays, gamma rays, ultrasound, radio frequency radiation and magnetic sources. In this field, physicists advise on protocols & technology used for creation of images that are generated by these diagnostic methods.