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Dielectric Constant:

It is the 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.   



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


Distance travelled by an object in a specified direction.


Dielectric strength:

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


Debroglies Hypothesis:

Hypothesis put forward by French scientist Debroglie, as per which materialistic particle possess both wave and particle characteristics


Debroglie wavelength:

Ratio of Planck’s constant to momentum of particle. 


Drift velocity:

The steady average velocity imposed by electric field on the charged particle. 


Dulong & Petit’s law:

The atomic heat capacity for all solids is same and is nearly equal to 25 Jmol-1K-1 and is independent of temperature. 


Dispersion of light:

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


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. 


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. 

Davisson – Germer experiment:

Experiment which gave evidence of wave nature possession by materialistic particle (electron) for the first time.



It is unit of electric dipole moment.

1 Debye = 3.33 x 10-30 Coulomb-meter 


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. 



It is the total length of actual path covered by a moving body.


Dilute Magnetic Semiconductor:

Alloy between semiconductor and magnetic elements. The information is not only carried by charge but also spin of electrons on additional degree of freedom. The compounds are useful for novel electronic devices.


Dispersive power of grating:

It is defined as rate of variation of angle of diffraction with wavelength.

Derived Air Concentration: 
It is defined as radioactive concentration level in the air such that if a worker works for whole year(assumed to be 2000 hours) in such an atmosphere , he or she would get exposed to annual limit intake set by “international Commission on radiological Protection”. 

Dose equivalent:  The effect of a particular radiation on a biological system depends on absorbed dose ‘D’ and on quality factor Quality Factor ‘QF’ of radiation. Dose equivalent is product of D & QF.

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


Damped vibration:

This is type of vibration in which amplitude decreases continuously with time and finally dies out because of dissipation of energy due to frictional forces.


Deewar flask:

A double walled vessel, in which the space between two walls is kept at high vacuum and walls provided with silver lining to give good insulation for thermal, electrical & radiation. 

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.  

Dulong & Petit’s law: The atomic heat capacity for all solids is same and is nearly equal to 25JMol-1K-1 and is independent of temperature.



It is the mechanical property which is a measure of degree of plastic deformation that has been sustained at fracture. It is expressed quantitatively as either percent elongation or percent reduction in area from a tensile test.



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