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Vacancy Diffusion

The diffusion mechanism where in net atomic migration is from lattice site to an adjacent vacancy.


 A normally occupied lattice site in crystal from which an atom or ion is missing.

Vacuum Pressure

If pressure of a fluid is below atmospheric pressure, it is designated as gauge pressure.


According to definition of American vacuum society, “vacuum” refers to a given space filled with gas at pressures below atmospheric i.e. having a density of molecules less than about 2.5 x 1019 molecules /cm3

Valence Band

 For solids, the electron energy levels band that contains valence electrons. These electrons lie in outermost orbits of atoms at absolute zero temperature.


Valence Electrons

 The electrons occupying outermost orbits which usually participate in interatomic bonding are called as valence electrons.

Van De Graff Generator

 It was invented by American physicist Robert J. VandeGraff in 1929. It is a machine capable of building up potential difference of few million volts, and fields close to breakdown field of air which is about 3x106 V/m. The potential is build up with help of static change by friction. 

Vanderwaal’s Gas

 See real gas

Vanderwalls Bond

A secondary interatomic bond between adjacent molecular dipoles, which may be permanent or induced is called Vanderwall’s bond. They are formed as a result of dipole attractions, the dipoles being formed as a result of unbalanced distribution of electrons in asymmetrical molecules.


 Gaseous state below the critical temperature of substance is known as vapor.

Vapor Degreasing

The process in which solvent is heated to boiling point, and parts to be cleaned are hung in the chamber exposing to hot vapors. The vapors get condensed on metal surfaces during which dissolves oil & grease and flows back to solvent condition.


The variance gives us a sense of how closely concentrated a set of values is around its average value, and is calculated in the following manner: Subtract the mean of the distribution from each of the values. Square each result. Add all of the squared results. Divide the result by the number of values minus 1.

Vector Product

 See cross product.


 It is a physical quantity having both magnitude and direction and which obey laws of vector addition.

Velocity Addition Theorem

The addition of any velocity to the velocity of light merely reproduces again the velocity of light. Thus velocity of light is maximum range of velocity attainable in nature.

Velocity Gradient

 It is rate of change of velocity of liquid layers with distance, the distance being taken normal to the direction of flow.


 It is the to & fro motion of a particle from one extreme position to the other and back again. It is motion of a particle from mean position to one extreme position, then to other extreme position and finally back to mean position.

Vibration-Rotational Spectrum

 Molecular spectra obtained in near infrared region.

Virtual Image

A virtual mage is formed at a position where the paths of principal rays cross when projected backward from their paths beyond the lens. Although a virtual image does not form a visible projection a screen, it is no sense “imaginary” i.e. it has a definite position and size and can be “seen” or imaged by the eye, camera or other optical instrument.


When a beam of light diverging from a point after reflection (or refraction) appears to diverge from second point, then the second point is called as virtual image of first. The virtual image cannot be formed on screen.