A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Dissociation
It is phenomenon
of infinite separation of atoms of a molecule.
Distance
of Closest Approach
It is the
distance from the nucleus of an atom at which the alpha particle comes to rest
and its kinetic energy is totally converted into electrostatic potential
energy.
Distance
It is the total
length of actual path covered by a moving body.
Distortion
The image of a
plane square like object placed perpendicular to the principal axis is not of
the same geometrical shape as the object. This aberration is called as
distortion and arises due to variation of magnification with lateral distance
of an object point from the axis of lens.
Diathermanous
Substances which
are transparent for thermal radiations are called the diathermanous.
Divergence
It represents the
total amount of flux escaping an infitesimal volume at a point in vector field.
D-lines
of Sodium
The bright
doublet (line spectrum) which is responsible for yellow light from Sodium lamp
is called as D-lines of sodium. The line emission spectrum of lamp consists of
two wavelengths 5890 Ao & 5896Ao. The transition
which gives rise to doublet is from 3P3/2 & 3P1/2 to
3S1/2 levels. The doublet is due to splitting of 3P level into state
with angular momentum 3/2 & 1/2 by magnetic energy of electron spin in
presence of internal magnetic field caused by orbital motion of electron.
Domain
Theory
Theory in
magnetism as per which any ferromagnetic or ferri-magnetic material that is at
a temperature below curie temperature, is composed of small volume regions in
which there is mutual alignment in the same direction of all magnetic dipole
moments. Such a region is called a domain, and each one is magnetized to its
saturation magnetization. Adjacent domains are separated by domain boundaries
across which the direction of magnetization gradually changes. Normally,
domains are macroscopic in size and for polycrystalline specimen; each grain
may consist of more than a single domain. Thus, in a macroscopic piece of
material, there will be large number of domains, and all may have different
magnetization orientation. The magnitude of magnetization field for entire
solid is vector sum of magnetizations of all domains, each domain contribution
being weighted by its volume fraction.
Donor
It is an impurity
atom when added to host crystal lattice (semiconductor), contributes an
electron to conduction bond after formation of covalent bonds with four valence
electrons.
Doping
The intentional
addition of foreign atoms (acceptors/donors) into semiconductor with controlled
concentrations.
Doppler
Effect (Light)
Change in
frequency of light radiation when source or observer move with respect to each
other.
Doppler
Effect (Sound)
Apparent change
in the pitch of sound due to relative motion between source and observer is
called as Doppler effect.
Dose
(Radiation)
Materials other
than air exposed to ionizing radiation will differ in their rate of energy
absorption. It is therefore necessary to have a standard for defining energy
absorption by ionization in different materials. The quantity for this purpose
is called absorbed dose ‘D’ and measures energy deposited by ionizing radiation
per unit mass of material. The SI unit is Gray equal to absorption of 1 joule per Kg of material.
Dose
Equivalent
The
effect of a particular radiation on a biological system depends on absorbed
dose ‘D’ and on quality factor ‘QF’ of radiation. Dose equivalent is product of
D
& QF.
Dosimeter
Device which
measures absorbed dose deposited in its sensitive volume by ionizing radiation.
Dosimetry
Subject which
deals with measurement of absorbed dose or dose rate resulting from interaction
of ionizing radiation with matter.
Dot
Product
Dot product of
two vectors is the product of the magnitudes of two vectors and cosine of angle
between them.
Double
Refraction
Some crystals
have the property that the light rays incident on them splits into two
refracted rays inside the crystal. The property is called double refraction.
Doublet
Combination of two simple lenses is called as doublet.
Drift
Tube
It is used in
linear accelerator in which there is arrangement of dynodes at varying potential
to drift electrons to higher velocities.
Drift
Velocity
Velocity attained
by charge carriers under influence of electric field. The velocity is directly
proportional to electric field and inversely proportional to mass.
Dry
Bulb Temperature
It is the temperature
which is usually referred to as simply ambient air temperature.
Dry
Cell
It is a galvanic
electric cell in which the chemicals in the electrolyte are made into a paste such
as ammonium chloride and manganese dioxide
so that they cannot easily
spill from their container. A
chemical reaction within the battery creates an electrical charge that flows
from inside to an outer circuit that is connected to an electrical device.
