Saturday, June 27, 2009

6.ATOMIC PHYSICS.

6.ATOMIC PHYSICS.
1.How electric current passes through gases?
Electric current may be passed through a gas if by some mechanism, charged particles are produced in the gas. This can be done
(i) by applying a large potential difference across a gas column at very low pressure and (ii) by allowing X-rays to pass through the gases.
2.Define positive coulumn.
As the pressure is reduced to the order of 10 mm of Hg, the irregular streaks broaden out into a luminous column extending from the anode, almost upto the cathode. This column is known as the positive column.
3.Define Crooke’s dark space.
With further reduction in pressure to around 0.01 mm of Hg, the positive column disappears and Crooke’s dark space fills the whole tube.
4.What are cathode rays?
When the pressure reduced to around 0.01 mm of Hg, the positive column disappears and Crooke’s dark space fills the whole tube.At this stage,the walls of the glass tube fluoresce with green colour. This greenish glow in the final stage of the gaseous discharge is found to be a fluorescence of the glass produced by some invisible rays emanating from the cathode . These rays are called cathode rays and are found to be electrons.
5.State the properties of cathode rays.
(i) They travel in straight lines.
(ii) Cathode rays possess momentum and kinetic energy.
(iii) Cathode rays produce heat, when allowed to fall on matter.
(iv) Cathode rays produce fluorescence when they strike a number
of crystals, minerals and salts.
(v) When cathode rays strike a solid substance of large atomic
weight, X-rays are produced.
(vi) Cathode rays ionize the gas through which they pass.
(vii) Cathode rays affect the photographic plates.
(viii) The cathode rays are deflected from their straight line path
by both electric and magnetic fields. The direction of deflection shows
that they are negatively charged particles.
(ix) Cathode rays travel with a velocity upto (1/10)th of the velocity
of light.
(x) Cathode rays comprises of electrons which are fundamental
constituents of all atoms.
6.State the Properties of Canal rays
(i) They are the streams of positive ions of the gas enclosed in the
discharge tube. The mass of each ion is nearly equal to the mass of the
atom.
(ii) They are deflected by electric and magnetic fields. Their
deflection is opposite to that of cathode rays.
(iii) They travel in straight lines.
(iv) The velocity of canal rays is much smaller than the velocity of
cathode rays.
(v) They affect photographic plates.
(vi) These rays can produce fluorescence.
(vii) They ionize the gas through which they pass.
7.State the principle of Millikan’s oil drop experiment
Millikan’s oil drop method is based on the study of the motion of uncharged oil drop under free fall due to gravity and charged oil drop in a uniform electric field. By adjusting uniform electric field suitably, a charged oil drop can be made to move up or down or even kept balanced in the field of view for sufficiently long time and a series of observations can be made.
8.Define Thomson atom model.
According to Thomson, an atom is a sphere of positive charge having a
radius of the order of 10-10m. The positive charge is uniformly
distributed over the entire sphere and the electrons are embedded in
the sphere of positive charge . The total positive charge inside the atom is equal to the total negative charge carried by the electrons, so that every atom is electrically neutral.
9.State the drawbacks of Thomson,s atom model.
(i) According to electromagnetic theory, the vibrating electron
should radiate energy and the frequency of the emitted spectral line
should be the same as the electron. In the case of hydrogen atom,
Thomson’s model gives only one spectral line of about 1300 Å. But the
experimental observations reveal that hydrogen spectrum consists of
five different series with several lines in each series.
(ii) It could not account for the scattering of α-particles through
large angles.
10.State the observations and conclusions of Rutherford,s atom model.
(i) Most of the α particles either passed straight through the gold
foil or were scattered by only small angles of the order of a few degrees.
This observation led to the conclusion that an atom has a lot of empty space .
(ii) A few α particles were scattered in the backward direction, which led Rutherford to conclude that the whole of the positive charge was concentrated in a tiny space of about 10-14m. This region of the
atom was named as nucleus. Only a small number of particles approaches the nucleus of the atom and they were deflected at large angles.
11.Define :Distance of closest approach
An alpha particle directed towards the centre of the nucleus will move
close upto a distance ro, where its kinetic energy will appear as electrostatic potential energy. After this, the α particle
begins to retrace its path. This distance ro is known as the distance of
the closest approach.
12. State the results of Rutherford alpha scattering experiment.
Based on the results of α-particle scattering experiment,
Rutherford suggested the following.
(i) Atom may be regarded as a sphere of diameter 10-10m, but
whole of the positive charge and almost the entire mass of the atom is
concentrated in a small central core called nucleus having diameter of
about 10-14m
(ii) The electrons in the atom were considered to be distributed
around the nucleus in the empty space of the atom. If the electrons
were at rest, they would be attracted and neutralized by the nucleus.
To overcome this, Rutherford suggested that the electrons are revolving
around the nucleus in circular orbits, so that the centripetal force is
provided by the electrostatic force of attraction between the electron
and the nucleus.
(iii) As the atom is electrically neutral, the total positive charge
of the nucleus is equal to the total negative charge of the electrons in
it.
13.State Bohr,s quantization condition.
An electron cannot revolve round the nucleus in all possible
orbits. The electrons can revolve round the nucleus only in those
allowed or permissible orbits for which the angular momentum of the
electron is an integral multiple of(h/2π) (where h is Planck’s constant =
6.626 × 10-34 Js). These orbits are called stationary orbits or nonradiating orbits and an electron revolving in these orbits does not
radiate any energy.This is called Bohr’s quantization condition.
14.State Bohr,s frequency condition.
An atom radiates energy, only when an electron jumps from a stationary orbit of higher energy to an orbit of lower energy. If the electron jumps from an orbit of energy E2 to an orbit of energy E1, a photon of energy hν = E2 – E1 is emitted. This condition is called Bohr’s frequency condition.

15.Define Lyman series
When the electron jumps from any of the outer orbits to the
first orbit, the spectral lines emitted are in the ultraviolet region of
the spectrum and they are said to form a series called Lyman
series .Here, n1 = 1, n2 = 2,3,4 …
16.Define Balmer series
When the electron jumps from any of the outer orbits to the
second orbit, we get a spectral series called the Balmer series. All the
lines of this series in hydrogen have their wavelength in the visible
region. Here n1=2, n2 = 3,4,5 …
17.Define Paschen series
This series consists of all wavelengths which are emitted when
the electron jumps from outer most orbits to the third orbit. Here n2
= 4,5,6 … and n1 = 3. This series is in the infrared region with the
wave number given by
18.Define Brackett series
The series obtained by the transition of the electron from
n2 = 5, 6... to n1 = 4 is called Brackett series. The wavelengths of these
lines are in the infrared region.
19.Define Pfund series
The lines of the series are obtained when the electron jumps from
any state n2 = 6, 7... to n1=5. This series also lies in the infrared region.
The wave number =
20.Define excitation potential,ionisation potential.
The energy required to raise an atom from its normal state into an excited state is called excitation potential energy of the atom.
The ionisation potential is that accelerating potential which
makes the impinging electron acquire sufficient energy to knock out
an electron from the atom and thereby ionise the atom.
21.Define critical potential.
The excitation potential and ionization potential are called as
the critical potentials of the atom. The critical potential of an atom, is
defined as the minimum potential required to excite a free neutral
atom from its ground state to higher state.
22.What are two main modifications that Sommerfeld introduced in Bohr’s theory?.
(i) According to Sommerfeld, the path of an electron around the
nucleus, in general, is an ellipse with the nucleus at one of its foci.
(ii) The velocity of the electron moving in an elliptical orbit varies
at different parts of the orbit. This causes the relativistic variation in
the mass of the moving electron.
23.What are the drawbacks of Sommerfeld atom model.?
(i) Though Sommerfeld’s modification gave a theoretical
background of the fine structure of spectral lines of hydrogen, it could
not predict the correct number of observed fine structure of these lines.
(ii) It could not explain the distribution and arrangement of
electrons in atoms.
(iii) Sommerfeld’s model was unable to explain the spectra of
alkali metals such as sodium, potassium etc.
(iv) It could not explain Zeeman and Stark effect.
(v) This model does not give any explanation for the intensities of
the spectral lines.
24.What are the basic requirement for the production of X–rays ?
(i) a source of electrons, (ii) effective means of accelerating
the electrons and (iii) a target of suitable material of high atomic weight.
25.What are the characteristics of anode used in x-ray production?
The characteristics of anode used in x-ray production are
(i) high atomic weight – to produce hard X-ray
(ii) high melting point – so that it is not melted due to the
bombardment of fast moving electrons, which cause lot of heat
generation.
(iii) high thermal conductivity – to carry away the heat generated.
26.What are Soft X–rays?
X–rays having wavelength of 4Å or above, have lesser frequency
and hence lesser energy. They are called soft X – rays due to their low
penetrating power. They are produced at comparatively low potential
difference.
27.What are Hard X–rays ?
X–rays having low wavelength of the order of 1Å have high
frequency and hence high energy. Their penetrating power is high,
therefore they are called hard X–rays. They are produced at
comparatively high potential difference.
28.State four Properties of X–rays
(i) X–rays are electromagnetic waves of very short wave length.
They travel in straight lines with the velocity of light. They are invisible
to eyes.
(ii) They undergo reflection, refraction, interference, diffraction
and polarisation.
(iii) They are not deflected by electric and magnetic fields. This
indicates that X-rays do not have charged particles.
(iv) They ionize the gas through which they pass.
(v) They affect photographic plates.
(vi) X–rays can penetrate through the substances which are
opaque to ordinary light e.g. wood, flesh, thick paper, thin sheets of
metals.
(vii) When X–rays fall on certain metals, they liberate photo
electrons (Photo electric effect).
(viii) X-rays have destructive effect on living tissue. When the
human body is exposed to X-rays, it causes redness of the skin, sores
and serious injuries to the tissues and glands. They destroy the white
corpuscles of the blood.
(ix) X–rays do not pass through heavy metals such as lead and
bones. If such objects are placed in their path, they cast their shadow.

29.Why gratings are not used for X-ray diffraction?
Diffraction effects can only be observed if the spacing between the lines ruled on the grating is of the order of magnitude of wavelength of the wave used. Thus, in order to diffract X–rays, grating with much finer rulings, having distance between rulings comparable to the wave length of X–rays are required.
It is impossible to construct a grating of such fine dimensions
artificially.
30.Why crystals are used for X-ray diffraction?
In a crystal, the atoms or molecules are arranged symmetrically in a three dimensional space. Any plane containing an arrangement of atoms is known as lattice plane or cleavage plane. The spacing between the atoms is of the order of 10-10 m, comparable to the wavelength of X-rays. & suggested that the regular arrangement of atoms or molecules in the cleavage planes of a crystal might provide a grating element suitable to diffract X–rays. The crystal might serve as a three dimensional grating, whereas optical grating is a two dimensional one.
31.State Moseley’s law.
Moseley,s law state thatthe frequency of the spectral line in the characteristic X-ray spectrum is directly proportional to the square of the atomic number (Z) of the element considered. This is known as Moseley’s law. i.e ν α Z2 or ν =a(Z −b) where a and b are constants depending upon the particular spectral line.
32.State the applications of Moseley’s law
(i) Any discrepancy in the order of the elements in the periodic table can be removed by Moseley’s law by arranging the elements according to the atomic numbers and not according to the atomic weights.
(ii) Moseley’s law has led to the discovery of new elements like
hafnium (72), technetium (43), rhenium (75) etc.
(iii) This law has been helpful in determining the atomic number
of rare earths, thereby fixing their position in the periodic table.
below:
33.State the Applications of X–rays in Medical field.
(i) X–rays are being widely used for detecting fractures, tumours,
the presence of foreign matter like bullet etc., in the human body.
(ii) X–rays are also used for the diagnosis of tuberculosis, stones
in kidneys, gall bladder etc.
(iii) Many types of skin diseases, malignant sores, cancer and tumours have been cured by controlled exposure of X-rays of suitable quality.
(iv) Hard X–rays are used to destroy tumours very deep inside the
body.
34.State the Industrial applications of X-rays?
(i) X–rays are used to detect the defects or flaws within a material
(ii) X–rays can be used for testing the homogeneity of welded
joints, insulating materials etc.
(iii) X-rays are used to analyse the structure of alloys and the
other composite bodies.
(iv) X–rays are also used to study the structure of materials like
rubber, cellulose, plastic fibres etc.
35.State the Scientific research applications of X-rays?
(i) X–rays are used for studying the structure of crystalline solids
and alloys.
(ii) X–rays are used for the identification of chemical elements
including determination of their atomic numbers.
(iii) X–rays can be used for analyzing the structure of complex
molecules by examining their X–ray diffraction pattern.
36.What is Laser? State its characteristics.
The word ‘Laser’ is an acronym for Light Amplification by Stimulated
Emission of Radiation
Characteristics of laser
The laser beam (i) is monochromatic. (ii) is coherent, with the
waves, all exactly in phase with one another, (iii) does not diverge at
all and (iv) is extremely intense
37.Define normal population.
In a system of thermal equilibrium, the number of atoms in the ground state (N1) is greater than the number of atoms in the excited state (N2). This is called normal population
38.Define stimulated absorption.
Consider a sample of free atoms, some of which are in the ground state with energy E1 and some in the excited energy state with energy E2.
If photons of energy hν = E2-E1 are incident on the sample, the photons can interact with the atoms in the ground state and are taken to excited state. This is called stimulated or induced absorption
39.Define pumping and optical pumping.
The process by which the atoms in the ground state is taken to
the excited state is known as pumping.
I f the atoms are taken to the higher energy levels with the help of light, it is called optical pumping.
40.Define population inversion.
If the atoms in the ground state are pumped to the excited
state by means of external agency, the number of atoms in the excited
state (N2) becomes greater than the number of atoms in the ground state (N1). This is called population inversion.
41. Define metastable state.
The life time of atoms in the excited state is normally 10-8 second. Some of the excited energy levels have greater life times for atoms (10-3s). Such energy levels are called as the metastable states.
42. Define spontaneous emission.
If the excited energy level is an ordinary level, the excited atoms return
to the lower (or) ground energy state immediately without the help of any
external agency. During this transition ,a photon of energy
E2-E1 = hν is emitted. This is called spontaneous emission.
43.Define stimulated emission.
If the excited state is a metastable state, the atoms stay for some
time in these levels. The atoms in such metastable state can be
brought to the lower energy levels with the help of photons of energy
hν = E2 – E1. During this process, a photon of energy E2 – E1 = hν is
emitted. This is known as stimulated emission (or) induced emission.
44.State the Conditions to achieve laser action.
(i) There must be an inverted population i.e. more atoms in the
excited state than in the ground state.
(ii) The excited state must be a metastable state.
(iii) The emitted photons must stimulate further emission. This is
achieved by the use of the reflecting mirrors at the ends of the system.
45.State the Industrial applications of laser
(i) The laser beam is used to drill extremely fine holes in
diamonds, hard sheets etc.,
(ii) They are also used for cutting thick sheets of hard metals
and welding.
(iii) The laser beam is used to vapourize the unwanted material
during the manufacture of electronic circuit on semiconductor chips.
(iv) They can be used to test the quality of the materials.
46.State the Medical applications of laser.
(i) In medicine, micro surgery has become possible due to narrow
angular spread of the laser beam.
(ii) It can be used in the treatment of kidney stone, tumour, in
cutting and sealing the small blood vessels in brain surgery and retina
detachment.
(iii) The laser beams are used in endoscopy.
(iv) It can also be used for the treatment of human and animal
cancer.
47.State the Scientific and Engineering applications of laser.
(i) Since the laser beam can stay on at a single frequency, it can
be modulated to transmit large number of messages at a time in radio,
television and telephone.
(ii) The semiconductor laser is the best light source for optical
fiber communication.
(iii) Narrow angular spread of the laser beam makes it a very
useful tool for microwave communication. Communication with earth
satellites and in rocketry. Laser is also used in accurate range finders
for detecting the targets.
(iv) The earth-moon distance has been measured with the help of
lasers.
(v) It is used in laser Raman Spectroscopy.
(vi) Laser is also used in holography (three dimensional lensless
photography)
(vii) Laser beam can determine precisely the distance, velocity
and direction as well as the size and form of the objects by means of
the reflected signal as in radar.

48. State about Holography.
When an object is photographed by a camera, a two dimensional
image of three dimensional object is obtained. A three dimensional
image of an object can be formed by holography. In ordinary
photography, the amplitude of the light wave is recorded on the
photographic film. In holography, both the phase and amplitude of the
light waves are recorded on the film. The resulting photograph is called
hologram.

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