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Chapter 20 Atomic Spectra Physics 12th Class Punjab Short Questions

Question No. 1 - Distinguished between stimulated emission and spontaneous emission.

Answer No. 1 - Stimulated emission: If a atom is excited for a longer lifetime of about 10-30 Sec than an incident photon of energy equal to the difference of two energy levels induces the atom to decay by emitting a photon that travel in the direction of incident photon. This process is called stimulated or induced emission.

Question No. 2 - What are the difference between laser light and ordinary light?

Answer No. 2 - The laser light is intense, unidirectional and phase coherent, so it does bit spread as compared to ordinary light.
The energy of laser can be focused at a point to get enough heat for welding which is not possible with ordinary light.
A laser beam is used as a surgical tool for welding detached retina which ordinary light can not do.

Question No. 3 - Can the electron in the ground state of hydrogen atom absorb a photon of energy 13.6 eV greater than 13.6 eV?

Answer No. 3 - Yes an electron in the ground state of hydrogen atom can absorb a photon of energy 13.6 eV and greater than 13.6 eV.
Ionization energy of hydrogen atom in ground state is 13.6 eV. So if hydrogen absorb a photon of energy greater than 13.6 eV than the surplus energy of photon appear as K.E of electron.

Question No. 4 - Write use of x-rays.

Answer No. 4 - X-rays are used:
1. To visualize the interior of the material opaque to
ordinary light.
2. In computerized axial tomography.
3. In photographic films.

Question No. 5 - What is meant by line spectrum? How line spectrum can be used for the identification of elements?

Answer No. 5 - When an electron jumps from higher energy state to lower energy state than emits energy and make a spectral line.
E2- E1=hf
This is called line spectrum.And different elements emit line spectrum of different wavelengths, So they can be identified easily.

Question No. 6 - Write down use of laser in industry.

Answer No. 6 - It can be used for telecommunication in fiber optics.
It is used to read bar codes.
It is used to generate 3D image of objects by holography.

Question No. 7 - What is CAT scanner?

Answer No. 7 - A vastly improved x-rays technique is computerized axial tomography. The corresponding instrument is called CAT scanner. In CAT scanning a number of different orientations. Computer is attached to construct picture. Density difference of the order of one percent can be detected. Tumors and other anomalies much too small to be seen with older techniques can be detected.

Question No. 8 - Define continuous spectrum.

Answer No. 8 - The radiation emitted by continuous media such as "Black Body" forms a continuous spectrum. In continuous spectrum wavelengths of radiations can not be observed in discrete lines.

Question No. 9 - What do we mean when we say that the atom is excited?

Answer No. 9 - When energy from some external source is provide to an atom in its normal state, then its electron will jump from lower energy state to higher energy state. And atom is said to be excited.

Question No. 10 - What is meant by lasing action?

Answer No. 10 - When a large percentage of atom or sample is in population inversion then large number of coherent photons along same direction of motion could be obtained to form laser light. The combination of spontaneous emission of first and then stimulated emission cause the laser to generate coherent beam of light at a single frequency which is called lasing action.

Question No. 11 - Differentiate between Line and Band spectrum.

Answer No. 11 - Line spectrum: When electron jumps from higher orbit to lower orbit, it emits energy,This emission of energy constitutes spectral line. This is called the line spectrum.

Question No. 12 - Bohrs theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?

Answer No. 12 - Bohrs first assumptions contradicts classical physics. Bohr said that electron do not radiate energy during revolving while according to classical physics, electron radiate energy during revolving and fall into the nucleus which is impossible.

Question No. 13 - How does a Kax-rays differ KbX-rays?

Answer No. 13 - When an electron from L -shell jumps to occupy the hole in K- shell Kax-rays is emitted.
When an electron from M-shells jumps to occupy the hole in K-Shell KbX-rays is emitted.

Question No. 14 - In which region of electromagnetic spectrum does the following series fall a Lyman series b Balmer series?

Answer No. 14 - Balmer series falls in the visible region.
Lyman series falls in the ultraviolet region.

Question No. 15 - Define characteristics of X-Rays and continuous X-Rays.

Answer No. 15 - Characteristics X-Rays: The x-rays emitted from inner shell transitions are called characteristics x-rays and their energy depends on the type of target material.
Continuous X-Rays: The x-rays emitted having continuous range of frequencies due to bremsstrahlung effect are called continuous x rays.

Question No. 16 - Write two properties of X-Rays.

Answer No. 16 - 1. They cause ionization.
2. They have very short wavelength.

Question No. 17 - Give two uses of laser in medicine and industry.

Answer No. 17 - Laser is used:
1. For welding detached retinas
2. To destroy cancerous and precancerous cells

Question No. 18 - Explain why laser action could not occur without population inversion between atomic levels?

Answer No. 18 - When a large percentage of atom or sample are in population inversion, then large number of coherent photons along same direction of motion could be obtained to form laser light. It is impossible without population inversion.

Question No. 19 - Is energy conserved when an atom emits a photon of light?

Answer No. 19 - Yes energy is conserved when an excited atom emits a photon of light. When atom is excited ,energy is supplied. The same energy is emitted in the form of photon when it return back to its ground state.

Question No. 20 - List of color of line spectra of an excited hydrogen atom.

Answer No. 20 - The color of line spectrum of hydrogen atom are:
. Red
. Blue
. Blue-Green
. Violet
. Ultraviolet

Question No. 21 - What do you mean by inner shell transition?

Answer No. 21 - A transition in which an electron from higher orbit, emits energy and accommodate a hole in the lower orbit. Such a transition is called inner shell transition. The photons emitted in such transition are called characteristic x-rays.

Question No. 22 - Briefly describe continuous x-rays.

Answer No. 22 - Continuous X-rays is due to an effect known as Bremsstrahlung effect when fast moving electron bombarded at the target, they are suddenly slowed down on impact with the target. And due to deceleration their kinetic energy is converted into X-rays photons.

Question No. 23 - How can spectrum of hydrogen contain so many lines, whereas hydrogen atom contain one electron?

Answer No. 23 - The single electron in hydrogen atom occupies ground state but it can be excited to several states by absorbing energy. During de-excitation, it can emit several lines of different wavelength.

Question No. 24 - What is Spectroscopy?

Answer No. 24 - It is that branch of physics which deals with the production, measurement and interaction of electromagnetic radiation emitted or absorbed by atom is called Spectroscopy.

Question No. 25 - Can electron reside inside the nucleus?

Answer No. 25 - No, electron cannot reside inside the nucleus. If electron resides inside nucleus then uncertainty in position.

Question No. 26 - Can X-Rays be reflected, diffracted polarized just like any other eaves? Explain.

Answer No. 26 - Yes X-Rays can be reflected, refracted diffracted by crystal only and polarized just like any other waves.

Question No. 27 - Define population inversion and metastable stable.

Answer No. 27 - A condition of matter in which more electrons are in a high energy state than in a lower energy state is called population inversion.
A particular excited state of an atom that has longer lifetime than the ordinary excited states is called metastable state.

Question No. 28 - Define normal population and population inversion.

Answer No. 28 - In normal population, the lower energy state has a greater population than the higher energy state.
Population inversion occurs when no more electron are in a higher energy state than in lower energy state.

Question No. 29 - Write two postulate of Bohrs model of H-atom.

Answer No. 29 - . An electron, bound to the nucleus in an atom, can moved
around the nucleus in certain circular orbits without
radiating. These orbits are called the discrete
stationary orbits.
. Only those stationary orbits are allowed for which
orbital angular momentum is equal to an integral multiple
mvr=nh/2π.

Question No. 30 - What are the advantages of laser over ordinary light?

Answer No. 30 - Laser light has many advantages over ordinary light such as laser light is:
1. Coherent
2. Intense
3. Monochromatic
4. Unidirectional

Question No. 31 - What is fluorescence?

Answer No. 31 - Fluorescence is a property of absorbing radiant energy of high frequency and re emitting energy of low frequency in the visible region of electromagnetic spectrum.

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