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12th Class Chapter 9 Physics Short Questions

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Question: 1
Write two postulate of Bohr's model of H-atom.
Answer: 1
1-31
  • 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: 2
How can spectrum of hydrogen contain so many lines,whereas hydrogen atom contain one electron?
Answer: 2
2-31
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: 3
What is meant by line spectrum? How line spectrum can be used for the identification of elements?
Answer: 3
3-31
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 identifiedeasily.
Question: 4
Differentiate between Line and Band spectrum.
Answer: 4
4-31
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: 5
What do we mean when we say that the atom is excited?
Answer: 5
5-31
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: 6
List of color of line spectra of an excited hydrogen atom.
Answer: 6
6-31
The color of line spectrum of hydrogen atom are
  • Red
  • Blue
  • Blue-Green
  • Violet
  • Ultraviolet
Question: 7
Bohr's theory of hydrogen atom is based upon several assumptions.Do any of these assumptions contradict classical physics?
Answer: 7
7-31
Bohr's 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: 8
Can the electron in the ground state of hydrogen atom absorb a photon of energy 13.6 eV greater than 13.6 eV?
Answer: 8
8-31
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: 9
What is fluorescence?
Answer: 9
9-31
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.
Question: 10
In which region of electromagnetic spectrum does the following series fall (a)Lyman series (b)Balmer series?
Answer: 10
10-31
Balmer series falls in the visible region.
Lyman series falls in the ultraviolet region.
Question: 11
Define continuous spectrum.
Answer: 11
11-31
The radiation emitted by continuous media such as ''Black Body" forms a continuous spectrum,In continuousspectrum wavelengths of radiations can not be observed in discrete lines.
Question: 12
What do you mean by inner shell transition?
Answer: 12
12-31
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: 13
Give two uses of laser in medicine and industry.
Answer: 13
13-31
Laser is used
  1. For welding detached retinas
  2. To destroy cancerous and precancerous cells.
Question: 14
Briefly describe continuous x-rays.
Answer: 14
14-31
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: 15
Can x-rays be reflected,diffracted polarized just like any other eaves?Explain
Answer: 15
15-31
Yes x rays can be reflected,refracted diffracted by crystal only and polarized just like any other waves.
Question: 16
Define characteristics of X-rays and continuous x-rays.
Answer: 16
16-31
Chararcterstics 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: 17
Write use of x-rays.
Answer: 17
17-31
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: 18
what is Spectroscopy?
Answer: 18
18-31
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: 19
Explain why laser action could not occur without population inversion between atomic levels?
Answer: 19
19-31
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 withput population inversion.
Question: 20
How does a Kax-rays differ KbX-rays?
Answer: 20
20-31
When an electron from L -shell jumps to occupy the hole in K- shell Kax-rays is emitted .
And
When an electron from M -shells jumps to occupy the hole in K-Shell KbX-rays is emited.
Question: 21
Is energy conserved when an atom emits a photon of light?
Answer: 21
21-31
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: 22
Write two properties of x-rays.
Answer: 22
22-31
  1. They cause ionization.
  2. They have very short wavelength.
Question: 23
What are the advantages of laser over ordinary light?
Answer: 23
23-31
Laser light has many advantages over ordinary light such as laser light is
  1. Coherent
  2. Intense
  3. Monochromatic
  4. Unidirectional
Question: 24
Define population inversion and metastable stable.
Answer: 24
24-31
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: 25
Distinguished between stimulated emission and spontaneous emission.
Answer: 25
25-31
Stimulated emission: If a atom is excited for a longer lifetime of about 10-3sec 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: 26
What is meant by lasing action?
Answer: 26
26-31
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 laster 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: 27
Can electron reside inside the nucleus?
Answer: 27
27-31
No,electron cannot reside inside the nucleus .If electron resides inside nucleus then uncertainty in position.
Question: 28
What are the difference between laser light and ordinary light?
Answer: 28
28-31
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: 29
What is CAT scanner?
Answer: 29
29-31
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: 30
Write down use of laser in industry.
Answer: 30
30-31
It can be used for telecommunication in fibre optics.
It is used to read bar codes,
It is used to generate 3D image of objects by holography.
Question: 31
Define normal population and population inversion.
Answer: 31
31-31
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.

12th Class Physics Chapters Short Questions