VISUAL PHOTOMETRY

1. Visual photometry:-
Visual photometry is the science of measuring brightness or relative luminous intensities of light emitted by different sources using certain standards and techniques.

2. What is the total solid angle for the sphere
The total solid angle for the sphere is
Ω=4П sr
Where,
Sr=steradians.

3. Give examples of self-luminous bodies
Sun, candle, incandescent bulb.

4. Give the vale of luminosity of incandescent lamp
luminosity value of incandescent lame is “14 1m/W”.
Note:-
1. Luminosity value of fluorescent lamp is 43 1m/W.
2. Luminosity value of Halogen lamp is 58 1m/W.
3. The value of Luminous intensity of 100 watt incandescent bulb is about 118 cd (candela).

5. Explain the terms luminous flux and luminous intensity. Give their units
Luminous flux (or) Light flux:-
Luminous flux is the amount of radiant (light) energy flowing from a source of light per unit time.
Units:-
ergs/second.

Luminous intensity (or) illuminating power:-
Luminous intensity of a point source of light is defined as the luminous flux emitted from it per unit solid angle.
Units:-
Candela(cd).

6. Define Solid angle
When normal lines are drawn from the boundary of a portion on the surface of a sphere such that the lines meet at its centre and form a cone, then the angle enclosed by the conical surface is defined as solid angle.
Or
Solid angle is defined as a ratio of the area of a sphere, the portion of a sphere enclosed by the conical surface forming the angle to the square of radius of a sphere.

7. What is lumen
Lumen(lm) is the amount of light energy emitted per second by a uniform (point) source of one candela with in a cone of unit solid angle.
Or
One lumen is the amount of luminous flux emitted within unit solid angle by a source of light of luminous intensity of one candela.
Or
A lumen is the amount of light flux radiating from a uniform source of one candela throughout a solid angle of such size as to surround a unit area at a unit distance from the source.

8. Define Candela
Candela(cd) is defined as the luminous flux emitted per unit solid angle along normal to the surface by one sixtieth ( ) square-cm area of a black-body radiator (source) kept at the temperature (2046K) of solidifying platinum.

9. Candle power:-
Candle power of a light source, in a given direction, is the amount of luminous intensity of the source in that direction expressed in terms of candela.

LASER- A SPECTACULAR LIGHT

1. Laser:-
Laser is an extra-ordinary light emitted under stimulated and amplified conditions. So that the beam is characterized by high intensity, specific directionality, high monochromacity and high degree of coherence.

2. LASER:-
OR
What is the full form of LASER
The full form of LASER is “Light Amplification by Stimulated Emission of Radiation”.

3. What are the special properties of laser light?
The characteristic properties of laser light are:
1. Coherence.
2. Directionality
3. Monochromacity and
4. High intensity.

4. Explain coherence

1. Visible light energy is emitted when excited electrons in atoms under go transitions to the ground state. In ordinary light sources, these transitions take place at random in time and the light waves received at a point on a screen bear no definite phase relation among them.

2. But, in a laser source, electronic transitions take place in an ‘orderly way’ and the light waves emitted have a consistent phase relation which does not change with time. This is called ‘temporal coherence’ and is the most important characteristic of laser light.

3. Lack of coherence makes ordinary light an ‘optical noise’. But coherence makes a laser light ‘optical music’.

4. Because of this coherence, tremendous amount of power of the order of 1013 watts can be concentrated in a narrow space of linear dimension of 10-6 m.

5. Explain the property of directionality of laser

1. The conventional sources like lamp, torch light and sodium lamp emit light in all directions. This is called ‘divergence’. Laser on the other hand, emits light only in one direction. This is called ‘directionality of laser light’.

2. For example, in a powerful search light, if the beam from it travels a distance of 1km, it spreads to about a kilometer in diameter.

3. If the laser travels a distance of 1km, it spreads to a diameter less than 1cm as shown in the figure.

4. The directionality of laser enables us to focus the light to a point on a target at large distance.

6. What do you mean by ‘Monochromacity’ of laser beam? Explain
1. Light from sodium lamp is monochromatic i.e., of single colour or of single wavelength of about 5893 .
2. When we say that the wavelength (λ) of sodium light is 5893 , it means simply that intensity is maximum at this λ value.

3. However, intensity is not zero for wavelengths above and below 5893 upto even 500 on either side.
4. This spread of wavelength (or frequency) about the wavelength of maximum intensity is called ‘band with’ (or range).

5. The band width for ordinary monochromatic light is shown in the figure. The band width (Δλ) of a conventional monochromatic light is of the order of 1000 .

Band width of a conventional monochromatic light

6. On the other hand, the band width of an ordinary laser is of the order of 10 and for a high quality laser it is only (Δλ=) 10-8 at 6000 .

7. This narrow band width of a laser light is called ‘high monochromacity’.

8. Because of this monochromacity, large energy can be concentrated into an extremely small band width.

7. Compare the intensities of an ordinary and laser light

1. The intensity (I) of a wave is the energy per unit time flowing through a unit normal area. The light from an ordinary light source spreads out uniformly in all directions and forms spherical wave fronts around it.

2. If you look at a 100 watt lamp filament from a distance of 30 cm, the power entering your eye is less than of a watt. This is shown in the figure.

Comparison of intensity of (a)ordinary light (OL) (b)laser light

3. In the case of a laser light, energy is emanated in small region of space and in a small wavelength range and hence is said to be of great intensity.

4. If you look directly along the beam from a laser (which you should never do), then all the power in the laser would enter your eye. Thus, even a 1 watt laser would appear many thousand times more intense than 100 watt ordinary lamp.

5. For certain lasers, the intensity is so enormous that a power of 1015 watt can be concentrated into an area of 1 square centimeter.

8. What are the basic processes involved in the working of a laser
The basic processes involved in the working of a laser are:
1. Absorption.
2. Spontaneous emission.
3. Pumping and population inversion and
4. Stimulated emission of electro magnetic radiation.

9. Explain the absorption principle of working of a laser
1. Consider two energy states of an electron, the ground state Eg and the excited state Ex.

2. When an electro magnetic energy, equal to Ex-Eg =hν in the form of light-photon of frequency ν, is incident on the atom, the electron readily absorbs this energy and hence goes from Eg to the excited state Ex as shown in figure.

3. In a system, like a gas or a solid, large number of atoms participate in this kind of absorption of electro magnetic energy from an external source.

10. Explain the spontaneous emission principle of working of a laser

1. The electrons excited to higher energy state Ex will not remain for long time in that state. They return to the ground state on their own as shown in the figure releasing energy in the form of photons which have no correlation in phase.

2. These photons constitute ordinary incoherent light, which is due to spontaneous emission.

11. Explain the induced or stimulated emission of working of a laser

1. Assume that a coherent light beam of energy hν=E2-E1, from an external source is incident on the active system.

2. Such photons induces or stimulates all the (N2) electrons in the state 2 to undergo a simultaneous transition to state 1.

3. In this process, large number of photons of the same energy (hν) are simultaneously emitted.

4. Then the light beam emitted will have the property of ‘temporal coherence’.

5. This process is called ‘induced or stimulated emission’.

12. What is population inversion

1. Consider three electronic states 1,2,3 in an active medium in the figure.

Pumping and population inversion

2. Normally, large number of electrons exist in the lowest energy state called ground state.

3. Let the number be N1. When photons of energy equal to the difference between E3 and E1 are incident, the electrons from the state 1 are excited to state 3, because of absorption.

4. But the property of the state 3 is such that the life time (the time for which electrons exist in that state) of electrons in the state is very short, of the order of 10-8 sec. Of course, most atoms return to state 1 from state 3 by spontaneous emission.

5. But some electrons go to the state 2 also.

6. Therefore, the electrons from the state 3 under go a quick transition to the state 2 by emitting only a small energy (E3-E2) which is simply absorbed as thermal energy by the medium itself.

7. The transition of electrons from E3 to E2 that is E3→E2 is called non-radiative (invisible) transition.

8. Then the property of the energy state 2 is such that the life-time of the electrons in this state is relatively large, that is, of the order of a few milli-seconds
(3×10-3 seconds). Such an electronic state is called ‘metastable state’.

9. The transition E3→E2 makes the number of electrons in N2 in the state 2 increase rapidly.

10. At a particular instant of time, N2 becomes greater than N1. This condition N2>N1, is called ‘the population inversion’.

11. The process of achieving populating inversion is called ‘pumping’.

13. Explain pumping and population inversion
Refer to 12th answer.

14. Explain actual laser and its working

Parts:-

R-cavity resonator, M1,M2- partly reflecting mirrors, E-external light source
A-Active medium

1. An actual laser consists of the active medium (A) in a special cell called cavity resonator, R.

2. Two partly reflecting mirrors M1 and M2 are arranged at each end of the resonator as shown in the figure.

3. The light generated within the active medium A is made to bounce back and forth in the resonator.

4. This stimulates other atoms so that they too emit identical light.

5. This amplified light escapes through the partly reflecting mirror.

6. The intense flash of light that emerges is our laser beam. In the figure, E is the external light source which causes ‘pumping’.

15. What are the important applications of laser light in science and technology
1. It helped in studying the Brownian motion of particles.

2. With the help of He-Ne laser, it was proved that the velocity of light is same in all directions.

3. He-Ne laser helped in determining the rate of rotation of the earth accurately.

4. The counting of atoms in a substance became possible because of a laser.

5. Lasers have been used successfully to separate isotopic species in a substance containing mixture of isotopes of an element.

6. Lasers have been used to study the nature of plasma state of matter and also to achieve high temperatures necessary to cause thermo-nuclear fusion of atoms.

7. Lasers are employed in a special three dimensional photography called ‘holography’.

8. In astronomy, lasers helped in extending the distance of observation of stellar objects and study their nature.
9. The high intensity and directionality of lasers led to the development of a new branch of science called micro-Raman spectroscopy which analyses small quantities of biological and bio medical samples.

10. In computers lasers are used to retrieve stored information from a compact disc (CD).

11. It is the development of lasers which helped in establishing a new revolutionary method of communications called ‘fibre-optic communication’.

12. A new branch called ‘Atmospheric optics’ uses laser to measure pollutant gases and other contaminants of the atmosphere.

16. Give the applications of laser in the field of medicine
1. Lasers are used for bloodless surgery.

2. The liver and lung diseases could be treated by lasers.

3. Lasers are used in fibre-optic endoscope to detect ulcers in the intestines.

4. Lasers are used extensively in the treatment of eye-diseases, particularly to reattach a detached retina.

17. Mention the uses of lasers in industry
1. Lasers are used now for cutting, drilling and welding of metals and other materials.

2. Laser light is used to collect the information about the prefixed prices of various products in shops and business establishments from the bar-code printed on the product.

18. What are the applications of lasers in defence and space science
1. Lasers are used in various guided missiles and also for detection of enemy targets.

2. Lasers are also used in space communications, in radars and in satellites.

19. What is population inversion
The process in which the number of electrons (N2) in a higher energy state, called ‘metastable state’ of an active medium is increased to a value greater than the number (N1) in the ground state (i.e.,. N2>N1) is called population inversion.

20. What is spontaneous emission
The process in which the electrons in the exited atoms are released on their own from their higher energy state to the ground state is called spontaneous emission.

21. What are the main parts of an actual laser
The main parts of an actual laser are:-
1. Active medium(A).
2. Cavity resonator (R).
3. Partly reflecting mirrors (M1 and M2).
4. External light source.
22. Mention various types of lasers that are now in operation
1. Solid state lasers.
2. Liquid and dye lasers.
3. Gaseous lasers.
23. Explain the term ‘temporal coherence’
In a laser source, electronic transitions take place in an ‘orderly way’ and the light emitted have a consistent phase relation which does not change with time. This is called ‘temporal coherence’.

24. What is divergence of light
The emission of light from conventional sources in all directions is called divergence of light.

25. What is meant by directionality of a laser light
The property of a laser emitting light only in one direction is called directionality of a laser light.

26. Explain band width
The spread of wave length (or frequency) about the wavelength of maximum intensity is called band width or range.

27. What is monochromacity of laser
The band width of a high quality laser is only 10-8 . This narrow band width of a laser light is called high monochromacity.

28. Name the place in India where different lasers are in operation.

In India, we have different lasers in operation at Centre for Advanced Technology (CAT), Indore under the control of Department of Atomic Energy(DAE) Government of India.

29. Expand the term ICBM
ICBM stands for Inter-Continental Ballistic Missile.

30. Give an example for solid state laser
Ruby laser.

31. Give an example of gaseous state laser
Helium(He)-Neon(Ne) laser.

32. What are the uses of Helium-Neon lasers
1. Helium-Neon lasers helped in determining the rate of rotation of the earth accurately.
2. With the help of Helium-Neon laser it was proved that the velocity of light is same in all directions.

33. Distinguish between spontaneous and stimulated emission of radiation

Spontaneous emission Stimulated emission
1. The process of electrons being released on their own from the excited states in atoms and emitting incoherent light is called spontaneous emission. 1. When an external coherent beam is incident on the active system in a state of population inversion, the electrons are induced or stimulated to undergo simultaneous transition from the excited state to the ground state emitting amplified, coherent radiation. This process is called Stimulated emission.

II. Match the following:-

1.     Group-A                                              Group-B

1. Corpuscles                                           (b) a) Maxwell.

2. Waves                                                  (e) b)Newton’s theory of light.

3. Electro magnetic theory                      (a) c)source at a large distance.

4. Spherical wave front                            (g) d)demonstration of waves

5. Cylindrical wave front                          (f) e)Huygens’ theory of light

6. Plane wave front                                  (c) f)linear source of light.

7. Ripple tank                                          (d) g)point source of light.

 

2.        Group-A                                              Group-B

1. Luminous flux                                         (d) a)lumen/watt

2. Luminous intensity                                  (e) b)candela/m2.

3. Illumination                                             (b) c)Lumen/m2.

4. Luminosity                                               (c) d)lumen

5. Brightness                                               (a) e)candela.

3.        Group-A                                                  Group-B

1. Brownian motion                                    (g) a)Charles H.Townes

2. Fibre-optic endoscopy                            (c) b)CAT

3. Holography                                              (f) c)To detect Ulcer.

4. Founder of laser                                       (a) d)achieving population
inversion.

5. Pumping                                                    (d) e)Maxwell.

(f) Three dimensional photography.

                                                                     (g)Cells or particles.

4.     Group-A                                                  Group-B

1. Population inversion                                  (d) a)Ruby laser.

2. Solid laser                                                   (a) b)Helium-Neon laser.

3. Holography                                                 (e) c)rate of rotation of earth.

4. Helium-Neon laser                                      (c) d)pumping.

5. Gaseous laser                                              (b) e)three dimensional
photography.

III. Fill in the blanks:-

1. The wave theory of light was proposed by Huygens.

2. Light is treated as consisting of quanta of radiations called photons.

3. The stream of tiny, light, elastic particles considered to be the constituents of light are called corpuscles.

4. Corpuscular theory of light could not explain the phenomenon of light called interference, polarisation, diffraction.

5. Subtle and elastic hypothetical medium supposed to be all pervading is ether.

6. The imaginary three dimensional surface formed by the envelope of the particles of medium which are vibrating in the same phase is called wave front.

7. A linear source of light produce cylindrical wave fronts.

8. The imaginary line drawn normal to any wave front that represents the path along which it travels is called light ray.

9. If the normals drawn to the spherical wave fronts all meet at a point, the wave front is called converging spherical wave front.

10. If the normals drawn to the spherical wave fronts diverge from a point, the wave front is called diverging spherical wave front.

11. According to wave theory of light, difference in colours is due to difference in wave lengths.

12. If ‘c’ is the velocity of light in air and ‘v’ is the velocity of light in glass then .

13. If ‘i’ is the angle of incidence in rarer medium and ‘r’ is the angle of refraction in denser medium then .

14. The mirror that converts an incident plane wave front into converging spherical wave front is concave mirror.

15. The mirror that converts an incident plane wave front into diverging spherical wave front is convex mirror.

16. The crest of a water wave in a ripple tank behaves like a convex lens.
17. The trough of a water wave in a ripple tank behaves like a concave lens.
18. In a ripple tank when water waves are produced, a trough forms a dark band.
19. The velocity of water waves depend on depth of water.
20. In a ripple tank when water waves are produced, a crest forms a bright band.
21. When two or more wave motions combine in space at a time, one of the several physical effects that takes place is interference.

22. The combined effect of disturbance caused by each individual wave at the same place and at the same time is known as interference.

23. When crest falls on crest and trough falls on trough the displacement produced by superposition of waves is maximum.

24. When maximum displacement occurs due to the superposition of waves they are said to superpose constructively.

25. During super position of two waves when crest falls on trough and vice-versa the displacement produced is minimum.

26. When minimum displacement occurs due to superposition of waves, they are set to superpose destructively.

27. The physical effect of superposition of waves from two sources vibrating with same frequency and amplitude is called interference.

28. One of the characteristic phenomenon that relates all waves including light waves is interference.

29. The property of two light sources by virtue of which the waves travelling from them and interfering at a point in the screen have the definite phase that remain constant is called coherence.

30. Two sources emitting light of single wave length are called monochromatic sources.

31. For the observation of sustained interference of light the screen must be at a distance of 50 cms.

32. The bending of wave fronts or its deviation from original direction of propagation when it meets a small obstacle is called diffraction.

33. The wave length of visible light is 5000 .

34. When a monochromatic light ray is incident on a filament of hair its geometrical shadow consists of a few dark and bright fringes. This is because of diffraction.

35. The science of measuring brightness or relative luminous intensities of light emitted by different sources is called visual photometry.

36. The amount of radiant energy flowing from a source of light in unit time is luminous flux.

37. Luminous flux emitted by the source per unit solid angle is called luminous intensity.

38. Unit of luminous intensity is candela(cd).

39. Luminous intensity of a source of light in a given direction expressed in candela is called candle power.
40. Unit of candle power is candela (cd).

41. Luminosity of flourescent lamp is 43 lumen/watt.

42. The luminosity of incandescent lamp is 14 lumen/watt.

43. If I1 and I2 are the illuminating powers of two sources kept at a distance of r1 and r2 from a screen then .

44. The basic scientific principle behind a laser was first put forward by Charles H.Townes.

45. Coherence makes a laser light an optical music.

46. The order of power that can be concentrated in a narrow space of linear dimension of 10-6 m is 1013 Watts.

47. Laser emits light only in one direction. This property is called directionality.

48. The property of laser that enables us to focus light to a point on a target of large distance is directionality.

49. Wave length of light emitted by sodium lamp is 5893 .

50. The band width of a conventional monochromatic light is of order of 1000 .

51. The energy per unit time flowing through a unit normal area is called intensity of a wave.

52. The power that enters our eye if we look at a 100 watt lamp at a distance of 30 cms is

53. If Eg and Ex are the energies of electrons in ground and excited states respectively then Ex-Eg=hν.

54. The process in which the electrons in the excited atom are released on their own from their higher energy states to ground state is called spontaneous emission.

55. The process of achieving population inversion is called pumping.

56. An example of solid state laser is Ruby laser.

57. The laser which helps to prove that velocity of light is same in all directions is Helium-Neon Laser.
58. Wave length of Helium-Neon laser is of the order of 6328 .

59. The counting of atoms in a substance became possible because of the discovery of Laser.

60. Laser is helpful in the study of various plasma states of matter.

61. A special three dimensional photography in which lasers are employed is Holography.

62. The branch of science which deals with analysis of small quantities of biological and medical samples is Micro-Raman Spectroscopy.

63. The branch of science that measures pollutants and other contaminants of atmosphere is Atmospheric Optics.

64. Lasers are used in bloodless surgery.

65. Lasers are used in treatment of diseases of liver.

66. The process of detecting ulcers in intestine is fibre-optic endoscopy.

67. To reattach a detached retina, lasers are used.

68. In guided missiles, lasers are used.

69. The process of electrons being released on their own from excited state in atoms and emitting incoherent light is spontaneous emission.

70. In absorption process the electrons in ground state in atom absorbs incident energy and goes to excited state.

71. The extra-ordinary light emitted under stimulated and amplified conditions is laser.

72. In computers lasers are used to retrieve stored information from Compact Disc(CD).

73. A power of 1015 watts can be concentrated by laser into an area of 1 sq cm.

74. For a high quality laser, the band width is of order of 10-8 .

75. Newton proposed corpuscular theory of light .

76. Corpuscles are repelled by reflecting surface.

77. Corpuscles are attracted by refracting surface.

78. The size of corpuscles are different for different colours.

79. Wave theory of light was proposed by Huygens.

80. According to corpuscular theory velocity of light in denser medium is greater than that in rarer medium.

81. Corpuscular theory of light could not explain interference, polarisation, diffraction.

82. The corpuscles are emitted by luminous Source.

83. For constructive super position of waves, phase difference between the waves should be equal to zero or even integral multiple of П.

84. When corpuscles fall on retina of eye, they cause sensation of sight.

85. When corpuscles of light strike plane surface, their parallel component of velocity is constant.

86. Electro magnetic theory helped for further under standing nature of light .

87. Maxwell proposed electro magnetic theory.

88. In case of phenomenon of reflection, angle of incidence is equal to angle of reflection.

89. When corpuscles of light approach refracting surface normal component of velocity increases in a denser medium.

90. A Luminous source sends out light energy uniformly in all directions.

91. The wave front can be of different shapes depending on size, shape of the light source.

92. A sodium vapour lamp is a linear source.

93. Huygens’ principle states that every point of wave front behaves as a source of secondary wavelets.

94. Huygens’ wave theory simply states that light energy is propagated as waves.

95. Huygens’ wave theory was later modified by Thomas Young and Fresnel.

96. Young and Fresnel assumed that light waves are transverse.

97. Wave theory would prove that velocity of light in denser medium is less than that in rarer medium.

98. The rectilinear propagation of light is explained by motion of corpuscles in straight lines.

99. According to wave theory of light, colours of light is due to difference in wavelength.

100. In Huygens’ theory, the reflection and refraction are explained by the construction of Secondary wave fronts.

101. A series of water waves consists of a set of crests and troughs.

102. The reflection and refraction of water waves can be demonstrated using ripple tank.

103. The pattern of successive bright and dark bands seen on the paper of ripple tank represents water waves in ripple tank.

104. The deeper the water, the higher the velocity of wave.

105. The bending of waves or change in velocity at the boundary separating two media is due to the phenomenon of diffraction.

106. When two or more waves cross one another in a same point in a medium they are said to interfere.

107. When two or more waves travel through same portion of medium simultaneously resultant displacement at any point is vector sum of displacement due to individual waves.

108. Interference is the combined effect of disturbance caused by each individual wave of same phase.

109. When two waves superpose such that crest falls on crest and trough falls of trough, resultant displacement is more.

110. When two sources vibrate with same amplitude say ‘Y’ then the resultant displacement ‘R’ is maximum and is equal to 2Y.

111. Constructive super position is obtained between two waves when phase difference between them at that point is zero or even integral multiple of П.

112. When two waves vibrate with same amplitude and when crest of one wave falls of trough of other wave or vice-versa minimum displacement is produced.

113. If two sources vibrate with same frequency and amplitude, the super position of wave from them results in well defined maximum and minimum displacement of space.

114. The formation of antinodal and nodal line results in relatively bright and dark regions called interference pattern of wave.

115. The physical effect of superposition of waves from two sources vibrating with same frequency and amplitude is in the form of variation in amplitude of resultant wave is called interference.

116. Monochromatic sources are necessary for observing sustained interference.

117. A white light source gives coloured fringes.

118. Diffraction of light requires obstacle of small size for experimental observation.

119. The unit lumen is employed for luminous flux.

120. Latest unit of luminous intensity is Candela(cd)

121. When normal lines are drawn from boundary of a portion on the surface of a sphere such that the line meets at the centre, angle enclosed is called solid angle.

122. Solid angle is equal to the ratio of area of conical surface and square of radius of sphere.
123. If dΏ is the solid angle enclosed by area ‘A’ and radius of sphere is ‘r’ then .

124. Luminous intensity of a point source of light is defined as the luminous flux emitted per unit solid angle.

125. Lumen is the amount of light energy emitted per second by a uniform source of 1 candela within a cone of unit solid angle.

126. Candle power of a light source in given direction is the luminous intensity of source in that direction expressed in terms of candela.

127. A source having an illuminating power of candela in a given direction is said to be of one candle power.

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