Engineering Physics Interview Questions and Answers:
1. Define stress on a body.
Ans. The internal force developed per unit area in the body during the application of external force or load is called stress. Within the elastic limit, the stress is equal to the external force per unit area. Its unit is Nm– 2 or pascal.
2. Define strain in the
Ans. The ratio of change in dimention to the original dimension of the body during the application of load is called strain.
3. Define elastic limit of a body.
Ans. Elastic limit is the maximum amount of stress upto which it can regain its original state when the deforming forces are removed from it. It is also equal to the minimum amount of stress from which there is permanent set or deformation eventhough the deforming forces are removed from it.
4. Define yield strength and ultimate tensile strength.
Ans. Yield strength of the body is the value of stress at the yield point at. which the material begins to deform appreciably without an increase of load and there is a considerable permanent straining.
Ultimate tensile strength UTS is the maximum amount of stress withstand by the material without any fracture.
5. State Hooke’s law
Ans. Hooke’s law states that within the elastic limit the stress developed in the body is directly proportional to strain in it.
6. Define modulus of elasticity.
Ans. Modulus of elasticity is the property of the material by which it has a little deformation eventhough a large amount of deforming force applied on it. Thus it is the property by which it resists the applied force or load to undergo deformation.
7. What is the effect of temperature on elasticity?
Ans. Elasticity is inversely. proportional to temperature. If the temperature increases, even a highly elastic material losses its elasticity and becomes plastic. But the decrease of elasticity with the increase of temperature can be reduced to a smaller value by increasing its creep resistance through dispersion hardening and selecting coarse grained materials.
8. Give the importance of safety factor in designing engineering
Ans. Safety factor is an important parameter for each engineering material from which the life of the structure can be evaluated.
The working stress is always within the elastic limit to safeguard the structure. For good engineering material like steel, it is about 4. But when the engineering material is not elastic like brick, then its safety factor is large and is about 10.
9. What are the uses of stress-strain curve?
Ans.
- It is used to determine the elastic strength, yield strength, ultimate tensile strength and compressive strength of an engineering material.
- It is useful to identify the material as ductile or brittle or hard.
- One can study the effect of heat treatment on the improvement of mechanical properties using stress-strain curve.
- For manufacturing the engineering products, one can determine the amount of energy required to deform the material elastically or plastically.
10. What are the three modulii of elasticity? What is the relation between them?
Ans.
11. What is Poisson’s ratio? What are its limiting values?
Ans.
12. Define modulus of torsion or torsional rigidity of a wire.
Ans. Modulus of torsion or torsional rigidity of a wire is defined as the twisting couple required to produce a twist of one radian in the wire.
13. What are the requirements of a good shaft?
Ans.
- It should transmit the couple without any appreciable twist in it.
- Its torsional rigidity should be large by selecting the shaft material with high rigidity modulus of elasticity and large radius.
14. What is torsion pendulum?
Ans. Torsion pendulum is a system consisting of a heavy circular disc which is suspended by a metallic wire clamped to a rigid support. It makes torsional oscillations periodically about the wire as axis such that at any time, its angular acceleration is directly proportional to its angular displacement and is always directed towards the mean position.
15. Define neutral axis and bending amount of a beam.
Ans. Neutral axis of the bent beam is the axis in the median plane and it does not undergo any compression or extension due to bending. Thus its length is always constant and equal to the unbent beam length. Bending moment is the moment of the internal restoring couple and is equal to E/R Ig
where E = Young’s modulus, Ig = AK2 = geometrical moment of inertia and R = radius of curvature of the bent beam. Shaft is a energy transferring device from cylinder to the wheel.
16. Define uniform bending and non uniform bending of a beam.
Ans. Uniform bending of a beam is the bending due to the application of uniform loading of the beam such that the bending stress developed at each and every point of the beam will be the same. Non uniform bending of a beam is the bending due to the application of nonuniform loading of the beam such that the load is applied at any point of the beam and so the action of the load is nonuniform throughout the beam.
17. Why do we prefer I shape girders rather than solid girders?
Ans. I shape girders are preferred because they are cheaper than the solid girders due to usage of lesser amount of material for making I shape girders with no loss in strength. Secondly due to their low weight it would not undergo depression by its weight and due to their larger depth, the depression produced is almost negligible which enhances the life of the girder.
18. Briefly explain the measurement principles of high vacuum.
Ans.
- Pirani gauge and conductivity gauges are based on the fact that at low pressures the thermal conductivity of a gas is directly proportional to pressure. It is used to measure pressures upto 10-3 torr.
- Penning gauge or ionisation gauge is based on the fact that at low pressure with high potential difference, the discharge current or ionisation current produced is directly proportional to pressure. It can measure pressures from 10-3 torr to 10-6 torr.
- Radiometric gauge or Kundsen gauge is based on the fact that at very low pressures, the mechanical force exerted between a cold surface and a hot surface placed very close to each other and maintained at a constant temperature difference is directly proportional to pressure. It is used to measure very low pressures upto 10-15 torr.
19. Why do we require force vacuum for the operation of diffusion pumps and penning gauge?
Ans. Diffusion pump is based on the principle of diffusion such that a gas diffuses from a region where its partial pressure is higher to a region where its partial pressure is lower irrespective of the total pressure in the two regions. During diffusion, the high pressure mercury vapours push the low pressure gas. molecules towards the outlet and so the pressure of gas molecules in the evacuated vessel is further reduced. Therefore the pressure of gas molecules should be reduced to a pressure level of 10-3 torr, Then only diffusion pump effectively works. Similarly the penning gauge is based on the ionisation produced during discharge of electricity through gases. For effective ionisation, the pressure of the discharge tube should be reduced. Therefore the penning gauge can measure the low pressure from 10-3 torr to 10-6 torr.
20. Define coefficient of viscosity and viscous force.
Ans. Coefficient of viscosity of a liquid is defined as the tangential force acting per unit area of the liquid when there is a unit velocity gradient between two layers of the flowing liquid. It’s unit is Ns-m-2. Viscous force is the resisting force for the fluid flow. It is always acting in the opposite direction of the flow. So that, it is similar to the frictional force.
Viscous force 
Where η is the coefficient of viscosity A is the area of the liquid layer and (v1-v2/x) is the velocity gradient between any two layers of the main liquid.
21. What is Reyonld’s number? Give its importance.
Ans. Reyonld’s number ‘K’ is a dimensionless constant and is used to determine whether the liquid flow through a tube or pipe is stream lined or turbulent.
where vc is the critical velocity of the liquid flow, ρ is the density of the liquid and r is the radius of the tube or pipe.
22. Give the variation of viscosity of liquid with temperature.
Ans. Generally the viscosity of a liquid decreases with the rise of temperature such that
where a and b are constants for a given liquid.
23. What is the principle behind the comparison of viscosities of two liquids?
Ans. When the volume of flow and temperature of two liquids remain the same and maintained as constant, then
Thus η ∞ ρ t where ρ1 and ρ2 are th.e densities of two liquids and t1 and t2 are the time taken by the first and second liquids for a constant volume flow through a capillary tube.
24. Define the term reverberation time. What is the unit of reverberation time?
Ans. The time required for the intensity of the sound pulse to fall below the audible limit after the source is cut off is known as the reverberation time. Thus by Sabine’s law, it is defined as the time taken by sound to fall to one millionth of its original intensity after the source of sound is cut-off. The unit of reverberation time is second.
25. What is Piezo electric effect?
Ans. When certain crystals like quartz are stretched or compressed along certain axis called mechanical axis of the crystal, an electrical potential difference is produced along a perpendicular axis called electrical axis of the crystal. This is called Piezo electric effect. Thus it refers to the development of e.m.f. or voltage by the application of mechanical pressure.
26. What is inverse Piezo-electric effect?
Ans. The converse of Piezo-electric effect is called inverse piezo electric effect. i.e. when an alternating potential difference is applied along the electric axis of the crystal, the crystal is set into mechanical vibrations along the corresponding mechanical axis. Thus by means of application of high frequency a.c. voltage on a piezo electric crystal the ultrasonic vibrations can be developed.
27. Explain briefly how are ultrasonics used to detect flaws in a metal.
Ans. When the ultrasonic pulses are transmitted into the metal, the pulse is reflected by the flaw as well as at the top and bottom surfaces of the metal. Using the receiver transducer these echoes are collected and are displayed in the CRO in the amplitude mode. Using the display pattern the size and location of the flaw inside the metal can be determined.
28. Explain is the phenomenon of magnetostriction.
Ans. The change in the dimension of a ferromagnetic material by the application of a magnetic field is known as magnetostriction. When a rod of a ferromagnetic material such as iron or nickel is placed in an alternating magnetic field parallel to its length, there is a small change in length of the rod and hence the rod vibrates. This phenomenon is called magnetostriction. If the rod is suitably magnetised before being inserted in the alternating magnetic field and the natural frequency of vibration of the rod is equal to the frequency of the alternating magnetic field resonant ultrasonic vibrations with higher amplitude are emitted from the ends of the rod.
29. What is meant by radiation resistance?
Ans. The characteristic impedance offered by the medium for the propagation of sound waves is called radiation resistance.
It is represented by Z such that
where ρ0 = equilibrium density of the medium and C = velocity of sound in that medium.
30. Define absorption coefficient of a material. Give its unit.
Ans. The absorption coefficient of a material is defined as the ratio of the sound energy absorbed by certain area of the surface of the material to that of an open window of same area. It can also be defined in the other way.
Absorption coefficient of a material is defined as the reciprocal of its surface area which absorbs the same sound energy as absorbed by an unit area of an open window. Absorption coefficient is measured in open window unit (OWU) or sabine.
31. How are ultrasonic waves used to measure the depth of sea?
Ans. Ultrasonic waves are sent from the surface of the sea through sea water and are reflected from the bottom of the sea. The reflected waves are received at the surface of the sea and are displayed on CRO. Using CRO the time taken ‘t’ for the ultrasonic waves to travel from the top surface to the bottom surface of sea and back to top surface of the sea is calculated.
This is called, as one round trip time.
where V is the ultrasonic velocity in sea water.
32. Mention the applications of ultrasonics.
Ans. Ultrasonics are used
- to detect the flaws in materials
- to find the depth of sea and submarines
- to do sound navigation and ranging (SONAR)
- to do drilling, welding and grinding
- to remove body pains and for cleaning
- to study the structure of solids and liquids.
33. What is a decibel?
Ans. A decibel (dB) is the unit of the intensity level of a sound. Intensity level is measured in decibel scale (or) logarithmic scale because the response of human ear to sound is found to be logarithmic.
where I0 = standard intensity = 10 -12 W/m2 and I = intensity of sound.
Thus I dB of intensity level corresponds to
Hence 26% change in intensity corresponds to 1 dB of difference in intensity levels.
Further dB unit is also used to express the gain of the electronic amplifiers.
34. What is meant by sonar? Name two uses of it.
Ans. Sonar means Sound navigation and ranging. This is similar to radar. Suppose a submarine or any underwater object like iceberg is to be located. Ultrasonic waves are sent into the sea water and the waves reflected from the submerged objects are received. The time interval between sending and receiving the waves ‘t’ is determined. Since the ultrasonic velocity ‘V’ in the sea water is known, the distance of the submarine or iceberg from the surface of the sea = Vt/2
This is the principle of sonar. Further the transmitted information carried by ultrasonic waves can be received by the people in the submarine and this provides navigation. Thus it. is mainly used for
- guiding the submarines or ships in seas,
- measuring the depth of the sea and
- to do sound signalling.
35. Name three sound absorbing materials used in an auditorium
Ans.
- fibrous (straw) plaster material
- wood
- cork
- rubber tile.
36. What is meant by resonance effect in acoustics?
Ans. Hollows and crevices select their natural frequencies from the sound produced in the hall and reinforce them there by producing resonance. This creates a jarring effect and non-uniform distribution of sound energy in the hall. This is called resonance effect in acoustics.
37. What is meant by structure-borne noise?
Ans. Structure borne noises are the noises produced inside the structure of hall or working machine and are conveyed through the structure. Any loose fittings or sliding one surface with the other surface of various machine parts produce structure borne noises. Further drilling, hammering and working of machinery inside the building also produce structure borne noise. For the reduction of these noises, working machineries are placed on absorbent pads and the walls, floors and ceilings of the building are padded with sound absorbing materials.
38. What is air-borne noise?
Ans. The noise carried by air that enters into ears directly or from outside through open windows of the hall is called air borne noise. The ascending and decending aeroplane, siren or any moving vehicle are the main sources of air borne noise. To reduce air borne noise, it is better to use ear muffs or ear plugs when we walking on the road and to construct air conditioned halls or rooms.
39. What are ultrasonic waves?
Ans. Ultrasonic waves are the inaudible sound waves having frequencies more than 20000 Hz.
40. Explain a method of detecting ultrasonic waves?
Ans. When one pair of faces of quartz crystal is subjected to ultrasonic waves, an e.m.f of frequency equal to the ultrasonic frequency is produced on the other pair of faces which are perpendicular to the first pair of faces. The magnitude of produced e.m.f. is proportional to the intensity of ultrasonic waves. Thus by detecting the produced e.m.f. in the piezo electric crystal one can detect ultrasonic waves.
41. What do you mean by acoustic analysis and correction?
Ans. Acoustic analysis means the measurement of reverberation time and absorption coefficient of the hall or room. Using these data and by sound absorbing materials, an uniform distribution of sound energy and adequate reverberation time are made in the hall or room for good This is called acoustic correction.
42. What do you meant by acoustics of buildings?
Ans. Design of the building to provide good uniform intensity of audible sound to every audience in the building is called acoustics of buildings or architectural acoustics.
43. What are the requirements for good acoustics?
Ans.
- The hall should have proper reverberation time about 1 to 1.5 second.
- No echo should be present
- Resonance effect (jarring effect of sound) should be avoided.
- There should be no echelon effect.
- There is no path for entering of outside noises.
- The loudness of the sound should be uniform throughout the hall.
44. What is acoustic grating? What is its use?
Ans. When ultrasonic waves travel though a liquid compressions and rarefactions are formed. The lines of compression act as opaque and lines of rarefaction act as openings for light waves. This arrangement is called acoustic grating. Using the acoustic grating, ultrasonic wavelength in the liquid can be calculated since the distance between two consecutive compressions and rarefactions in the acoustic grating is equal to the wavelength of ultrasonic waves. Knowing the frequency of ultrasonic waves and their wavelength in the liquid, one can determine the velocity of ultrasonic waves in that liquid.
45. Define 1 phon.
Ans. 1 phon is the loudness level of a given sound, provided it has equal loudness with the standard tone having frequency 1000 Hz and intensity level 1 dB above the standard intensity (10-12 Wm-2)
46. What is Echelon effect?
Ans. The sound waves reflected from the flight of steps with equal width in a room will consist of echoes with regular phase difference. These echoes which produce a separate musical note. So far a listener, this creates confusion. To eliminate this effect, steps should be made of unequal width or the steps should be covered with sound absorbing material.
47. Define coefficient of thermal conductivity.
Ans. Coefficient of thermal conductivity is defined as the quantity of heat conducted per second through unit area of the material when unit temperature gradient is maintained. Unit : Wm-1 K-1.
48. What are the characteristics of good and bad conductors?
Ans.
Good conductors :
- They have low specific heat
- They can be easily heated or cooled.
- They have high thermal and electrical conductivity
- (Examples : Metals like copper, silver and gold)
Bad conductors :
- They have high specific heat
- They can not be easily heated or cooled.
- They have low thermal and electrical conductivity
- (Examples : Non metals like glass, wood and mica)
49. Define thermal resistance.
Ans. Thermal resistance is the ratio between the temperature gradient and thermal current flow. It gives the amount of obstruction offered by the material for the thermal energy flow through the material.
50. Explain the terms steady state and thermal diffusivity.
Ans. Steady state means the condition of the body at which the temperature of the body remains constant and the amount of heat conducted through the body is completely radiated out without heating the body. Before the steady state is reached, the rate of heat flow is determined only by the thermal diffusivity ‘h’ of the body. It is the ratio between the thermal conductivity and the thermal capacity per unit volume of the body
i.e. h = K/ρS
where S is the specific heat of the body and ρ is its density. After steady state is reached, the rate of heat flow is deterinined by thermal conductivity alone.
51. Give the reasons for the higher thermal conductivity of metals.
Ans. In metals, thermal conduction takes place not only by lattice vibrations but also by free electrons. The amount of thermal conduction by free electrons is very larger than the thermal conduction by lattice vibrations. Thus metals have high thermal conductivity.
52. Is there any heat conduction through vacuum?
Ans. No, since vacuum has no atoms or molecules to transfer heat. Thermal conduction takes place only in material medium and vacuum is a thermal insulator.
53. What are the required properties of thermal insulating materials? Give some examples for them.
Ans.
- Thermal insulating materials should have a fibrous, granular or porous structure.
- They should have high specific heat and low thermal conductivity.
- They should have high thermal resistance
54. State Zeroth law of thermodynamics.
Ans. It states that if each of the two systems is in thermal equilibrium with a third, they are also in thermal equilibrium with each other. From the zeroth law, we are able to give precise meaning to temperature.
55. Give the important features of First and Second Law of thermodynamics.
Ans. First law is a generalisation of the principle of conservation of energy to include heat. From the first law, we are able to define internal energy. Second law gives the impossibility of conversion of all the heat into work. It also gives the efficiency with which heat may be converted into work. From the second law we are able to define entropy.
56. State carnot’s theorem.
Ans. The efficiency of all reversible engines operating between the same two temperatures is the same and no irreversible engine working between the same two temperatures can have a greater efficiency than this.
57. Explain entropy.
Ans. Entropy is a thermodynamical property related to the amount of disorder in a system. If a substance which is undergoing a reversible change, takes in a quantity of heat dQ at a temperature ‘T’ its entropy is said to increase by an amount dQ/T.
58. In what way entropy and disorder of the system are related?
Ans. All irreversible systems proceed towards an equilibrium state of the system. The equilibrium state of a system is the most probable macro state with maximum entropy. When we mix two gases, there is an increase of disorder. Meanwhile entropy of she system also increases. Thus entropy is very much related to the disorder of the system. Greater the disorder in a system, greater is its entropy and vice versa.
59. What are coherent waves?
Ans. Two waves are said to be coherent if they have same amplitude, same frequency and constant phase difference among them.
60. State Brewster’s Law. What is the use of that law?
Ans. Brewster’s Law states that when the angle of incidence is at the polarising angle of the medium, the reflected ray is completely polarised and the tangent of the polarising angle is equal to the refractive index of the medium. The law is used to get polarised light by reflection.
61. What is meant by crossed nicols?
Ans. In the case of crossed nicols, the plane of polarisation of the polarising nicol and the plane of polarisation of the analysing nicol are at perpendicular directions. Hence the intensity of the transmitted light through the analysing nicol is zero.
62. What is photoelasticity?
Ans. Photoelasticity is the property of a material by which it becomes a doubly refracting one when it is mechanically strained. This is also called artificial double refraction. This property is used to measure stresses in solids and to study the distribution of stresses at various points on the transparent model of an engineering structure like a dam or bridge.
63. What is the principle behind the photoelastic stress analysis?
Ans. A transparent model of the given engineering structure like a dam or bridge is made from bakelite which becomes doubly refracting under mechanical strain. Then it is subjected to mechanical stresses by applying load on it. The model is placed between the crossed nicol prisms. The plane polarised light produced by one of the nicol prism is converted into elliptical or circularly polarised light. Further the light through the strained model produces fine interference fringes called stress or induced birefringence. From this interference paltern we can calculate the magnitudes and directions of the stresses at all points of the model.
64. State stress-optic law.
Ans. When the material is mechanically strained, it becomes doubly refracting and the changes in the indices of refraction are linearly proportional to the stresses or strains.
65. What are isoclinics and isochromatics?
Ans. Isoclinics and isochromatics are the interference fringes observed through the analyser when the photoelastic strained model is placed in between the crossed polariser and analyser. Isoclinics are the loci of points where the principal stress direction coincides with the axis of polariser.
Isochromatics are the loci of points of equal principal stress difference or where the relative angular phase shift of the emerging wave is 2 n π where n = 0, 1, 2, 3 ….
66. What are the merits and demerits of transmission electron microscope over optical microscope?
Ans.
Merits : It has high resolving power and high magnification power. Hence the objects can be studied upto cellular levels.
Demerits : It can work under vacuum. So the living objects can not be studied. Further its cost is very large.
67. What is the advantage of polarising microscope over ordinary metallurgical microscope?
Ans. If the specimen under study is optically active, then one can easily identify the different minerals present in the sample when we view the bright and dark with different colours in the field of view. Further the polarising microscope uses the polarised light and the transmitted light is used to analyse the structure. In the case of ordinary microscope if the specimen surface is etched properly, it is able to identify the grain boundaries only. Further the reflected light from the specimen surface is used to analyse the structure.
68. Define luminous flux and luminous intensity of a light source.
Ans. Luminous flux is the total amount of light energy emitted from the light source in all directions per second (Unit lumen). Luminous intensity is the luminous flux radiated by the source through one square metre area kept at one metre from the light source.(Unit : candela)
69. What are the characteristics of laser radiation?
Ans. Laser radiation has high intensity, high coherence, high monochromatism and high directionality with less divergence.
70. Mention the communication based advantages of light waves over microwaves or radiowaves.
Ans.
- Due to high frequency of light, more number of data can be sent per second.
- Light waves have increased antenna gain or high S/N ratio since the signal energy received at the receiver is directly proportional to square of carrier frequency.
- Light waves require antenna with small size due to their short wavelengths.
- Light waves have negligible divergence due to smaller wavelengths.
71. Mention three advantages of optical fiber as waveguide over conventional metallic waveguide.
Ans.
- Optical fiber is made up of dielectric (glass) so there is no electromagnetic interference or cross talk.
- Optical fiber cable is in small size with less weight. Hence it is flexible and it can be laid at any place without any congestion.
- Optical fiber has low transmission loss. The transmittal signal through the fibers does not radiate like metallic conductors.
72. What is meant by coherence in laser light?
Ans. A coherent light is a pure sine wave and during its transmission it can maintain constant phase difference between any two points in space as well as in a given time interval at any point along the transmission path. Thus it is travelling in a continuous manner without undergoing any abrupt change in phase. Thus the laser light is a coherent light.
73. Which gives the special characteristics for laser light?
Ans. Stimulated emission is responsible for the characteristics of laser light stimulated emission means the emission of light photon by the inducement or stimulation of an atom to undergo laser transition through a photon whose energy is equal to the emitted photon’s energy or equal to the energy difference between the laser transition levels.
74. What is the necessity of cladding for an optical fiber?
Ans.
- To provide proper light guidance inside the core
- To avoid leakage of light from the fiber
- To give mechanical strength for the fiber
- To protect the core from scratches and other mechanical damages.
75. What are the uses of optical fibers?
Ans.
- To transmit the information which are in the form of coded signals of the telephone communication, computer data, etc.
- To transmit the optical images [Example : Endoscopy]
- To act as a light source at the inaccessible places.
- To act as sensors to do mechanical, electrical and magnetic measurements.
76. What is the principle used in the working of fibers as light guides?
Ans. The phenomenon of total internal reflection is used to guide the light in the optical fiber. To get total internal reflection, the ray should travel from denser to rarer i.e. from core to clad region of the fiber) and the angle of incidence in the denser medium should be greater than the critical angle of that medium.
77. What are step index and graded index fibers?
Ans. In the case of step index fiber, the refractive index of core is a constant and is larger than the refractive index of the cladding. The light propagation is mainly by meridional rays. In the case of graded index fiber (GRIN fiber) the refractive index of the core varies parabolically from the centre of the core having maximum refractive index to the core-cladding interface having constant minimum refractive index. Here the light propagation is by skew rays.
78. Define acceptance angle and numerical aperture of the fiber.
Ans. The maximum angle ‘Φmax‘ with which a ray of light can enter through the entrance end of the fiber and still be totally internally reflected is called acceptance angle of the fiber. Numerical aperture (N.A) of the fiber is the light collecting efficiency of the fiber and is a measure of the amount of light rays that can be accepted by the fiber. It is equal to the sine of the angle of acceptance.
where n1 and n2 arc the refractive indices of core and cladding respectively.
79. Why do we prefer step index single mode fiber for long distance communication?
Ans. Step index single mode fiber has (a) low attenuation due to smaller core diameter, (b) higher bandwidth and (c) very low dispersion.
80. Define relative refractive index difference ‘Δ’
Ans.
Thus relative refractive index difference is the ratio between the refractive index difference (of core and cladding) and refractive index of core.
81. What are meridional rays?
Ans. Meridional rays are the rays following Zig Zag path when they travel through fiber and for every reflection it will cross the fiber axis.
82. What are skew rays?
Ans. Skew rays are the rays following the helical path around the fiber axis when they travel through the fiber and they would not cross the fiber axis at any time.
83. What is Vnumber of fiber or normalised frequency of fiber?
Ans. Vnumber of fiber or normalised frequency of the fiber is used to find the number of propagating modes through the fiber.
For example in the case of step index fiber Number of modes propagating through the fiber = V2/2.
84. What are the conditions for total internal reflection?
Ans.
- Light should travel from denser medium to rarer medium.
- The angle of incidence sho ald be greater than the critical angle the denser medium.
85. Give the relation between numerical aperture of skew rays and meridional rays.
Ans. (N.A)skew = cos γ (N.A)meridional when the fiber is placed in air.
Here γ is the half of the angular change in every reflection inside the fiber in the case of skew rays.
86. What is meant by threshold condition for laser oscillation?
Ans. There should be a minimum amount of population inversion from which laser oscillation starts. This is called threshold condition for laser oscillations. Therefore to start, the laser oscillation, the gain coefficient should exceed the threshold value.
87. What are direct band gap and indirect band semiconductors?
Ans.
Direct Band Gap Semiconductors:
- Here electron can directly recombine with a hole
- Life time of charge carriers is less
- Example InP, Ga As
Indirect Band Gap Semiconductors:
- Here electron can not directly . recombine with hole due to traps and recombination centres in the band gap.
- Life time of charge carriers is more.
- Example Si, Ge.
88. What are the drawbacks of homojunction laser diodes?
Ans.
- Threshold current is very large.
- The output beam has large divergence.
- Coherence and stability are poor
- Optical confinement is very poor.
89. What are the features of fiber optic sensors?
Ans.
- Highly reliable and secure due to immunity of the sensed signal to electromagnetic interference.
- Since the fiber optic sensors are made up of silica, they are free from fire or electric sparks. So they can be used safely in the explosive and nuclear environments.
- These are mostly suitable for remote sensing and telemetry.
- These have high accuracy and sensitivity.
- These have small size and less weight.
90. What are active and passive sensors?
Ans. Active sensor or intrinsic sensor is a sensor in which sensing and simultaneous measurement of the variable are taking place. They do not require any extra signal processor. Single mode fibers are widely used as active sensors which utilize intensity, phase and polarisation modulation methods. Passive sensor or extrinsic sensor is the sensor which acts merely as a transmission channel. for the light radiation. The sensed signal is transferred to a remote point for signal processing. Mostly multimode fibers are used as passive sensors.
91. What is the basic principle of interferometric sensors? What are their uses?
Ans. The basic principle of interferometric sensors is based on the phase shift produced by the given signal and the corresponding change in the interference pattern obtained from the superposition of reference beam and sensing beam. These are widely used to measure temperature, displacement, pressure, electric field and magnetic field intensities.
92. What is the principle of microbending sensor?
Ans. Microbending sensor is based on the production of microbending in the fiber by the given variable and the measurement of intensity of the transmitted light through the fiber. The microbending produces phase shift and coupling between different modes present in the transmitted light. This will lead to reduction in the intensity of the transmitted light.
93. What is the principle of laser doppler velocity sensor?
Ans. Based on the doppler effect, the velocity of the particle in the fluid flow can be measured from the doppler frequency shift produced in the laser beam.
94. What is Pockels effect?
Ans. Pockels effect refers to the change of refractive index of the medium by the applied electric field. Due to that, the phase shift or phase retardation is produced in the transmitted polarised light and hence there is a change in the intensity of the transmitted light.
95. What is Faraday rotation?
Faraday rotation refers to rotation of the angle of polarisation by a magnetic field or current.