AVS Engineering College
Department of Physics
Engineering Physics – I
PART-A Questions & Answers
UNIT –I ULTRASONICS
(1) How are sound waves classified?Sound waves are classified into three categories on the basis of frequency. (1) Infrasonics (frequency below 20 Hz)
(2) Audible sound (frequency in between 20Hz to 20,000Hz) (3) Ultra sound (frequency above 20,000Hz)
(2)What are the methods used to produce ultrasonics?
There are three methods used to produce ultrasonics (1) Mechanical generator (or) Galton’s whistle. (2) Magnetostriction oscillator method.
(3) Piezo-electric oscillator method.
(3) State the magnetostriction principle.
When an alternating magnetic field is applied to a rod of ferromagnetic material such as nickel, iron, cobalt then the rod is thrown into longitudinal vibrations. When the frequency of the vibrating rod is equal to the natural frequency of vibration, resonance occurs. Thus produces ultrasonic waves under resonance condition.
(4)Write the condition of resonance in a magnetostriction generator
Frequency of the oscillatory circuit = Frequency of the vibrating rod
1/ 2п√L1C1 = 1/ 2l√E/ρ
l is the length of the rod
E is the young’s modulus of the rod ρ is the density of material of the rod.
(5) State the merits and demerits of magnetostriction oscillator. Merits :
(1) It is mechanically versatile. (2)Cost is low.
(3)It can produce large acoustical power with high efficiency.
Demerits :
(1)It can produce frequencies upto 3 MHz only.
(2) It is not possible to get a constant single frequency, because it depends on the temperature and degree of magnetization.
(3)As the frequency is inversely proportional to the length of the vibrating rod, to
(6)What is piezo-electric effect?
When pressure or mechanical force is applied along certain axis (mechanical axis) with respect to optic axis of the crystals like quartz, tourmaline, Rochelle salts etc then equal and opposite charges are produced along the perpendicular axis (electrical axis) with respect optic axis of the crystal. This effect is called piezo-electric effect.
(7)What is inverse piezo-electric effect?
When the potential difference or e.m.f. is applied along certain axis (electrical axis) with respect to optic axis of the piezo-electric crystals then the crystal starts vibrating along the perpendicular axis (Mechanical axis) with respect to the crystal. This effect is called as inverse piezo-electric effect.
(8) Write the condition of resonance in a piezo-electric generator.
Frequency of the oscillatory circuit = Frequency of the vibrating crystal
1/ 2п√L1C1 = P/ 2l√E/ρ
l is the length of the crystal
E is the young’s modulus of the crystal ρ is the density of material of the crystal. P is the number of overtones. (P=1, 2, 3….)
(9) List the methods of ultrasonic detection.
(1) Kundts tube method. (2) Sensitive flame method. (3) Thermal method.
(4) Piezo-electric detector.
(10)Mention the properties of ultrasonic waves.
(1) They are highly energetic.
(2) They travel through long distances.
(3) They are reflected, refracted and absorbed similar to ordinary sound waves.
(4) When ultrasonics are passed through liquid, it produces stationary wave pattern and makes the liquid to behave as acoustic grating element.
(5)When an object is exposed to ultrasonics for a longer time it produces heating effect.
(11)What is Cavitation? Mention its use.
Cavitation is the process of creation and collapse of bubbles due to the negative local pressure created inside the bubble.
Uses:
(1) It is used to produce shock waves.
(12) What is meant by acoustic grating? Where it is used?
When ultrasonic waves are passed through a liquid, the density of the liquid varies layer by layer due to the variation in pressure and the liquid will act as a diffraction grating, so called acoustical grating. Under this condition when a monochromatic source of light is passed through the acoustical grating, the light gets diffracted. Then by using the condition for diffraction, the velocity of ultrasonic waves can be determined.
(13) Mention the applications of ultrasonics.
(1) Ultrasonic welding and soldering (2) Ultrasonic drilling and cutting (3) Ultrasonic cleaning and drying (4) Sound signaling
(5) Sonar.
(14) What is Non-Destructive Testing (NDT) method?
NDT is a method of testing the material, without destructing (or) damaging the material, by just passing X-rays (or) ultrasonics (or) any other radiations through the material.
(15) Compare destructive and non-destructive testing.
(16) Name the different scanning methods used in ultrasonics.
The different types of ultrasonic scanning methods are (1) A-scan (or) Amplitude modulated scan. (2) B-scan (or) Brightness modulated scan. (3) T-M scan (or) Time motion scan.
(17) Define A-scan display in ultrasonics.
A-scandisplay gives one dimensional information about the specimen. A-scan is also called amplitude
modulation. In this type the imaging is done by means of change in amplitude only.
(18) Define B-scan display in ultrasonics.
B-scan display gives two dimensional images. B-scan is also called as brightness modulation. The signals are received from the object in the form of varying intensity. The reflected images with varying intensity are studied.
S.No Destructive Testing Non-Destructive Testing
(1) This method is applied only to
the sample.
This method is applied directly on production item.
(2) Tested parts are damaged during testing.
Testing parts are not damaged during testing.
(3) Different tests cannot be
performed on the same item.
Different tests can be performed on the same item.
(19) Define C-scan (or) T.M. scan display in ultrasonics.
This mode can be used to obtain the information about the moving object. When the object moves during the scan, the dots also move at a lower speed. The pattern of movement is recorded and this will appears as a trace.
(20) What is sonogram? Mention its applications.
Sonogram is an instrument used to monitor and visualize the image of the interior parts of the body using high frequency ultrasonic sound waves.
Applications:
(1)The sonogram is used to monitor health and development of the fetus. (2)It is used to confirm pregnancies and ensure that fetus is growing normally.
(21) Mention a few medical applications of ultrasonic waves.
The ultrasonic waves are used in medicine in the following areas, (1) Ultrasonic imaging system.
(2)Cardiology. (3)Neurology. (4)Dental.
(22) Write the principle of sonar in ultrasonics.
It is based on the principle of Echo-sounding. When ultrasonic waves are transmitted through water, it is reflected by the objects in the water and will produce an echo signal. The change in frequency of the echo signal, due to doppler effect helps us to determine the velocity and the direction of the object.
(23) Draw the block diagram of SONAR and list its parts.
(24) An ultrasonic generator consists of a quartz plate of thickness 0.7 mm and density 2800Kg/m3. Find the fundamental frequency of ultrasonic waves if the Young’s modulus of quartz is 8.8*1010 N/m2.
Given values
Thickness of the crystal (t) = 0.7*10-3m.
Density of the quartz crystal (ρ) =2800Kg/m3.
Young’s modulus of the quartz crystal (E) = 8.8*1010 N/m2
Solution:
Frequency (f) = (P/ 2t)*√E/ρ
P=1
f = (1/ 2* (0.7*10-3))*(√8.8*1010 /2800)
The frequency of vibrating crystal f = 4.0043 MHz.
(25) A quartz crystal with 1mm thickness is vibrating at resonance. Calculate the fundamental frequency. Given Young’s modulus for quartz=7.9*1010 N/m2 and density for quartz=2650Kg/m3.
Pulse/frequency generator Timing
section CRO Amplifier
Transducer
(Transmitter) Object
Given values
Thickness of the crystal (t) = 1mm=1*10-3m.
Young’s modulus of the quartz crystal (E) = 7.9*1010 N/m2.
Density of the quartz crystal (ρ) =2650Kg/m3.
Solution:
Frequency (f) = (P/ 2t)*√E/ρ
P=1
f = (1/ 2* (1*10-3))*(√7.9*1010 /2650)
The frequency of vibrating crystal f =2.7299 MHz.
(26)What is PCG? What are its uses?
Phonocardiography deals with graphic recording of heart sounds. Uses:
(1)Phonocardiography is used to record the pumping action of heart. (2)The damaged valves can be identified.
(3)Fetus growth can be monitored by picking up of fetal heart sounds.
(27)Differentiate ECG& PCG.
(28)Compare piezo electric method and magnetostriction method. `
UNIT – II LASER 1. What are the characteristics of laser?
i. It is highly directional
ii. It has high intensity
iii. The beam is purely monochromatic
iv. It has coherence
2. What are coherent sources?
S.No Electrocardiography (ECG) Phonocardiography (PCG)
(1) The activity of heart such as rhythmic
disturbances can be found with help of electrical signals
Recording the sound of pumping heart is done with help of sound signals
(2) It is not possible to find valvular
defects.
Valvular defects can be identified clearly by using PCG.
S.No Piezo electric method Magnetostriction method
(1) It generates very high Frequencies
(500 MHz). It generates low frequency Ultrasonic waves (3 MHz).
(2) Frequency of oscillation is
Coherent sources are the sources which have same wavelength and frequency. It has correlation with the amplitude and phase at any point with any other point.
3. Define coherent length and time? How are they related to each other?
The maximum length up to which two waves trains have correlation with the amplitude and pulse is called coherent length and the time up to which they are correlated is called coherent time. Coherent time = Coherent length / Velocity of light
4. State some of the applications of lasers in engineering and industry field
i. High power lasers are useful to blast holes in diamonds and hard steel
ii. They are used to test the presence of pores, cracks flows, blow holes etc in the material.
iii. They are used for welding and cutting
5. Explain the importance of holographic storage?
i. Each point of an object is recorded on the whole hologram.
ii. From each piece of a hologram we can reconstruct the image.
iii. Laser is used for recording and reconstruction processes.
iv. The phase and amplitude information in the reflected waves from the object are completely
recorded in the hologram.
6. What is holography?
Holography deals with image construction by means of interference techniques without using lenses. Here the distribution of amplitude and phase is recorded in 3D manner so as to get complete information of the object to be photographed.
7. What are the different methods of achieving population inversion?
i. Optical pumping
ii. Direct electron excitation
iii. Inelastic atom-atom collision
iv. Direct conversion
v. Chemical process
8. What is the function of helium in He-Ne laser?
Helium atom delivers its energy to Neon atoms and it causes population inversion.
9. Define meta stable state:
It is the state for which the life time is more than the excited sate that is, it is the more stable state, which lies between the excited state and the ground state.
10. What is the principle of semi conductor laser?
The electron in conduction band combines with a hole in the valance band and hence the recombination of electron and hole produces energy in the form of light. This photon, in term may induce another may induce another electron in the conduction band to the valance band and there by stimulate the emission of another photon.
s.no Spontaneous emission Stimulated emission
1. The atom in the excited state returns to
Ground state there by emitting a photon, without any external inducement is called as Spontaneous emission
An atom in the excited state is
induced to return to ground state there by emitting two photons of same frequency and energy is called as Stimulated emission
2. The emitted photons can move randomly The emitted photons can move in
same direction and is highly directional.
3. The radiation given out is of less in
intense and are incoherent
The radiation is highly intense, monochromatic and coherent.
4. The photons are not in phase The photons are in phase
5. The rate of transmission is given by
R21 (SP)=A21 N2
The rate of transmission is given by R21 (ST)=B21 ργ N2
12. Distinguish between Ruby laser and Nd-YAG laser
s.no Ruby laser Nd-YAG laser
1. This is a three level laser This is a four level laser
2. Elliptical cavity resonator is used Elliptical cavity resonator is not used
3. Xenon flash lamp is wound around the
ruby rod Here the Nd-YAG rod and Xenon flash lamp are placed at the two foci
of Elliptical cavity
4. Power output is 104 -105 watts Power output is 2×104 watts
5. Wavelength of output is 6943 A˚ Wavelength of output is 1.064 µm
13. Classify different types of lasers based on active medium, with one example for each:
s.no Type Example
1. Solid state laser Ruby laser-Active medium is ruby rod
2. Gas laser CO2 laser-active medium is the mixture
of water vapour
3. Liquid laser Europium chelate laser-active medium is
benzoyl accetonate dissolved in alcohol
4. Dye laser Coumarin dye laser-active medium is
coumarin compound.
5. Semi conductor laser Ga As laser- active medium is P-N
junction diode made up of Ga and As
14. Will laser be called as a non material knife? Justify.
Yes .because in laser surgery, without knife or hammer, bloodless operation, cutting tissues, etc. can be made.
s.no LED LASER
1. It requires low current density. It requires high current density.
2. Junction of diode need not be polished. Junction of diode should be highly
polished.
3. Minority carrier injection should take
place.
Stimulated emission will take place
4. Power output is low. Power output is high.
5. Intensity is less. Intensity is very high.
16. Will laser be called as a non material knife? Justify.
Yes .because in laser surgery, without knife or hammer, bloodless operation, cutting tissues, etc. can be made.
17. Give the Importance of population inversion:
When population inversion is achieved, the majority of atoms are in the excited state. So, the absorption coefficient will be negative. The negative absorption coefficient causes the amplification of the incident beam by stimulated emission. Thus the laser beam is produced. Hence, population inversion is a must for the production of laser beam.
18. Define population and population inversion:
The no of atoms in the ground state will be more then that of the atoms in the excited state and it is called as usual population.
A state of achieving more no of atoms in the higher energy level than that of the lower energy level is called population inversion.
19. Can a two level system be used for the production of laser? Why?
No, two level system cannot be used for the production of laser, because for population inversion to be achieved at least three levels are required.
20.Distinguish between holography and photography:
s.no holography photography
1. Laser light is used Ordinary light is used
2. The distribution of amplitude and
phase is recorded.
The variation of amplitude alone is recorded.
3. It gives 3D picture. It gives 2D picture.
4. No lens is used lens is used
21. What are the two types of transitions that are possible in a CO2 Gas laser?
i. Transition from asymmetric mode (00˚1) to bending mode (02˚0) will emit laser of wavelength
9.6µm.
ii. Transition from asymmetric mode (00˚1) to symmetric mode (10˚0) will emit laser of
wavelength 10.6µm.
In CO2 laser the nitrogen helps to increase the population of atoms in the upper level of CO2
while He helps to depopulate the atoms in the lower level of CO2 and also to cool the discharge tube.
23. What is Stimulated emission of radiation?
An atom in the excited state is induced to return to ground state there by emitting two photons of same frequency and energy is called as Stimulated emission.
24. Distinguish between homo junction and hetero junction lasers:
s.no homo junction hetero junction
1. Homo junction laser is made by a
single crystalline material.
Hetero junction laser is made by different crystalline material.
2. Power output is low Power output is high.
3. Pulsed output. Continues output.
4. It has high threshold current density. It has low threshold current density.
5. Cost is less. Cost is more.
6. Life time is less. Life time is more.
7. Exam: Ga-As and InP. Exam: Ga-As/GaA/As and InP/InA/P.
25. What are the advantages of gas laser over solid state laser?
It produces more directional and highly monochromatic output than solid state lasers.
The output can be increased by extending the length of gas tube.
26. What is pumping action?
The process of raising more number of atoms to excited state by artificial means is called as pumping process.
27. Distinguish between ordinary light and laser light
s.no Ordinary light Laser light
1. Intensity is lesser Higher intensity
2. Directionality is very less Directionality is high
3. It is not a coherent source It is a coherent source
4. Angular spread is more Angular spread is less
5. It is a polychromatic source
Eg: sun light
It is a monochromatic source Eg : laser, sodium vapour lamp etc.,
28. What is the condition for laser action?
Laser action can be achieved when the stimulated emission dominates spontaneous emission that is population inversion must be achieved in a system for the emission of laser action.
29. What are different types of holography?
Optical holography
Acoustical holography
X- ray holography
Microwave holography
Holography is used in,
Production of Photographic masks
Production of Diffraction grating
NDT
Identification of finger prints
Data processing
Optical signal processing
31. Define active medium and active centre.
The medium in which the population inversion takes place is called as active medium.
The material in which the atoms are raised to excited state to achieve population inversion
is called as active centre.
32. Give the principle of laser action.
Due to stimulated emission the photons multiply in each step giving rise to an intense beam of photons that are coherent and moving in the same direction.
Hence the Light is Amplified by Stimulated Emission of Radiation termed as LASER.
33. What are the basic phenomenon’s involved in laser action?
Process of absorption
Spontaneous emission and
Stimulated emission
34. What is optical resonator or resonance cavity?
Resonant cavity is a set of reflecting surfaces. One of the surfaces is fully reflecting and the other is partially reflecting. The photons moving back and forth between the reflecting surfaces leads laser action.
35. What are the different modes of stretching in CO2 laser?
The fundamental modes of vibration of the CO2 molecules are,
Symmetric stretching mode
Bending mode
Asymmetric stretching mode
36. Give the principle of CO2 laser
The nitrogen atoms are excited by electric discharge. These excited nitrogen atoms
transfers energy to CO2 molecules which has closest energy level. Then the transition takes place
between the vibration energy levels of CO2 molecules and hence laser beam is emitted.
37. Name some applications of Nd-YAG(Solid state ) laser
Nd-YAG laser is used in,
Transmitting signals to longer distances
Long Haul communication system
Endoscope applications
Remote sensing applications
A combination of two types of gases used, say A and B both having same excited states A* and B*.
During elastic discharge the A atom gets excited A* due to collision with electrons. The excited A* atoms now, collide with B atom, so that B goes to excited state B* as,
eˉ + A -> A* A* + B -> B* + A
eg. He-Ne laser
39. Give some applications of laser in medical field
Laser is used to
Drill minute holes in cell walls of human body
Treat cancer and tumour
Treatment of detached retina
Carry out microsurgery and bloodless operation
Shatter the kidney stones
Cut the bones precisely
40. What are the precautions to be followed while constructing a hologram?
i) The distance travelled by the reference beam and the object beam should be almost equal.
ii) Ratio of light intensity between the reference beam and the object beam should be 3:1.
iii) Total darkness should be maintained, while loading the holographic Plate.
iv) The total setup has to be kept in a vibration less stable, to avoid vibrations due to the
natural seismic vibrations of the earth.
41. What are the applications of laser in communication and military fields?
(a) Communication
Transmitting signals to longer distances
Long Haul communication system
(b) military
It is used for,
Producing laser guns
Annihilate objects like airplanes and missiles.
UNIT –III FIBER OPTICS & APPLICATIONS 1 .What is an optical fiber?
For an efficient communication system, the information carried out by light waves requires a guiding medium though which light can be transmitted safely. This guiding medium is called optical fiber. The optical fibre consists of a central tube called core and an outer tube called cladding with different refractive indices.
2. Define acceptance angle?
The maximum angle at which the ray of light can enter into the fibre so that the light will be totally internally reflected inside the fibre is called acceptance angle.
3. Define numerical aperture of a fibre.
NA = √ n2 1 - n22
4. What is a single mode fibre?
Single mode fibre is the fibre, in which only one mode of light can be propagated. It has smaller core diameter and difference in refractive indices of core and cladding is small.
5. What is multimode fibre?
The fibre in which more than one mode of light is allowed to pass through a core is called multimode fibre. It has large core diameter and difference in refractive index of core and cladding is large.
6. What is splicing? List out the different types of splicing.
Splicing is a method used to connect the fibres permanently to carry the information. They are two types namely
I. Fusion splicing.
II. Mechanical splicing.
7. How the scattering loss occurred in opticalfibre?
Glass is the material used for fabrication of fibres. By nature, glass is a disordered structure in which the material density fluctuation is observed. This leads the variation of refractive index and causes scattering loss.
8. What is radiation loss in fibre optics?
The radiation loss occurs in fibre due to bending of curvature. The bending of fibre is classified into microscopic bending and macroscopic bending.
9. What is the role of cladding in an optical fiber?
Cladding is a transparent dielectric material in cylindrical form next to the core, which has slightly lower refractive index (n2) than core material (n1). Cladding is used to produce total internal reflection within the fiber
by keeping n1>n2. It is also used to produce gradient in refractive index.
10. Features of optical fibers.
1. It is light in weight.
2. It is smaller in size and is flexible, so that it can bend to any position. 3. It is non-conductive and non-inductive.
4. It has high bandwidth and low loss.
5. There is no short circuiting as in metal wires. 6. There is no internal noise / cross talks.
7. It can withstand to any range of temperature and moisture condition. 8. There is no need to ground and hence no voltage problem occurs.
11. What are the conditions to be satisfied for total internal reflection?
(i) Light should travel from denser medium to rarer medium.
(ii) The angle of incidence(ф) on core should be grater than the critical angle (фc).
i.e. ф > фc
(iii) The refractive index of the core (n1) should be greater than the refractive index of the cladding (n2) .
12. How will you classify optical fibres?
Optical fibres are classified in to three major categories based on (i) Material (ii) Number of modes (iii) Refractive index profile.
(i) Based on the material it can be classified in to
(a) Glass fibre (b) plastic fibre
(ii) Based on number of modes they are classified as
(a) Single mode fibre (b) Multi mode fibre
(iii) Based on refractive index profiled they can be classified as.
(a) Step-index fibre (b) Graded index (GRIN) fibre
13. What are the losses that occur during optical fiber communication?
During the transmission of light through the optical fiber, three major losses will occur, viz., attenuation,
distortion and dispersion.
Attenuation is mainly caused due to the absorption, scattering and radiation of light inside the fibers.
Distortion and dispersion occurs due to spreading of light and also due to manufacturing defects.
14. How an LED can be converted into laser diode?
In a semiconductor diodes, if the emission is not stimulated, the device is called light emitted diode (LED).
To convert LED in to a laser diode, high current density is required to achieve population inversion and the opposite surfaces should be polished.
15. State some of the application of optical fibres in medical field.
1. Fibre optic endoscopes are used in medical diagnosis. 2. It is used to visualize the inner organs of the body.
3. Fibres as endoscope are used in various medical fields such as cardioscopy, Laproscopy, Cytoscopy etc.
16. What is meant by endoscope?
A medical endoscope is a tubular optical instrument, used to inspect (or) view the internal parts of human body which are not visible to the naked eye. The photograph of the internal parts can also be taken using endoscope.
17. Mention the advantage of optical fibre communication over radio wave communication?
(i) Optical communication can be made even in the absence of electricity. (ii) The optical signals are not affected by any electrical signals or lightening. (iii) Optical fibre communication is free form electromagnetic interference (EMI).
` (iv) This type of communication is suitable to any environmental conditions.
(v) Easy maintenance, longer life, economical and high quality signal transmission are the additional features of optical fibre communication.
Mode of propagation r3epresents the number of possible directions or the path of propagation of light through the optical fibers. When single ray of light propagate through a path then it is called single mode and when many rays propagates through different direction it is called multimode.
19. Mention the properties of detectors used in fibre-optic communication.
The detectors should possess the following properties, viz.
(i) Ability to convert optical signal in to electrical signal
(ii) Fast response time
(iii) Zero dark current and
(iv) Cost effective.
20. What is meant by attenuation?
It is defind as the radio of the optical power output (Pout) from a fiber of length ‘L’ to the power input
(Pin).
-10 Pin
Attenuation (α) = --- log --- dB/km
L Po
21. Distinguish between step-index and graded index fibres.
S.No Step-index fibre Graded index fibre
1
The difference in refractive indices between the core and the cladding is obtained in a single step and hence called as step-index fibre.
Due to non uniform refractive indices, their difference in refractive indices between the core and the cladding gradually increases form centre towards interface and hence called graded-index fibre.
2 The light rays propagate as meridinal rays and pass through fibre axis.
The light propagation is in the form of skew rays and does not cross the fibre axis.
3 It follows a zig-zag path of light propagation. It follows a helical path (i.e spiral manner) of light propagation.
4 It has a low bandwidth It has high bandwidth.
5 Distortion is more in multimode step-index fibre. Distortion is very less and is at most Zero due to self focusing effect.
22. What is meant by a photo detector?
Photo-detector is a device to detect the light falling over it. It converts the light energy in to electrical energy.
Examples : (i) PIN photo diode
(ii) Avalanche photo diode (APD)
S.No Single mode fibre Multimode fibre
1 In single mode fibre only one mode
can be propagated.
The fibre in this case allows large number of modes of light to propagate through it.
2
The single mode fibre has a smaller core diameter and difference in
rerfactive index of core and cladding is small.
Here, since the core diameter is large, the core and cladding
refractive index difference is also large.
24. What are the types of sensors used in the fibre optics?
There are two types of sensors used viz.,
(i) Intrinsic sensors (or) Active sensors :
Here fibre itself acts as a sensing element.
(ii) Extrinsic sensors (or) Passive sensors :
Separate sensing system collects the light from the fibre.
25. Calculate the numerical aperture and the acceptance angle of an optical fibre from the following data. Refractive index of core is 1.55, Refractive index of cladding is 1.50.
Given Data
Refractive index of core is n1 = 1.55
Refractive index of cladding is n2 = 1.50
Solution : Formula:
(i)Numerical aperture (NA) = √ n2
1 - n22 = √0.1525
NA = 0.3905 (No unit)
(ii)Acceptance angle sin Im = NA
Im = sin-1 (NA)
Im = Sin-1 (0.3905)
Im = 22˚59 ́
The numerical aperture and acceptance angle of the optical fiver is 0.3905 and 22˚59 ́ respectively.
26. Estimate NA when the core refractive index is 1.48, relative refractive index is 2. Also calculate the critical angle.
Solution: Δ = 0.02
We Know, NA = n1(2Δ)1/2
= 1.48 (0.04) 1/2
= 0.296
n1 - n2 n2
n1 n1
n2
Therefore --- = 1- Δ = 1-0.02 = 0.98
n1
n2
The critical angle фc = sin-1 ---- = sin-1 (0.98)
n1
The critical angle фc = 78.52˚
27. A single of 100 mW is injected into a fibre. The out coming signal from the other end is 40 mW. Find the loss in dB?
Solution: Given Data:
Power input (Pin) = 100mW
Power output (Pout) = 40mW
Formula:
Pout
Power loss (PL) = 10 log --- dB
Pin
40x10-3
= 10 log --- dB
100x10-3
PL = 10 log (0.4) dB
PL = -3.979 dB
Here the negative sign indicates the loss of power, with respect to distance traveled by the signal through the fibre.
The power loss PL = -3.979 dB
28. Calculate the refractive indices of core and cladding materials of an optical fibre if its numerical aperture is 0.22 and relative refractive index differences are 0.012.\
Solution;
NA = 0.22 Δ = 0.012
Formula:
Numerical aperture NA = n1 √2 Δ
NA n1 =
√2 Δ 0.22 n1 =
---√2x0.012 n1 = 1.42
We know the relation n2 = n1 ( 1- Δ )
The refractive index of core is = 1.42 And the refractive index of cladding = 1.40
UNIT-IV - QUANTUM PHYSICS 1. Define Kirchoff’s law of radiation?
Ratio of emissive power to the coefficient of absorption of any given wavelength is the same for all bodies at a given temperature and is equal to the emissive power of the black body at that temperature.
i.e eλ/aλ =E
2. What are the postulates of Planck”s quantum theory?
(i) The electrons in the black body are assumed as simple harmonic oscillators. (ii) The oscillators will not emit energy continuously.
(iii) they emit radiation in terms of quantas of magnitude ‘hυ’, discretly. i.e. E= nhυ where n= 1, 2,3…..
3. What is Compton wavelength? Give its value.
The shift in wavelength corresponding to the scattering angle of 90° is called Compton
wavelength.
We know Compton shift Δλ = h(1-cosѲ)/m0c
Since Ѳ=90°, cos Ѳ=0
So, Δλ = h/m0c= 6.625*10-34/(9.11*10-31)*(3*108)
Δλ = 0.02224Å.
4. Give the concept of dual nature of light.
We know nature loves symmetry, since the light exhibits the dual nature (i.e) it can behave
both as a particle and the wave, debrogile suggested that an electron, which is particle can also behave as a wave and exhibits the dual nature.
Thus the wave associated with a material particle (electron) is called as matter waves. If v is the
velocity and m is the mass of the particle then de Brogile wavelength λ=h/mv.
5. Give the physical significance of wave function?
1. The probability of finding a particle in space, at any given instant of time is characterized by a function Ψ (x,y,z) called wave function.
2. It relates the particle and the wave statistically. 3. It gives the information about the particle behaviour. 4. It is a complex quantity.
5. Ψ2 represents the probability density of the particle, which is real and positive.
6. What is a black body and what are its characteristics?
In practice no body is perfectly black. We have to coat the black colour over the surface to make a black body.
It is said to be a perfect absorber, since it absorbs all the wavelengths of the incident radiation.
The black body is a perfect radiator, because it radiates all the wavelength absorbed by it. This phenomenon is also called black body radiation.
7. Define Stefan- Boltzmann’s law.
It is defined as “The radiant energy (E) of the body is directly proportional to the fourth power of
the temperature (T) of the body”. i.e. E∞T4
E= σ T4 where, σ is the Stefan constant.
8. State Wien’s displacement law. Give its limitation.
It is defined as, “The product of the wavelength (λm) of maximum energy emitted and the
absolute temperature (T) is a constant.
i.e. λmT= constant and also the Emax∞T5
or Emax=constant. T5
Limitation: it holds good only for shorter wavelength.
9. Define Rayleigh-Jeans law. Give its limitation.
It is defined as, “The energy is directly proportional to the absolute temperature and is inversely
proportional to the fourth power of the wavelength”
i.e. Eλ ∞T/λ4 or E= 8ΠKBT/ λ4 where KB is the Boltzman constant.
Limitation: It holds good only for longer wavelengths. i.e. E=nhυ n=0, 1, 2,3….
10. What is meant by photon?.Give its two properties.
Photons are discrete energy values in the form of small quantas of definite frequency or
wavelength.
Properties: 1. They do not have any charge and they will not ionize. 2. The energy and momentum of the photon is given by E=nhυ and p=mc.
Where υ is the frequency m is the mass of photon c is the velocity of photon h is the Planck’s constant.
11. What is meant by wave function?
Wave function (Ψ) is a variable quantity that is associated with a moving particle at any position
(x, y, z) and at any time ‘t’. It relates the probability of finding the particle at that point and at that time. Since Ψ is a complex quantity, it has no meaning and hence the probability function Ψ2 = Ψ* Ψ is found,
which is real and positive and has physical meaning, which is a measurable quantity too.
12. Define Eign function and Eign value.
Eign function is defined as the wave function of the particle and is denoted by the letter ( Ψn).
13. What is meant by degenerate and non-degenerate states?. Give example.
Degenerate state: For various combinations of quantum numbers if we get same eign value
(Energy levels) but different eign functions, then it is called degenerate state.
Example: For three combination of quantum numbers say (112), (121) and (211), we get same energy values i.e. E112=E121=E211 but different eign functions i.e. Ψ112≠Ψ121≠ Ψ211
Non-degenerate state: For various combinations of quantum numbers if we get same eign values (Energy levels) and same eign functions, then it is called Non-degenerate state.
Example: For three combination of quantum numbers say (222) we get same energy value E222 and same
eigen function Ψ222.
14. State magnifying power.
The magnifying power (M) of a microscope is defined as the ratio between the angle subtended
by the final image (β) at the eye to the angle subtended by the object (α) at the eye kept at the neat point. M= Angle subtended by the final image at eye/Angle subtended by the object at eye kept at the near
point. (or) M= β/α
15. Give the principle of an optical microscope.
A suitable etched specimen which does not have any scratches is placed in the bed plate of the
microscope. Now, the light is allowed to fall over the specimen and image formed due to the reflected light from the specimen is examined. Thus the microstructure of the specimen can be determined.
16. What is the principle of electron microscope?
In an electron microscope a stream of electrons are passed through the object and the electrons
which carry the information about the object are focused by electric and magnetic lenses (or) electro magnetic lenses.
17. What is the principle of SEM?
Electrons are made to pass through the specimen and the specimen is scanned using scanning
coils. The transmitted beam is used to produce 3-dimensional bright field image and the diffracted beam can be used to produce the 3-dimensional dark field image in the CRO (cathode ray oscilloscope).
18. State the principle of TEM?
Electron is made to pass through the specimen and the image is formed in the fluorescent screen,
either by using transmitted beam (Bright field image), or by using diffracted beam (Dark field image).
19. Give any three advantages and disadvantages of TEM.
Advantages
(1) It can be used to examine the specimen of size upto 0.2 nm.
(2) The magnification is 1,000,000 times greater than the size of the object.
(3) It has high resolution
Disadvantages
(1) The specimen should be very thin (2) It is not suitable for thick samples
20. Give any three advantages and disadvantages of SEM. Advantages
(1) It has high resolution (0.1nm) and high contract (2) It is used to examine the nano materials of size 1nm (3) It is used to produce a 3-dimensional image of size 1nm Disadvantages
(1) Cost is very high
(2) Sample preparation is tedious
(3) Some specimen can loss its structural property due to the interaction of electrons with the sample.
21. Write down the Schroedinger’s time independent and dependent equations.
Schroedinger time independent equation is
2Ψ+ 8π2m(E-V)Ψ/h2=0
Then Schroedinger time dependent equation will be (-h2 2/8π2m +V2)Ψ=(ih/2π)∂/∂tΨ.
22. What is meant by nano diffraction in SEM technique.
The electron beam after a passing through the specimen produces a diffraction pattern so called
diffraction.
23. Mention the applications of electron microscope.
(1) It has a very wide area of applications in the field of biology, metallurgy, physics, chemistry, medicine and engineering.
(2) It is used to determine the complicated structures of crystals. (3) It is used in the study of celluoids.
(4) It is used to study the structure of micro organisms such as virus, bacteria etc.
24. What are the two different ways of scattering of x-rays?
Like ordinary light waves, X-rays are scattered by matter in two different ways. (a) Coherent scattering or classical scattering or Thomson scattering.
(b) Incoherent scattering or Compton scattering.
25. Give any two properties of matter waves.
(a) If the mass of the particle is smaller, then the wavelength associated with that particle is longer. (b) If the velocity of the particle is small, then the wavelength associated with that particle is longer. (c) These waves do not depend on the charge of the particles. This shows that these waves are not
electromagnetic waves.
26. Find the energy of an electron moving in one-dimension in an infinitely high potential box of width 0.1nm.
Given data a=0.1*10-9m,
We know that energy of an electron is En=n2h2/8ma2 where n=1, 2,3...
Substituting the ginen values, we have
E=(6.625*10-34)2/8*9.11*10-31*(0.1*10-9)2
E= 6.022*10-18/1.6*10-19
So, E= 37.69eV.
27. Calculate the number of photons emitted by a 100 watts sodium vapour lamp. (λ=5893*10-10m).
Energy=hυ= hc/ λ
So E=6.625*10-34*3*108/5893*10-10
=3.3726*10-19joule
Number of photons emitted=Power/Energy =100/3.3726*10-19
=2.965*1020
UNIT-V
CRYSTAL PHYSICS 1. What is a space lattice?
Space lattice is an infinite arrangement of points in three dimensions in which every point has an identical surrounding.
2. What is unit cell?
It is the smallest geometrical structure of a solid from which the entire crystal structure can be constructed by repetition in three-dimension.
3. Name the seven crystal systems.
(i) Cubic (ii) Tetragonal (iii) Orthorhombic
(iv)Monoclinic
(v) Triclinic
(vi) Rhombohedral
(vii) Hexagonal
4. What is primitive cell?
A primitive cell is the simplest type of unit cell which contains one lattice point per unit cell.
5. What are Bravais lattices?
There are only 14 ways of arranging points in space such that the environment looks same from
each point. i.e., there are 14 possible types of space lattices out of the seven crystal systems. These 14 space lattices are called as Bravais lattice.
6. Define coordination number.
It is the number of nearest atoms directly surroundings a particular atom in a crystal.
SC, BCC, FCC and HCP.
Type of Structure Number of atoms per unit cell
Simple Cubic 1
Body Centred Cubic 2
Face Centred Cubic 4
Hexagonal Close Packed 6
8. What are the coordination number for SC, BCC, FCC
and HCP and Diamond?
Type of Structure Coordination number
Simple Cubic 6
Body Centred Cubic 8
Face Centred Cubic 12
Hexagonal Close Packed 12
Diamond 4
9. Define atomic radius.
The half of the distance between nearest neighbouring atoms in a crystal is known as atomic radius. It is denoted by ‘r’ and it is usually expressed in terms of the cube edge ‘a’ (lattice parameter).
10. Define Packing factor. What is its unit?
It is defined as the ratio of the total volume occupied by the atoms in a unit cell to the total volume
of a unit cell.
Packing factor = Total volume occupied by the atoms in a unit cell (v)
Total volume of the unit cell (V)
= Number of atoms in a unit cell×Volume of one atom
Total volume of the unit cell
There is no unit since it is a ratio.
11. What are Miller indices?
Miller indices are three possible integers that have the same ratio as the reciprocals of the intercepts of
the plane concered on the three axes.
12. Sketch the (110) plane in a cubic system.
14. Obtain Miller indices of a plane whose intercepts are a,b/2,3c in a simple cubic unit cell.
Actual intercepts are a, b/2,3c
Numerical parameters are 1, 1/2, 3
Reciprocals of the above 1, 2, 1/3
Take L.C.M (i.e.) 3, 6, 1
Miller indices of the plane = (3 6 1)
15. What are lattice parameters?
The lattice constants or axial length a, b and c in a unit cell and the interfacial angels, α, β and γ along
three axes are called the lattice parameters.
16. What is a crystal structure?
A crystal structure is formed by associating abasis to every lattice point in a space lattice
17.What is crystal defect?
The deviation from the regularity of arrangement of atoms is called crystal imperfection or crystal defect.
18. What are various types of defects.
1. Point defects (Zero dimensional) a) Vacancies
I. Schottky defect II. Frenkel defect b) Interstitial
I. Self interstitial II. Foreign interstitial
c) Impurity
II. Interstitial impurity defect.
2. Line defects (one dimensional)
a) Edge dislocation b) Screw dislocation
3. Surface defects (Two dimensional)
i.Grain boundaries
ii.Twin boundaries
iii.Tilt boundaries iv.Stacking faults
4. Volume defects (three dimensional)
a. Cavities or voids. b. Cracks and holes.
19. What is Frenklel defect?
A vacancy associated with interstitial impurity is called Frenklel defect
20. What is surface defect? Classify this type of defect.
The defects in the materials arise form a change in the stacking of atomic planes on or across in boundary are known as surface defects. There are four different types of surface defects. They are:
i) Grain boundaries ii) Tilt boundaries iii) Twin boundaries iv) Stacking fault.
21. What is impurity defect? What are the types of impurity defect?
A foreign substance added to a crystal is called impurity. The impurity atom may fit in the structure in two ways giving rise to two kinds of impurity defects.
I. Substitutional impurity. II. Interstitial impurity defect.
22. What are vacancies?
Vacancies are empty atomic sites. Vacancies may occur as a result of imperfect packing during the original crystallization or they may arise from the thermal vibrational of atoms at higher temperature.
There are different kinds of vacancies like Frenkel defect, schottky defect, colour centres etc.
23. What is Schottky defect?
If an atom is missing from its lattice site, the vacancy is called schottky defect.
(OR)
It refers to the missing of a pair of positive and negative ions pair in an ionic crystals.
24. What is line defect? What are their types?
The defect along a line is called line defect. There are two types of line defects. a) Edge dislocation
b) Screw dislocation
The magnitude and the direction of the displacement due to edge dislocation are defined by a vector called burger’s vector.
26. What are twin boundaries?
If the atomic arrangement on one side of the boundary is the mirror image of the arrangement on
the other side the defect is called Twin boundaries.
27. What is stacking fault?
This defect arises due to defect in the stacking of atomic planes. In some cases a part of certain atomic plane will be missing where as in some other cases a portion of extra atomic plane is present, changing the sequence of arrangement of atoms.
28. Write the comparison between Edge dislocation and Screw dislocation.
S.No Edge dislocation Screw dislocation
1. These dislocations arise duo
introduction or elimination of an extra plane of atoms.
Screw dislocation results from a displacement of the atoms in one part of a crystal relative to the rest of crystal forming a spiral ramp around the dislocation line.
2. Region of lattice disturbance extends
along an edge inside a crystal.
Region of lattice disturbance extends in two separate planes at right angles to each other.
3. An edge dislocation can glide and
climb.
A screw dislocation can glide only.
4. Burger vector is always perpendicular
to the dislocation line.
Burger vector is parallel to the dislocation line.
5. These are formed during deformation
and crystallization. These are also formed during deformation andcrystallization.
29. Write the difference between slip and twinning boundaries.
S.No Slip Twinning
1. The shear deformation which moves
atoms in one crystal plane over the atoms of another crystal plane by many inter-atomic distances relative to their initial positions is called slip
The shear deformation in a solid crystalline material through a process by which a portion of the crystal takes up an orientation which makes that portion as a mirror image of the parent crystal is called twinning.
2. Lower stress is enough to produce slip Higher stress is required.
3. It occurs due to the movement of
dislocations This can be produced by mechanical deformation
4. The orientation of the crystal above and
below the slip plane is same before and after deformation.
Twinning results in an orientation differences across the twin plane.
5. Slip band is formed after several
milliseconds Twin can form in a time as short as fewmicroseconds.
(1 1 2), (0 0 1), (1 0 1), (2 0 0) and (1 0 0)
31. Figure below shows the three crystal planes. Compute the miller indices of those planes.
Fig (a) Fig (b) Fig (c) In Fig (a) the given plane is parallel to X and Z axes. Thus,
its numerical intercepts on these two axes are infinity.
The numerical intercept on Y axis is ½ (or) 0.5. Thus, the numerical intercepts of the plane is (∞, 1/2, ∞)
: . Miller indices of the plane is (0 2 0)
In fig (b) the given plane is parallel to Z axis. Thus, its numerical intercept of the Z axis is infinity.
The numerical intercept on X axis is 1 and Y axis is 1/2. Thus, the numerical intercepts of the plane is (1, 1/2, ).
:. Miller indices of the plane is (1 2 0)
In Fig (c) The given plane is parallel to Z axis. Thus, its numerical intercept on this Z axis is infinity. The numerical intercept on X and Y axis i/2 and ½. Thus numerical intercepts of the plane is (1/2, ½, ∞).
:. Miller indices of the plane is (2 2 0) 32. What is a edge dislocation?
It is a region of lattice disturbance extending along an edge inside a crystal due to the insertion of an extra plane of atoms.
33. What is screw dislocation?
Screw dislocation results from a displacement of the atoms in one part of a crystal relative to the rest of crystal forming a spiral ramp around the dislocation line.
34.Define Polymorphism and allotropy.
Many metals have different structures without any change in chemical composition when the temperature or pressure is varied is known as polymorphism.
If the change in structure is reversible, then the polymorphic change is called as allotropy.
.35.Name the crystal structure of the following.
(a) Gold – FCC (b) Germanium-Diamond cubic
(c) Barium – BCC (d) Zinc – HCP
36. Define inter-atomic distance and interplanar distance.
Inter atomic distance: The distance between the centers of any two nearest atoms is called inter-atomic distance.
Inter-planer distance: The distance between any two parallel planes is called inter-planer distance.
37.What is a Basis?
A unit assembly of atoms (or) molecules identical in composition, arrangement ant orientation is called the basis. A crystal structure is formed by associating a basis to every lattice point in a space lattice.
38. What is a crystal? (or) What are crystalline materials? Give example.
Crystalline solids (or) crystals are those, in which the constituent atoms (or) molecules are arranged in an orderly fashion throughout ina three dimensional pattern. Example: Copper, Silver.
39.Whart are the differences between crystalline and non- crystalline material.
S.No Crystalline material Non-Crystalline material
1 They have a definite and regular
geometrical shapes which extend through the crystal.
They don’t have definite geometrical shape
2 They are anisotropic They are isotropic
3 They are most stable They are less stable
40. State the conditions imposed on the cell parameters for crystal systems having the largest bravais lattices and the lease number of nearest neighbours.
i) System having largest number of Bravasis lattices is
orthorhombic. It’s cell parameters a≠b≠c; =β=γ=90◦
ii) System having least number of nearest neighbours is in
diamond cubic (4).It’s cell parameters a=b=c; =β=γ=90◦
PART-B QUESTIONS
1. Define coordination number and packing factor for a hexagonal close packed structure. Show that an HCP structure demands an axial ratio of 1.633 (April 2002)
2. Show that the packing factor for HCP and FCC equal (April 2003)
3. (i) Define packing factor of a unit cell.
(ii) Explain the term atomic radius, co-ordination number and packing factor.
(iii) Obtain the expressions for packing factor of FCC and BCC unit cells. (April 2003, May 2004)
4. (i) Obtain the packing factor for the hexagonal closed packed structure.
(ii) Show that for a cubic structure the interplanar distance‘d’ In terms of miller indices and a cell edge’ a’ is given by d= (Jan 2005)
5. (i) Describe the structure of HCP crystal
(ii) Calculate the axial ratio and atomic packing factor for HCP structure. (June 2005)
