New I SEM 2 Marks Question.docx

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Department of Physics

Engineering Physics – I

PART-A Questions & Answers


1. Name the seven crystal systems.

(i) Cubic (ii) Tetragonal (iii) Orthorhombic (iv)Monoclinic

(v) Triclinic (vi) Rhombohedral (vii) Hexagonal

2. What is a space lattice?

Space lattice is an infinite arrangement of points in three dimensions in which every point has an identical surrounding.

3. 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.

4. What is a primitive cell?

A primitive cell is the simplest type of unit cell which contains one lattice point per unit cell.

5. Name the crystal structure of the following:

(a) Gold - FCC (b) Germanium - Diamond cubic (c) Barium - BCC (d) Zinc - HCP

All the above elements exhibit the structures given above at 20C. But for barium other structures also exists at6 other temperatures.

6. 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.

7. Define coordination number.

It is the number of nearest atoms directly surroundings a particular atom in a crystal.

8. Give the values of number of atoms in unit cell of SC, BCC, FCC and HCP.

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).

Type of Structure Number of atoms per unit cell

Simple Cubic 1

Body Centred Cubic 2

Face Centred Cubic 4


10. What are the coordination number for SC, BCC, FCC and HCP and Diamond?

11. 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.

Total volume occupied by the atoms in a unit cell (v)

Packing factor =

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.

12. 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.

13. Sketch the (110) plane in a cubic system.

14. Sketch the (111) plane in a cubic system.

Type of Structure Coordination number

Simple Cubic 6

Body Centred Cubic 8

Face Centred Cubic 12

Hexagonal Close Packed 12


15. 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)

16. 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.

17. What is a crystal structure?

A crystal structure is formed by associating abasis to every lattice point in a space lattice

18. What is crystal defect?

The deviation from the regularity of arrangement of atoms is called crystal imperfection or crystal defect.

19. Draw the following planes in a cubic structure. (1 1 2), (0 0 1), (1 0 1), (2 0 0) and (1 0 0)

20. Figure below shows the three crystal planes. Compute the miller indices of those planes.


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) 21. Name the crystal structure of the following.

(a) Gold – FCC (b) Germanium-Diamond cubic

(c) Barium – BCC (d) Zinc – HCP

22. 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.

23. 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.

24. 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.

25. What 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

4 Example: Nacl, KCl Example: Plastic, glass, rubber

26. 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


UNIT - II PROPERTIES OF MATTER 1. Define Stress on a Body?

The internal forms Developed per unit area in the body during the applications of external force or load is called stress. Within the elastic limit, the Stress is the equal to the external force per unit area; its unit is Nm -2 or Pascal.

2. Define Strain in the body.

The ratio of change in dimension to the original dimension of the body during the application of load is called stain.

3. Define Elastic limit of a body?

Elastic limit is the maximum amount of stress up to 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 even though the deforming forces are removed from it.

4. Define yield Strength & ultimate tensile strength?

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 & 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.

Hooke’s law states that within the elastics limit the stress Developed in the body is directly proportional to strain in it.

i. e: Stress/Strain a constant called modulus of elasticity.

6. Define modulus of elasticity.

Modulus of elasticity is the property of the material by which is has a little deformation even though a large amount of deforming force applied on it. Thus it’s the property by which it resists the applied force or load to undergo deformation.

7. What is the effect of temperature on elasticity?

Elasticity is inversely proportional to temperature .If the temperature increases, even 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 factory in designing engineering structures.

Safety Factor is an important parameter for each engineering material from which the life of the structure can be evaluated.

Safety factor = Ultimate tensile stress /Working Stress

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 & is about 10.

9. What are the uses of stress –strain curve?

It is used to determine the elastic strength, yields strength, ultimate tensile strength & compressive

Strength of an engineering material.


One can study the effect of beat 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 modules of elasticity? What is relation between them? Young’s Modulus of elasticity

E = Linear Stress / Linear Strain

Bulk Modulus of elasticity

K = Bulk stress / Bulk strain

Rigidity Modulus of Elasticity

N =Shearing Stress/ Angle of shear 9/E=3/N+1/K

11. What is poison’s ratio? What are the limiting values?

Poisson’s Ratio = Lateral strain/Longitudinal strain and -1< O < 0.5

12. Define uniform bending & non-uniform bending of a beam

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 & every point of the beam will be the same. Non-uniform bending of beam is the bending due to the application of non-uniform loading of the beam such that the load is supplied at any point of the beam & so the action of the load is non –uniform throughout the beam.

13. Why do we prefer I-shape girders rather than solid girders?

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 & due to their larger depth; the depression produced is almost negligible which enhances the life of the girder.

14. Give the relation between the three modules?

Y = 9kn /3K+n

Y – Young modulus K - Bulk modulus n – Rigidity modulus

15. What is moment of a force?

The moment of a force about a point is defined the product of the magnitude the force & the perpendicular distance from the point to the line of action of force.

16. Define torque.

Torque is the rotating force & equal to the moment of the couple. Torque is the Product of one of the forces forming Couple & perpendicular distance between the two opposite forces.

17. Define a beam.

A beam is defined as a rod or bar (Rectangular) of uniform cross –section whose length is very much greater than its other dimensions Such as breath & thickness.

18. Define a cantilever.


19. Define I-shape girder.

The girder is the one in which the upper & lower sections are Broadened & middle of the section is tapered so that it can with stand heavy loads over it. Since the girder looks like letter I, it is called as I – Shape girder.

20. Give the applications Of I –shape girders?

1. They are used in the construction of bridges over the rivers.

2. They are very much useful in the production of iron rails which are employed in railway tracks. 3. They are used as supporting beams for the ceilings in the construction of buildings.

21. Define shearing strain?

It is defined as the angular deformation produced in the body due to the application of external tangential forces on it.

22. What is meant by annealing?

Annealing is a process by which the material is heated to very high temperature & then it’s slowly cooled it. Usually this process is adopted for a material to increase the softness & ductility of the materials. Annealing also reduce the elastic property of the material, due to the formation of large crystal grains in it.

23. Define yield point.

If the external stress applied is very large, then the body will lose its elastic property, even after the removal of the stress.

The point at which the body loses its elasticity (elastic nature) is called as yield point.

24. What are the factors affecting elasticity.

i) Effect of stress. ii) Effect of annealing. iii) Presence of impurities.

iv) Effect due to the nature of crystals. v) Effect of temperature

25. Define Elastic fatigue.

If body is continuously subjected to stress (or) strain, it gets fatigued (weak) called as elastic fatigue.

26. Define Rigidity Modulus

It is defined as the ratio between the longitudinal stress to the longitudinal strain within the elastic limit. F/A

n = --- Ф

= F/AФ, (N/ m2)

27. Define elastic limit & plastic limit

The maximum stress up to which a body can recover its original shape and size, after removing the external forces is called as elastic limit.

After elastic limit, if the elasticity of the body is completely lost, then the body will be in a limit called as plastic limit.

28. Define bulk Modulus .what is the relationship between bulk modulus & compressibility.

Bulk Modulus is defined as the ratio between the volume stress to volume strain within the elastic limit.

K = F/A v/V

K=PV/v (N/m 2) ( P = F/A)



1. Define co-efficient of thermal conductivity?

Co-efficient of thermal conductivity is defined as the Quantity of heat conducted per second through

with area of the material when unit temperature gradient is Maintained Unit: Wm-1K-1

2.The total area of glass window pane is 0.6 m2.Calculate how much heat is conducted per hour through

glass window pane if the thickness of glass is 5mm , the inside temperature is 20 º C & outside temperature is 2 C Thermal conductivity of glass is 1.1 Wm-1K-1

Q = KA θ1-θ2 t X

=1.1X 0.6 20-2 3600

5x10-3 = 8.554 J

3. Is there any heat conduction through vacuum?

No, since vacuum has no atoms or molecules to transfer heat. Thermal conduction takes place only in material medium & vacuum is a thermal insulator.

4. What are the characteristics of good & bad conductors?


1. They have low specific heat.

2. They can be easily heated or cooled. 3. They have high thermal & electrical Conductivity.

Ex : Metals like copper , silver & gold

1.They have high specific heat.

2.They cannot easily heated or cooled. 3.They have low thermal & electrical. Conductivity

Ex : Non –metals like glass, wood, & mica

5. Explain why specimen used to determine thermal conductivity of a bad conductor should have larger area & smaller thickness?

To get the accurate value of thermal conductivity of a bad conductor, the quantity of heat

conducted should be more. To get higher amount of heat conducted, the specimen with larger area & smaller thickness is used.

6. State the relation between heat & work.

dQ = dU + dW

If dU = 0, then dQ = dW. Here,

dQ is quantity of heat given to the body. dU is increase of internal energy.

dW is external work done by the body.

7. Give the reasons for the higher thermal conductivity of metals

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.

8. What are the required properties of thermal insulating materials? Give some examples for them.

1. Thermal insulating materials should have a fibrous, granular or porous structure. 2. They should have high specific heat & low thermal conductivity.


EX: wood (fibrous) , saw dust (granular), and foam tubular (porous) 9. Distinguish between conduction & convection:

Conduction :

It is the process in which the heat is transferred from hot end to cold end without the actual movement of the particles.

Convection :

It is the process in which the heat is transmitted from hot end to cold end by actual movement of particles.

10. What is meant by temperature gradient?

The rate of fall of temperature with respect to the distance is called as temperature gradient.

In general it is denoted as - dθ/dx . The negative sign indicates fall of temperature with the increase in distance.

11. Define Thermal Diffusivity.

It is defined as the ratio of thermal conductivity to the thermal capacity per unit volume of the Material.

Thermal diffusely (h) = Thermal conductivity / thermal capacity ,

Since thermal capacity is the product of specific heat capacity (s) & density of the material (ρ),We can

write, h = k/ρs m 2 / s-1

12.Define Newton’s Law of cooling

Newton’s Law: The rate of loss of heat of a body is directly proportional to the temperature difference between the body & its surrounding, of same nature.

13. Derive the unit for thermal conductivity.


The thermal conductivity of material is K = --- t

A (θ1-θ2)

Joules x meter Watts

= --- =

---(meter)2 x Kelvin x Second meter x Kelvin

The unit of thermal conductivity is Wm-1K-1

14.Explain radial flow of Heat.

In this method, heat flows from inner sphere (or) cylinder towards the outer sphere or cylinder along the radius & hence the heat is radiated radially across all layers. Thus it is called as radial flow method. This method is useful in determining the thermal conductivity of bad conductors.

15.What is meant by thermal resistance?

The thermal resistance of a body is a measure of its opposition to the flow of heat through it. (i.e) every body posses some resistive power when it is subjected to heat. This resistive power is termed as thermal resistance.

16. What is the Principle employed in Searle’s method?

Amount of heat conducted between the two points in a rod per second is calculated. This heat is absorbed by the water flowing through the tubes per second. Under steady state condition; the amount of heat flowing through any section of rod per second is equal to the amount of heat gained by per second.

Heat conducted by the section of the rod = heat gained by water.

17. What is the basic principle employed in lee’s disc method for bad conductor?


The amount of heat conducted through the bad conductor per second = Amount of heat lost per second by the disc.

18. Why the specimen used to determine thermal conductivity of a bad conductor should have a larger area & smaller thickness?

For a bad conductor with a smaller thickness & larger area of cross section, the amount of heat conducted will be more.

19. Give the methods of determining the thermal conductivity of good & bad conductors:

The methods used for determining the thermal conductivity of good conductors & bad conductors are:

1. Searle’s method = good conductors like metallic rod.

2. Forbe’s method = for determining absolute conductivity of metals.

3. Lee’s method = for bad conductors.

4. Radial flow method = for bad conductors.


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.


6. What is a black body and what are its characteristics?

A perfect black body is the one which absorbs and also emits the radiations completely.

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


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 value is defined as energy of the particle and is denoted by the letter (En).

Eign function is defined as the wave function of the particle and is denoted by the letter ( Ψn).


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.


(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


(1) The specimen should be very thin (2) It is not suitable for thick samples

(3) 3-dimensional image cannot be obtained.

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


(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


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... The lowest energy of the particle is obtained by putting n=1 Thus, E=12h2/8ma2

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




1. What are the factors affecting the acoustic quality of a building.

a) Reverberation time b) Focussing and interference c) Echoes and Echelon effect

d) Resonance e) Extraneous noise.

2. If the reverberation time is lower than the critical value, how will it affect the acoustical quality of a building.

When the reverberation time is lower than the critical value, sound becomes inaudible by the observer and the sound is said to be dead and if the reverberation time is too large, echoes are produced. Therefore, the reverberation time should have some optimum value.

3. Define reverberation time of an auditorium.

The persistence of audible sound, even after the source has stopped to emit the sound is called reverberation. The time during which the sound persists in the hall is called reverberation time.

4. Define absorption coefficient of a material.

The absorption coefficient of a material is defined as the ratio of the sound energy absorbed by the surface to that of the total sound energy incident on the surface.

Sound energy absorbed by the surface Absorption coefficient (a) =

---Total Sound energy incident on the surface

The absorption coefficient can also be defined as the rate of sound energy absorbed by a certain area of the surface to that of an open window of same area.

5. Write a note on noise pollution.

Noise pollution is one of the major factor which occurs in our day to day life. The noises

produced in a particular area creates harmful effects to the human beings. It produces mental fatigue and irritation. It diverts our concentration on work hence reduces the efficiency of the work. It Sometimes affects the nervous system and lowers the restorative quality of sleep. Some strong noises lead to damage the ear drum and makes the worker hearing impaired. Hence noise pollution should be reduced.

6. What is loudness? Give the relation between loudness and intensity of sound (or) State Weber-Fechner Law.

Loudness of sound is defined as the degree of sensation produced on the ear. This cannot be measured directly. So that it is measured in terms of intensity. Loudness is proportional to the logarithmic value of intensity.

L α log I L = K log I

This is also known as Weber – Fechner’s Law.

7. Define sound intensity level and write its unit.

Intensity level (IL) is equal to the difference in loudness, which is given by IL= L1-L0 = K log10 I1 – K log10Io.

Whereas L1 is the loudness of any sound of intensity IL and L0 is the loudness corresponding to the standard reference intensity I0.


IL = K log 10 I0 . Unit for intensity level is Bel.

8. Mention any four sound absorbing materials.


9. How are sound waves classified?

Sound waves are classified in to three categories on the basis of frequency.

1. Infrasonics (below 20 Hz)

2. Audible sound (between 20Hz to 20,000 Hz) 3. Ultrasound (above 20,000Hz)

Audible sound is further classified as

1. Musical sound which produces pleasing effect on the ear. 2. Noises which produces unpleasant effect on the ear.

10. We hear sound from a vibrating blade. If that sound is to be made louder, What should be done?

The sound from a vibrating blade can be made louder by the following ways.

(i) The size of the blade can be increased.

(ii) A resonant body should be kept near the vibrating blade.

(iii) By removing the sound absorbing material nearby the blade.

11. What is meant by quality of sound?

The quality of sound is that characteristic which enables us to distinguish between two notes of the same pitch and loudness produced by two different voices.

The loudness and pitch tell us whether it is a voice from a man or a woman. The quality will help us to recognize the particular person who is producing the sound without seeing him.

12. Give the relation between loudness and intensity.

S.No Loudness Intensity

1. It is the degree of sensation produced in the ear.

It is the energy of sound wave crossing per unit time through an unit area at right angles to the direction of propagation.

2. It is physiological quantity. It is purely a physical quantity.

3. It is difficult to measure. It can be easily and accurately measured.

13. What are units of loudness? Define them.

There are two units of loudness viz. Decibel, phon and Sone.

Decibel: It is the smallest unit compared to bel. It is the standard unit used to measure the loudness. One decibel is equal to one tenth of bel.

Phon : The measure of loudness in phon of any sound is equal to the loudness in decibels of an equally loud pure tone of frequency 1000 Hz.

Sone: The measure of loudness in sone of any sound is equal to the loudness of that particular sound, having a loudness of 40 phons.

14. State sabine’s law.

It states that the reverberation time is the time taken by the sound to fall from one millionth of its original intensity, after the source of sound is stopped.


If V is the volume of the hall, a is the average absorption coefficient and S is the total surface area, the reverberation time can be related as

T =

---15. What is meant by optimum reverberation time? Give its value for concert halls and theatres.

Optimum reverberation time is the persistant time of sound in hall, without causing echos (or) inaudibility.

(i) For concert halls it should be 0.5 seconds.

(ii) For small theatres it should be between 1.1 to 1.5 seconds and for large theatres it should be

between 1.5 to 3 seconds.

16. Give the importance of sabine’s law for a good auditorium.

(i) The sabine’s law can be used to calculated the reverberations time of an auditorium.

(ii) It is also used to find the absorption coefficient of any unknown material.

(iii) The reverberation time should not be too short and also should not be too long. If the

reverberation time is too short, the sound may not be sufficiently loud in all portions of the hall. If it is too long, echoes will be produced, so the reverberation time should be maintained with an optimum value for a good auditorium.

17. What is meant by echelon effect?

If there is a regular repeatation of echoes of the original sound received by the observer due to the presence of flight of stairs or set of railings, then the effect is called echelon effect.

18. What is meant by resonance effect in acoustics?

Sometimes, due to lack of rigidity the window-panes or sections of the wooden portions may vibrate with some audio frequency. When this frequency is equal to the frequency of original sound, ‘Resonance’ will occur. This matching of frequency of any sound with the standard sound is called as resonance.

19. What is meant by structure borne noise?

Some buildings may have motors, elevators etc., which generates enormous sound intensity called noise. This noise travels through the structure of the building, called as structure borne noise.

20. State the condition of good acoustics for an auditorium.

1. Sound should be sufficiently loud and intelligible in every part of the auditorium. i.e., optimum reverberation time should be maintained.

2. Sound of each syllable should decay soon so that the succeeding syllable may be heard distinctly. (i.e) the auditorium must be free from excessive reverberation.

3. There should not be any undesirable focusing of sound in any part of the hall. There should not be any zone of silence or regions of poor audibility any where inside the hall.

4. Resonance should be avoided and noise should be reduced.


21. What is air-borne noise?

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 vechicle 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 conditional halls or rooms.

22. What do you meant by acoustics of buildings?

Desgin 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.

23. Give the units for intensity of sound, intensity level of sound, loudness, loudness level, sound pressure and sound pressure level.

Intensity of sound – W/m2 Intensity level of sound – dB Loudness – sone Loudness level – phon

Sound pressure – Pa or Nm-2 Sound Pressure level – dB


(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


(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

increase the frequency, the length of the rod should be decreased which is practically impossible.

(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.


(1) It is used to produce shock waves.

(2) It is used to increase the temperature in liquids.

(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.

(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.


(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.

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.


(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


Frequency (f) = (P/ 2t)*√E/ρ


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.

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.


Frequency (f) = (P/ 2t)*√E/ρ


f = (1/ 2* (1*10-3))*(√7.9*1010 /2650)

The frequency of vibrating crystal f =2.7299 MHz.

(26)Compare piezo electric method and magnetostriction method. `



Pulse/frequency generator Timing

section CRO Amplifier


(Transmitter) Object

Transducer (Receiver)

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

independent of temperature.


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?

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. 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

6. What is the function of helium in He-Ne laser?

Helium atom delivers its energy to Neon atoms and it causes population inversion.

7. 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.

8. 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.

9. Distinguish between spontaneous and stimulated emission s.n


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

(22) 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


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

11. Classify different types of lasers based on active medium, with one example for each: 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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


ii. Transition from asymmetric mode (00˚1) to symmetric mode (10˚0) will emit laser of


18. What are the roles played by N2 and He in CO2 laser?

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.

19. 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.

20. Distinguish between homo junction and hetero junction lasers: 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.

21. 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.

22. What is pumping action?

The process of raising more number of atoms to excited state by artificial means is called as pumping process.

23. Distinguish between ordinary light and laser light 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.,

24. 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.

25. 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.

26. Give the principle of laser action.


Hence the Light is Amplified by Stimulated Emission of Radiation termed as LASER.

27. What are the basic phenomenon’s involved in laser action?

1. Process of absorption 2. Spontaneous emission and 3. Stimulated emission

28. 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.

29. What are the different modes of stretching in CO2 laser?

The fundamental modes of vibration of the CO2 molecules are,

1. Symmetric stretching mode 2. Bending mode 3. Asymmetric stretching mode

30. 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.

31. Name some applications of Nd-YAG(Solid state ) laser

Nd-YAG laser is used in,

1. Transmitting signals to longer distances 2. Long Haul communication system

3. Endoscope applications 4. Remote sensing applications

32. Explain inelastic atom-atom collision

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

33. 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

34. 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.


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.

The measure of amount of light rays that can be accepted by the fibre (or) the sine of acceptance angle of the fibre is called numerical aperture.

NA = √ n21 - n2 2

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. 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.

7. 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.

8. 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.

9. 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.

10. 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) . i.e. n1 > n2

11. How will you classify optical fibres?


(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

12. 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.

13. 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.

14. 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.

15. 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.

16. What is called mode of propagation in optical fibre?

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.

17. 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.

18. 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

19. Distinguish between step-index and graded index fibres.



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.

20. Differentiate between single mode and multimode fibre.

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.


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.

21. 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.

22. 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) = √ n21 - n22 = √0.1525 NA = 0.3905 (No unit)


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.

23. 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(2 Δ)1/2 = 1.48 (0.04) 1/2 = 0.296

n1 - n2 n2

The relative refractive index Δ = --- (or) 1-

n1 n1


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˚

24. 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



Power loss (PL) = 10 log --- dB



= 10 log --- dB


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

25. 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.\


NA = 0.22 Δ = 0.012


Numerical aperture NA = n1 √2 Δ NA n1 = √2 Δ


n1 = ---√2x0.012 n1 = 1.42 We know the relation n2 = n1 ( 1- Δ )

= 1.42 ( 1- 0.012 ) = 1.42 (0.988) n2 = 1.40

The refractive index of core is = 1.42





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