Questions and Answers of Engineering Physics

1. Explain some of the important advantages of optical fibres?  ( KU 2011)

·         Low transmission loss

·         Comparatively small volume and weight

·         They are immune to electromagnetic interference

·         Very cheap, as it is made of silica and glass, which is abundant in nature

·         Can be used in explosives as well as in high voltage environments.

2. What is double refraction? Explain the construction and working of a Nicol prism                                         

                                                                                                                       (KU 2011, 2007 CU 2011)

Solution:

  When a beam of natural light is passed through certain crystals (calcite, tourmaline etc.), it is split up into two refracted rays. The ray obeying laws of refraction and having vibrations perpendicular to the principle section is called ordinary ray (O-ray) and the ray which does not obey the laws of refraction and having vibrations in the principal section is called extra ordinary ray (E-ray). Both o-rays and E- rays are plane polarized. This is known as double refraction.

Nicol Prism

It is made from calcite crystal. The ends are cut down to more acute angle of 68⁰. It is cut into two pieces diagonally through one obtuse corner. The cut surface are polished flat and cemented together using Canada balsam. It offers refractive index =1.55

refractive index for O-ray =1.658

refractive index for E-ray = 1.486, offered by calcite crystal.

As such, O-ray for large angle of incidence will be totally reflected and E-ray will be transmitted.

Uses

1.      Can be used as polarizer and analyzer.

2.      Nicol prism with retardation plate can be used to produce elliptically and circularly polaristic beach.

From the above figure ACGE represents the principal section of the Nicol prism. When the ray of light is incident parallel to the long sides of the prism, it splits up into o-ray and e-ray. Inside the prism the two rays meet the Canada balsam layer.

As far as the o-ray is concerned, it tries to pass from a denser medium to a rarer medium at the Canada balsam surface. The angle of incidence at the Canada balsam surface is greater than the critical angle; the o-ray undergoes total internal reflection. It is absorbed by black paint coated on the surface. The e-ray passes straight and comes out through the other end of the crystal. Because µ_e< µ of Canada balsam. This is a plane polarized with vibrations parallel to the principal section of the crystal. Thus the Nicol prism can be used to produce plane polarized light. It can be used as an analyzer also.

3. Explain the construction and working of helium neon laser. (KU May 2011,2009,2008)

    In Helium neon laser, a suitable mixture of Helium and Neon gas at a pressure of about forms the lasing medium.

A mixture of He and Ne with pressure in the ratio 10:1 is filled in a glass discharge tube of about 80cm length and 1cm diameter. The ends of the tube are closed by 2 oblique quartz windows whose normal make polarizing angle ∝ with the axis of the tube so that tan ∝= n for stable resonant cavity action. Two spherical mirrors are arranged on either ends of the tube, one totally reflecting while the other is partially reflecting.

He-Ne laser is four level laser system. In the energy band diagram, F₁ is the ground level and F₂,F₃ are exited levels of Helium. E₁ is the ground level and E₂,E₃,E₄,E₅and H₆ are exited levels of Neon. The exited levels F₂ and F₃ of Helium coincides with the excited levels E₄ and E₆ of Neon which helps the population inversion.

            The electric discharge through the gas mixture causes excitation of the He and Ne by collision with accelerated high energy electrons in this discharge tube. The detestable level E₄ and E₆acts as upper lasing levels from which lasing transition take place to E₃ and E₅ levels.                        

The lower lasing levels E₃ and E₅ are depopulated by non-radiative transition to E₂. The continuous wave laser at three different wavelengths is emitted. The E₂ levels is depopulated further by non-radiative to E₁and the whole process repeats out of the three wavelengths, the desired one selected by optical resonant cavity action.

4. Describe some applications of nano materials? (KU May 2010)

  Some important applications of nano materials are:

a)      Nano materials are used for magnetic storage, magnetic memories, magnetic sensors, switches etc.

b)      Ferromagnetic nano materials are used for making hard permanent magnets.

c)      Their porous nature makes them suitable for clean air filters, purifiers and for waste water treatment.

d)      They have applications in biomedical field, micro electromechanical system, optics etc.

e)      Bucky balls can be used for bullet proofing. They are also used to fight against HIV virus.

f)       Creation of superconductors and insulators.

5. Plane polarized light of wavelength 600 nm is incident on a thin quartz plate cut with faces parallel for which the emergent light will be plane polarized. The refractive index of O-ray and E-ray are 1.544 and 1.533 respectively.     (KU MAY 2010)

Solution:

    =600×10^-9m

   

 

 Find the maximum thickness, t

 t=/2

                =600×10^-9/2 (1.544-1.533)

 =3.33×10^-5m

6. Explain the term stimulated emission and population inversion, metastable state in laser.                      (KU MAY 2010, 2005 CU 2011)

  Atoms, molecules, ions etc are excited to higher energy level by absorption energy. These particles then return to the lower energy levels by radiating energy, known as de-excitation transition,

The process of de-excitation transition with emission of radiation on finding an identical radiation or stimuli to trigger the emission process is known as stimulated emission. Rate of stimulated emission is proportional to the number of atoms in exited level.

    (dN₂/dt)∝N₂

    dN₂/dt=W₂₁N₂

     W₂₁=stimulated emission probability

Population inversion:
 The state in which the number of atoms in the exited level is more than that is ground level is known as population inversion. It is an essential condition for stimulated emission to override absorption population inversion can be achieved by the process of pumping

    In a distribution of atoms at thermal equilibrium, the number of atoms in higher energy state is less than the number of atoms in the lower energy states. It is possible to achieve a non-equilibrium condition in which the number of atoms in higher energy sates than the number of atoms in the lower energy state. This is called Population Inversion.

Metastable level: - The intermediate energy level between the excited level and ground level and having a narrow band width will have comparatively large mean life time. Such an intermediate level is known as metastable level.

7. With necessary theory explain how circularly and elliptically polarized light are produced and detected                                                                                                                         (KU MAY 2010)

Circularly and elliptically polarized light

Production of circularly polarized light:

A beam of unpolarized monochromatic light is allowed to fall on  Nicol prism N₁. The emergent beam from N₁ is plane polarized. Another Nicol prism N₂ is placed at a certain distance from N₁ in such a way that the two prisms are crossed. The field of view is dark. A QWP is introduced between N₁ and N₂. Now the field of view is not dark. The QWP is rotated till the field of view is again dark. This happens when the vibrations of light incident on the QWP are along its optic axis and so perpendicular to N₂. The QWP is rotated through 45⁰ so that the vibration of light incident on it makes an angle an angle of 45⁰ with its optic axis. At this position, the amplitude of O-ray and E-ray are equal but a phase difference of  π/2 is introduced between them. The emergent beam from the QWP is circularly polarized.

Production of elliptically polarized light:

A beam of unpolarized monochromatic light is allowed to fall on two Nicol N₁ and N₂in crossed position. The field of view is dark. A QWP is introduced between N₁ and N_2. The QWP is rotated in such a way that the plane of vibration makes an angle other than 45⁰ (say 30⁰)with its axis. The introduces a phase difference of π/2 between the O-ray and E-ray. Now the amplitude of O-ray and E-ray are unequal and the emergent beam from QWP is elliptically polarized.

Detection of elliptically polarized light:

Pass the given light through a QWP first and observe the variation in intensity through a rotating Nicol. If the intensity become a QWP converts elliptically polarized. This is because a QWP converts elliptically polarized light into plane polarized light.

Detection of circularly polarized light:

Pass the given light through a QWP and then observe the variation in intensity through a rotating Nicol. If the intensity becomes a maximum and zero, the light is circularly polarized. The reason is that a QWP converts circularly polarized light into plane polarized light.

8. Describe the construction and reconstruction process in holography?

                                                                                                (KU MAY 2010, 2005,CU 2010)

(i)  Recording the image
(ii)  Reconstruction

            Holography is a method of recording images of an object using interference principle. It was invented by Dennis Gabour. An ordinary photography is a two dimensional recording of a 3 dimensional image. The intensity of light reaching from the object is recorded.

            In holography the amplitude as well as phase of the waves scattered from the object are recorded by interference method. The recorded photographic film in holography is called hologram. Materials like photo thermoplastics are used to make holographic films.

            The ordinary photography is a point to point recording of the intensity of light that makes up an image. Each point of the object is related to a conjugate point in the image. But in holography each point in the film receives light from all parts of the object. So it contains full information. In photography each piece will give only partial information.

            No need of lens in holography. In hologram several images can be recorded and hence the information capacity of a hologram is high. While a 6mm x 9mm photograph stores up one printed page, a hologram of the same size can stores up 300 such pages. In hologram the information is recorded in the form of interference form.

Recording a hologram
            In this process the given object is illuminated with a coherent source of light. The waves scattered by the object are called object waves. Waves coming directly from the coherent source are called reference waves. The object wave and reference wave are allowed to superimpose in the plane of the photographic plate and interference takes place. The recorded interference pattern contains all the information about the object. It is called hologram.
            Laser light is split into two parts by beam splitter. One part of light incident directly on the film  (reference waves). Other part is allowed to incident on the object. The diffracted wave from the object (object wave) is also incident on the photographic film. These waves superimpose and interference recording takes place.

Reconstruction
                 It is the production of images on the hologram. In this process, the hologram is illuminated with the same kind of light similar to the reference wave (laser). This will produce different components of wave. One component of the wave is called reconstructed object wave. It will produce an exactly similar image of the object in its 3 dimensional form. This is a virtual image which can be viewed 3 dimensionally. Another component of the wave produces a real image which can be obtained on the screen.

Applications

1.      Non destructive testing of artificial heart valves can be done by holographic method.

2.      To study about the distribution of strain on objects.

3.      It is used to produce holographic masks for microelectronic circuits.

4.      The holographic computer memories are having very high storage capacity about 10 10 bits/mm3 with high speed access.

9. Derive expression for the numerical aperture of a step index fibre. (KU MAY 2010)

                                                                                      cladding

                               φ

n

              ∝                                                                                                       core

                           n₀

      Refractive index of core with respect to air

      But θ=

      For critical rays,∝=∝_m,

      Refractive index of core with respect to cladding is

      Thus numerical aperture

   (from equation(3)) If n₀=1, ie refractive index of air medium, then N.A =

                                      Numerical aperture of an optic fibre is a measure of its light gathering capacity and is defined as sine of maximum value of acceptable angle.

      N.A=sin ∝_m=(n₁²-n₂²)

       = refractive index of core

      n₂ = Refractive index of cladding.

10. Describe the construction of Ruby laser and explain with energy level diagrams the working of   the laser.                                                                                                 (KU May 2007, 2008,2009)   

      Construction

      Ruby laser is taken in the form of cylindrical rod 10 cm long, 1cm in diameter. The ends are polished flat. One end is silvered and the other is partially reflecting. A helical shaped xenon flash lamp kept around it.

      Working

      Xenon flash lamp is activated by the discharge of capacitor. The Cr ions selectively absorbed and green –yellow radiation and undergo excitation transitions to E₄ and E₃ levels. From these, they undergo fast non radiative transition to doublet  of E₂. The excess energy given out as heat energy.

      Population inversion takes place between E₂ and E₁ and lasing transition occurs out laser at 0.693 μm and 0.694 μm.

11. Explain half wave plate & quarter wave plates?                     (KU May 2008, 2006, CU 2009)  

 Half wave plate

When plane polarized light is incident normally on a crystal perpendicular to the optic axis, the ray splits up into o-ray and e-ray components. They travel in same direction but with different speeds.
When they came out of the crystal, there is a path difference between them depending on the thickness of the crystal. The thickness of the crystal is made in such a way that the path difference of the splited wave is ½ the total wave entering.

                                                 2(µ_e~µ_o)

(ii) Quarter wave plate

It is a crystal wave plate of a doubly refracting material of uniaxial crystal, cut in such a way that the optic axis is parallel to the refracting faces. When plane polarized light is incident normally on a doubly refracting crystal in a direction perpendicular to the optic axis, the ray splits up into ordinary and extra ordinary components. They travel in the same direction, but with different speeds. The quarter wave plate is made in such a way that the path difference of the polarized o-rays and e-rays is made ¼ times the total path difference of wave entering. i.e. If the thickness of the crystal is such that the path difference between the o-rays and e-rays is λ/4 the crystal is quarter wave plate. The thickness of a quarter wave plate is constant for given wavelength λ for a crystal.

                              4(µ_e~µ_o)
Applications
*The light coming out from the quarter wave plate is elliptical, the particles subjected to two simple harmonic motion right angles to each other. so it can be used to produce elliptically polarized light.
*When the vibrations are inclined at 45° with the principal section, the beam splits up with two equal amplitudes, phase difference π/2 with the components and comes out. i.e it can also be used to produce circularly polarized light.

13.  What you mean by Fermi level of a system?        (KU May 2008)        

      In the FD system at absolute zero, only one particle occupies the lowest state. The next particle goes to the next higher state, etc, thus filling, one by one, all the lower states up to a certain state. The highest filled level at absolute zero is called Fermi level.

14. Discuss the use of hologram for image storage?                            (KU May 2008)         

      The possibility of recording a very large number of images on a single hologram and the ability of even a part of the hologram to reconstruct the complete image opens up thee immense scope for storing large amount of information in a hologram.

15. Write a short note on fibre optic communication   (KU May 2008)         

      Major components are Optical transmitter, Optical fibre transmission line and an Optical receiver. In fibre optic communication, encoded optical signal is transmitted through optical fibres. At the receiver end, a semiconductors photo detector converts optical signals back into electric pulses. A decoder converts digital pulses into analogue signals and the sound is reproduced at the subscriber’s telephone.

16. Explain positive and negative crystal with reference to polarization. (KU 2007, CU 2010)

According to Huygens theory of double refraction, a source produces two types of secondary wave fronts in certain crystals and this causes double refraction. O-ray is produced due to spherical wave front and hence same velocity in all directions, e-ray is due to elliptical wave front and different velocities in different directions. In positive type crystals the o-ray travels faster than the e-ray. The ellipsoidal wave front lies inside the spherical wave front. The refractive index of e-ray is greater than the o-ray in positive crystals.

i.e. µ_e>µ_o         eg: quartz crystal.

In negative type crystals the e-ray travels faster than the o-ray. So that the spherical wave front lies inside the ellipsoidal wave front.  But both the wave fronts have same speed in the direction of optic axis and touch each other. The refractive index of o-ray is greater than the e-ray in negative crystals.
 i.e. µ_o>µ_e            eg: calcite crystal.

17. What are the characteristics of laser beam?             (KU MAY 2006)

      The most outstanding characteristics of the laser light are its high degree of directionality, intensity, monochromaticity and coherence.

Directionality:- The laser beam is highly directional. As the laser beam travels through space it diverges. This is because of the diffraction due to the infinite length of the wave front. The angle of divergence of a parallel beam is directly proportional to the wavelength and inversely proportional to the beam diameter. On account of high directionality of the laser bean it is possible to direct a laser beam to afar off object and obtain the reflection of the same.

Intensity:- The laser beam is highly intense as compared to ordinary sources of light. That is why it can be sued for such operations as welding of metals which involve high temperature. According to geometrical optics light travels along straight line. A point source of light can be focused into a point image. However light have a wave nature and this plays a major role in the propagation and focusing of laser beams.

Mono chromaticity:- An optical source of light does not emit light of single wavelength. The emission actually occurs over a range of wavelengths. The spectrum of light emitted by a source can be represented by a mathematical function g(v) which is called the line shape function.

Because of the line broadening mechanisms, the light emitted by any source is strictly not monochromatic instead there is always a spread in the frequency of emitted photons. However in the case of lasers, the line widths are much smaller compared to the ordinary sources and therefore are more coherent.

Coherence:-  The laser light is temporally and spatially to an extraordinary degree,. It is possible to observe interference effects from two independent laser beams. Temporal coherence is referred top as longitudinal coherence while the spatial coherence is called lateral coherence.

18. Mention few advantages of Optical fiber?     (KU MAY 2005)

      Low loss, high band width, high information carrying capacity, highly economic case in use and has more application. Avoid damages due to lighting and problems due to radioactive interference.

19. Derive Einstein’s Co-efficient relation between A and B

      Einstein proved at thermal equilibrium, the coefficient for induced absorption and the coefficient for stimulating emission are the same.

      ie    B₁= B₂

      The relation between the coefficient for spontaneous emission and the coefficient for stimulating emission is given by

     

where c the velocity of light and h is the Planck’s constant.

Proof: we cannot predict which particular atoms will make a transition from one energy sate to another energy state at a particular instant. Einstein calculated the probability of transition to an energy state assuming the atomic system to be in equilibrium with electromagnetic radiation. We considered that

1. Induced absorption transition rate

2. Spontaneous emission rate                      (1)

and            3. Stimulated emission transition rate

      where ρ is the energy density of incident radiation, N₁ is the number of atoms per unit volume in the lower energy state N₂ is the number of atoms per unit volume in the excited energy state

            B₁ is Einstein coefficient for induced absorption

            B₂ is Einstein coefficient for stimulated emission

            and A₂ is Einstein’s coefficient for spontaneous emission.

      At thermal equilibrium, the number of upward transition= the number of downward transition per unit volume per second.

                                      ∴ B₁ ρ N₁= A₂N₂ + B₂ N₂

      ρ(B₁ N₁- B₂  N₂) = A₂N₂

   

      Dividing with

      By Boltzmann’s theorem,

 and N₂=

      Where N₀ is the number of atoms per unit volume in the ground state

  

    Since E₂-E₁=h

      but the energy density of incident radiation ρ is given by Planck’s radiation energy distribution law as

      From (5) and (6) B₁ = B₂ and

      B₁ = B₂ ……….(7) ie coefficient of induced absorption= Coefficient of stimulated emission

      Where c is the velocity of light. This is the ratio of spontaneous emission to stimulated emission.

        Equation (8)gives the ratio of Einstein’s coefficient of spontaneous and stimulated emission.

20. Explain the construction and working of Nd-YAG laser. (CU 2011)

      This is a four level solid state laser in which YAG (Yttrium Aluminium, Garnet Y₃, Al₅ O₁₂) crystal is doped by a small amount (about 1%) of neodymium ions. The neodymium ions replace the yttrium in the crystal structure of YAG since they are of same size.

      The laser consists of an Nd:YAG rod of length about  0.1 m and diameter 4 to 6 mm and is kept in between two elliptically cylindrical reflectors along one of its focus lines. Krypton are lamp is used to excite the neodymium ions. The lamp is placed is placed along the other focus line of the reflectors to minimize the reflection loss. One reflector is fully polished and the other partially and they will act as optical resonators. The rod is kept in an glass jacket which is filled with a coolent to remove the excess amount of heat.

      The energy levels of Neodymium ions in the crystal are as shown.

      The flash lamp is switched on and the neodymium ions are excited

      To upper bands, since the life time of atoms in that level is very small (10^-9 sec) the atoms quickly make a non- radiative transition from the upper band E₄ to level E₃. The life time in level E₃ is of the order of milli second so the atoms will accumulative in that levels. E₃ is the metastable level. Thus population inversion is achieved between E₃ and E_2. Then the spontaneously emitted photons in the system will trigger the lasing action and powerful beam of laser is emitted from the partially polished reflector. The light output is continuous. The wavelength of emitted laser light is 1.06 μm. Nd:YAG lasers are available in different output power ranging from a few mill watts to as high as a kilowatt. 

      Advantages

            At ordinary temperatures the lower laser level will be almost unpopulated since it is sufficiently above the ground level. Hence population inversion can be easily done.

      Uses

1.      In military they are used as range finders

2.      Used in ophthalmology for surgery to correct vision.

3.      In fluid mechanics to visible the flow of fluids.

4.      Used in cosmetics to remove tattoos and hairs on skin

5.      Used for welding and drilling.

21.  Explain the construction and working of semiconductor laser, with band diagram. What are its merits and demerits.                                                (CU 2005,2008,2010)

Principle                       

      The light emitted by a LED is incoherent. It can be made coherent by using suitable material and proper biasing. When a particular value of current is passed through a highly doped pn junction stimulated emission take place producing laser light. A semiconductor laser is a pn junction diode formed by two heavily doped semiconductors. Generally there are two kinds of semiconductor laser. (1) Homojunction laser in which pn junction is formed on the same materials by proper doping. (2) Heterojunction laser in which the junction is made between two dissimilar semiconductors.

Construction

      Diode laser is very small in size in the order of nearly l mm. The end faces are parallel and perpendicular to the plane of junction. The region between p type and n type behaves as an active medium and it is very thin in the order of l μm. The top and bottom faces have metal coatings with current leads so as to pas s a current through it. A pair od opposite faces is polished well so that they behave like reflecting surfaces of resonant cavity. (left and right faces). The other pair of opposite faces is made rough to prevent laser action in that direction (frond and rear faces).

Working

      A simple way of achieving population inversion in a semiconductors laser to use to use it as a heavily doped pn junction and

       hence to make it forward biased. The energy band diagram of a heavily doped pn junction diode is as in figure. A part of conduction bands and complete part of valence band of a n region are filled with electrons. The Fermi- energy level E_Fn lies within the valence band. At thermal equilibrium the Fermi level is uniform across the junction (the same horizontal level as in fig). the small portion ABC in the depletion region is filled electrons in n side. Similarly small region DEF in the depletion layer is filled with holes in p side.

      When the pn junction is forward biased by passing a current through it, electrons are injected into the n region and electrons are removed away from a p region . i.e Number of electrons in n region gets increased and Fermi level splits up and rises in the n region (fig b). Similarly number of holes in p region gets increased and Fermi level falls down (as in fig b). As a result the electron concentration and hole concentration in depletion layer increase and depletion layer become very thin. So the size of regions ABC and DEF increases and at a particular stage DEF becomes just below the region ABC. Now the current flowing through diode is called ‘threshold current’. There is high population of electrons in the upper region of depletion layer and thus population inversion takes place due to injection of electrons and holes. Forward biasing (current) behaves as a pumping agent. Now it behaves as LED . When current increases the intensity of emission also increases.   

      Advantages

1.      It is miniature in size.

2.      It is highly efficient.

3.      Very easy to handle and operate.

4.      It is very simple and portable.

5.      It requires only a low power to operate.

6.      Laser output can be easily modulated by controlling the junction current.

Disadvantages

1.      Output is generally in the form of a wide beam.

2.      Purity and monochromatically are poorer than other type of solid state laser.

Applications

1.      Used in optical communication system.

2.      Used as a barcode reader and in UPC scanners.

3.      It is widely used to read the digitized data that is encoded on CD, DVD and Blue- Ray Discs (BD).

4.      As a range finding instrument.

5.      In printing technology for scanning the images and for high speed and resolution printing plate manufacturing. Used for laser printing, laser typesetting, laser fan machines and laser pointers.

22. Define (1) Polarization (2) Polarized light. Mention the application of Polarized light

(1)   The phenomenon of limiting the vibrations to a single plane is known as polarization.  Polarization is a property exhibited by transverse waves only. Hence polarizations of light shows that light waves are transverse. Sound waves cannot be polarized since they are longitudinal.

(2)   Since light is a transverse wave, according to Huggins’s wave theory either particles in the medium can vibrate in infinite directions perpendicular to the direction of propagation of the wave. If these vibrations are restricted in a single plane, then it is called plane polarized light. When the vibrations of the incident plane polarized light are parallel to the axis of crystal, the emergent light has max Julensity.

Applications of Polarized light

1)      Plane polarized light is used in 3D photography.

2)      Plane polarized light reduces glance. In foreign countries the wind screen in front of the driven is a polarizer. The light coming through this plane is polarized. Hence it reduces glare and thus accidents can be minimized.

3)      The analysis of polarized light coming from astronomical bodies gives considerable information about their physical state.

4)      The concentration of sugar and hence the quality of sugar cane is determined optically using polar meters.

5)      Polari meter is the main technique for studying the solar magnetic fields.

6)      Used in photo elasticity

7)      Polaroid’s are used in sun glass, head lamps, aero planes, trains etc. to cut off glare.

8)      Investigation of rocks and minerals can be done by observing the colours with very thin sheets of rocks placed between polarizer and quarter wave plate.

9)      Natural fibers are double refracting. Since silk contains a large number of fibers twisted in all directions. It behaves like a depolarizer. So quality of silk cloth can be tested.

10)  Polaroid are used to examine the surface condition of skin. Using polarizing filters glare and stray light are avoided and hence opticians can examine cornea of eye. Polarizing filters can be sued for diagnosis of certain eye defects

23. Explain the different types of fibres and list out their applications?

An optical fibre is a thin, flexible, transparent fibre that acts as a wave or light pipe to transmit light between the two lens of the fibre. Optical fibres are widely used in fibre optic communication which permits transmission over long distances and at higher bandwidths than other form of communication. Fibres are used instead of metal wires because signal travel along them with less loss and are also immune to electromagnetic interference.

Applications of optical fibre

1.      Military application:

Optical fibres are finding a lot of application in various and military operations.

During recent wars, fibre guided missiles have been extensively used. Sensors are mounted on missiles to collect the video information, this information is passed on to the ground control van which sends further commands to the missile through sensors. 

2.      Optical Fibre sensors

In this case the basic characteristics of the optical fibres are uses the variation of refractive index of the fibre under the action of external forces may be utilized f0r using the optical fibre as transducer

            Also fibre optics may be sue to measure pollution and foreign suspended particles in the air.

3.      Entertainment applications

A coherent optical fibre bundle can greatly enhance the size of the image on a T.V. screen. In case we have to use the conventional method of getting larger images, then the protection systems become quick bulky and unmanageable.

4.      Medical Applications

Fibre optics is finding large number of application in medical fields. A bundle of MMF is used illuminate a part of human organ and the other part to collect the reflected light. The fiberscope technique is employed for endoscopic applications, fibre optics can be employed to attach a detached retina or rectify other eye defects using lasers.

5.      Industry

They are used in electric railway system.

Used for signaling

For security and alarm system electrons instrumentation system, industrial automation and process control system.

They are used in computers in connection with CPU and memory.

Classification of optic fibre

 Optic fibres are generally classified into two main groups depending on the refractive of core and cladding. They are (a) Step index fiber and (b) graded index fiber (GRIN).

(a) Step index fibre or mono mode fibre

            The refractive index of the core n₁ and the refractive index of the cladding n₂ are constants. n_{1 >} n_2.

But there is a sudden decreases of refractive index at the core cladding boundary.

  A graph drawn with the refractive index on Y axis and the distance from the axis of the core on the X axis is called index profile. Here the index profile in the form of step and hence it is called step index fiber. This is reflective type fiber.

Step index fiber is again subdivided into types namely

1.      Single Mode fibre (SMF)

2.      Multi Node fibre (MMF)

     1. Single Mode Fiber (SMF)

            Single mode step index fiber has a very thin core with diameter about 1 to 10 μ m. typically core diameter is the order of 125 μ m. This is again covered with an opaque sheath with diameter about 150 μ m.

            The refractive index core n 1 and that of cladding n 2 are constants.n_{1 >} n_2. There is a sudden decreases of refractive index at the core- cladding interface. Hence the index profile is the shape of a step.     

      Since the core is very thin it supports only one mode for propagation. So this is called mono mode fiber. In SMF, the light is propagated almost along the axis of the core. This is called zero order mode of transmission.

Properties

1.      Bandwidth is high.

2.      It permits only one mode and this is travel along the axis of the core without any loss of  energy. So SMF is used for large distance communication purposes.

3.      Since there is zero order transmission there is no pulse broadening  effect and no intermodal dispersion.

4.      Since the core is very thin, the construction, handling, splicing etc. are very difficult.

5.      This is less expensive.

6.      NA is smaller since the core is thin.

7.      ∆ is also smaller. (∆=0.02)

2. Multi Mode Fiber (MMF)

      Multi mode step index fiber is very similar to SMF. But it has a very thick core with diameter 100 μm. It is surrounded by a cladding with diameter 125 μ m. Refractive index n1 of core and that of the cladding n₂ are constants. n_{1 >} n_2. The index profile in the shape of a step as in SMF.

Since the core is very thick, it permits a large number of modes for propagation. So it is multimode fiber. The number of modes that MMF can support is given by N_m =  where V is the V-number. They are travelling in a zig-zag manner through different paths and reach differently at the other end. This produces pulse broadening effect.

Properties

1.      Bandwidth is smaller and so this is used for short distance communication and illumination purposes.

2.      Since it permits large number of modes, there is pulse broadening effect and intermodal dispersion.

3.      Since the core is very thick, construction, handling, splicing etc are easier.

4.      It is less expensive.

5.      NA is larger. NA=0.3

6.      ∆ is also larger.

b) Graded Index Fiber- GRIN

Here refractive index of the core n 1 is not a constant. But the refractive index of the cladding n₂ is a constant.

n_{1 >} n_2. The refractive index of the core n₁ is varying. It is maximum along the axis of the core, it decreases radially outwards with the distance from the axis and it is minimum at the core boundary. i.e Refractive index is graded.

            Graded index fiber has a very thick core with diameter about 100 μ m. This is surrounded by a cladding with a diameter about 125 μm. This is again covered by an opaque sheath with a diameter about 150 μm. Core and cladding are made of glass. Since the core is very thick, it permits a large number of modes and hence it is a multimode type. The number of modes that a grain can support is given by
 where V is the V-number. The index profile is in the shape of a parabolic curve since the refractive index of the core  is varying

Since the refractive index of the core is varying, radially from the axis, the light rays are travelling along smooth parabolic curves. So this is refractive type fiber. The velocity is inversely proportional to the refractive index. Hence the axial rays (rays nearer to the axis) are travelling more slowly (shorter paths) and the marginal rays (rays away from the axis) are travelling more radidly (longer paths). As a result, all the rays are converging through different points on the axis. This produces periodic self focusing and hence pulse broadening effect and intermodal dispersion are eliminated.

            Since the refractive index of the core is decreasing radially acceptance angle and NA are also decreasing with the radial distance from the axis. NA is a function of radial distance r.

 where ‘r’ is the radial distance from the axis and ‘a’ the core radius.

                                                                                                 3

                                                                           NA    2

                                                                                                1

                                                                                                0

                                                                                                                Distance r/a

Variation

Properties

1.      Bandwidth is high and hence this is used for long distance communication purposes.

2.      Since there is periodic self focusing, this is free form public broadening effect and intermodal dispersion.

3.      Since the core is very thick, handling, splicing, coupling etc are easier.

4.      This is very highly expensive.

5.      NA is higher.

6.      ∆ is also higher.

7.      There is loss of energy due to attenuation.

24. Mention the application of lasers in various fields?

Application of laser light in medicine:- lasers have been used in medicine both for the capacities as well as for diagnosis. When a human issue is exposed to laser radiation its temperature raises. The extent of damage depends on the time for which the tissue is at elevated temperature. The nature of laser tissue interaction process may be divided into several regions determined primarily by the intensity of the laser beam and its interaction time with the tissue. Lasers are used for tissue cutting and removal at relatively high beam intensities with exposure times of milliseconds to seconds. This results in rapid deposition of heat and subsequent vaporization or decomposition. These are process that is characteristic of thermal or thermo acoustic ablation. Endoscopy can better be done using laser beams because of the coherence characteristics.

Application of laser in industry:-  the large intensity that is possible in the focused output of a laser beam and its directionality makes laser an extremely useful tool for a variety of industrial application.

Welding:- welding is the joining of two or more pieces into a single unit. If we consider welding of 2 metal plates, the metal plate are held in contact at their edges and a laser beam is made to move along the line of contact of the plates. The laser beam heats the edges of the 2 plates to their melting points and causes them to fuse together where they are in contact. Electronics industry:- it uses lasers in the manufacture of electronic compounds and integrated circuits. Lasers have been used to perforate and divide silicon likes having several hundred circuits. Trimming of thick and then fills resistors using lasers is a very common application.

Application of lasers in communication:- in the field of communication laser offer two unusual advantages. The first of these pertials to bandwidth. It is known that the rate at which information can be transmitted in proportional to bandwidth to the information carrier. Bandwidth is proportional is the carrier frequency.  Therefore if proper methods of modulation and demodulation are found for the laser light, an extremely potent information carrier could be achieved. The second consideration in connection with information transmission is the ability to aim it in proper direction. A point to point communication requires the concentration of the information carrier into a harrour beam. The high directionality of the laser beam.

One aspect of communication is the determinations of the position of distant object in short ranging.

Application of laser in science:- lasers offer a wonderful opportunity to investigate electromagnetic waves of atoms and molecules with electromagnetic waves of high intensity.

Laser isotope research lasers are used to separate various isotopes of an element. They are used for enrichment of ceramimum for nuclear power reaction.

It is used to accelerate chemical reactions to produce new compounds by destroying chemical bonds between molecules.

It is used to study the structure of molecules.

Used for genetic research.

Laser gyroscope is used for sensing temperature, pressure, current, magnetic fields, dectric fields, etc..

For spectrum analysis.

25. Explain half shade device & Laurent’s half shade polarimeter ?

    Laurent’s half shade plate is a device which consists of a semicircular half wave plate of a quartz cut with optic axis parallel to interfaces and at light angles to the vertical diameter, which is cemented together with another. Semicircular plate of glass G of thickness such that its absorption of light is forming a composite circular plate

                                                                Y

                                    B                             D

     
                                     X

                                                                                               

                                             C                               A

                                                          Y₁

Consider a beam of plane polarized light with its vibrations II^nd to CD is incident on the half shade. Light passing through the glass portion is unaffected when the light passes through quartz half plate is split into two components- vertical and other horizontal. Vertical component is E component (II^nd to optic axis 1/y’) and horizontal is o-component (Ir to optic axis ie, XX¹)

Inside a quartz V₀>V_E ie, the emergent o-component will gain a phase of  over E-component. Due to this phase difference the direction of O-component is reversed ie, OM is the initial direction then it final direction will be ON.

If the component of initial vibrations along OD in Quartz plate are OL and OM the emergent wave will be resultant of OL and ON, ie OB which makes an angle Q with Y axis as incident vibration along AB do but on other side

ie θ

Thus, this shows that the plate rotates the plane of polarization by an angle 2θ.

   When we notic two plane polarized beams, one beam emerging from glass half plate with vibrations in the plane OD and other beam from quartz half plate OB. If analyses N₂ has its principal plane along YY’ ie ,along direction which bisector 2

from both halves will be equal, The field view will be equally bright.

If N₂ is rotated to right of YY’, then the right half will be brighter as compared to left half. Portion and if N₂ is rotated to left of YY’, then the left half will be brighter as compared to right half.

Half shade serves the purpose of dividing the field of view into two halves.

Disadvantage with half shade device is that it can be used only for special wavelength for which it has been designed.

LAURENT’S HALF SHADE POLARIMETER

               

N₁  and N₂ are two Nicol prism. Where N₁ is polarizer and N₂ is analyzer. Behind N₁ there is a half wave plate of quartz which covers one half of the field of view while the other half G is a glass plate. G absorbs the same amount of light as quartz plate. T is the glass tube, containing optically active substance and closet at ends by cover slips and metal covers. It is mounted between N₁ N_2.There should be no air bubbles in the tube. If any  it will appears at upper portion of the wide bore T₁ of tube T. S is circular slit illuminated by monochromatic light. Light from slit pass through Let us L and the leus directs it towards the polarizing Nicol N₁. The light emerging from N₁ plane polarized with it vibrations in the principal plane of nicol. This plane polarized light passes through Laurent’s the tube containing optically active substance like sugar solution. The emergent light, after passing through nicol prism N₂ is viewed through Galilean telescope to which a scale V₁V₂ is attached.

Determination of Specific rotation of sugar

  To determine the specific rotation of an optically active substance, N₂  (analyser) is set in the position for equal brightness of the fields of view, first with water in the tube T. The reading in the vernier V₁ and V₂ are noted. Now water in the tube is placed by sugar solution. Due to this the vibrations from quartz half and glass half are rotated. The field of view is not equally bright. The analyse is rotated in clockwise direction and is brought to a position so that the entire field of view is equally bright. The new positions of vernier V₁ and V₂are recorded. The angle (θ) through which the analyser has been rotated gives the angle through which the plane of vibration has been rotated by sugar solution. The angle of rotation for different concentrations of sugar solution is recorded. A graph between concentration C of the sugar solution and the angle of rotation θ is plotted, which is a straight line. From the graph θ/C value is determined and specific rotation of sugar is calculated from the relation,

                                      S= 

 

                               θ

                                                                                                        

                                                          c              

Production of Polarized Light

 We can produce polarized light by various methods.

1.      By reflection

2.      By transmission through pile of plates

3.      By double refraction

4.      By selective absorption in crystals

5.      By scattering