Saveetha Engineering College, Thandalam, Chennai. Department of Physics. First Semester. Ph6151 Engineering Physics I (NOV/DEC 2014)

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3 Saveetha Engineering College, Thandalam, Chennai. Department of Physics First Semester Ph6151 Engineering Physics I (NOV/DEC 2014) Part A (Questions and Answers) 1. Distinguish between Crystalline and Amorphous material. S. No Crystalline Amorphous 1 Atoms are arranged in a regular pattern Atoms are arranged in a irregular pattern 2 They have definite geometric shapes They do not have definite geometric shapes 3 They are anisotrophic They are isotrophic 4 They are most stable They are less stable 5 They have sharp melting point They do not have sharp melting point 6 They have directional properties They do not have directional properties 2. Define Miller indices. Miller Indices is defined as the reciprocal of intercepts of a plane on crystallographic axes which are reduced to smallest number. (or) A set of three numbers to designate a plane in a crystal is known as Miller indices. 3. Name the factors which affect the elasticity of a body. Effect of temperature Effect of stress Effect of impurities Effect of hammering Effect of annealing Effect of crystalline solids

4 4. Define thermal conductivity. It is defined as the quantity of heat conducted per second normally across unit area of cross section of the material per unit temperature difference. 5. State Wien s displacement law. ( ) ie.., E T 5 The Product of wavelength and the absolute temperature is constant. i.e.., T = Constant The energy of a body is directly proportional to the fifth power of its absolute temperature. 6. What is the advantage of electron microscope over optical microscope? It can produce magnification as high as 1,00,000 times as that of the size of the object. The focal length of the microscope system can be varied. 7. Define sound absorption co-efficient of a material. The sound absorption coefficient of a material is defined as the ratio of sound energy absorbed by its surface to that of the total sound energy incident on the surface. 8. Mention four applications of ultrasonic waves. It is used to measure the depth of the sea. It is used to detect the flaws in materials. It is used in SONAR. It is used in drilling, welding and grinding. 9. What do you mean by population inversion? Population inversion (N 2 > N 1 ) is a situation in which number of atoms in the higher energy level is greater than the number of atoms in the lower energy level.

5 10. What are the uses of optical fibres? Fibre optic endoscopes are used for finding minor cracks, pores, etc.., in big machineries. It is also used to connect a monitoring station and a remote sensor in industries. It is used to visualize the inner organs of the human body. Part B 11. a) i) Show that the packing density of FCC and HCP structure are equal. (8)

The packing density of FCC and HCP are equal. Hence proved ii) Explain the physical basis of classifying crystals into seven systems and 14 bravais lattices.

6 The packing density of FCC and HCP are equal. Hence proved ii) Explain the physical basis of classifying crystals into seven systems and 14 bravais lattices. (8) Crystal systems can be classified into seven based on the values of lattice parameters. 1. Cubic system 2. Tetragonal system 3. Orthorhombic system 2. Monoclinic system 5. Triclinic system 6. Rhombohedral system 7. Hexagonal system

8 Bravais Lattices There are 14 possible ways of rearranging lattice points in space using 7 types of crystal system. These 14 space lattices are known as Bravais lattices (OR) (b) State the Principle and illustrations of i) Czochralski s technique ii) Bridgman technique for material preparation.

11 12 a) Derive an expression for the depression of cantilever at a distance x from the fixed end loaded at its free end, neglecting the mass of the cantilever. (16).

12 Depression of a cantilever: b) Lees Disc Method Thermal conductivity of a glass

13 1 2

14 2 r 2

15 ii) Rectilinear flow of heat

17 13 a) Explain Compton effect and its physical significance. Derive the relations giving the change of wavelength, the energy of recoil electron and recoil angle. What is Compton shift?

21 (OR) b) Derive Schrodinger s Time independent and time dependent equations for waves.

24 14. a) Explain in detail the acoustic demands of a hall. (16) Reverberation Reverberation is defined as the existence or prolongation or persistence of sound waves in a closed room even after the source of sound wave is turned off. Reverberation time Reverberation time is defined as the time taken for the existence or prolongation or persistence of sound waves in a closed room even after the source of sound wave is turned off. Sabine s formula for Reverberation time - Assumptions i) Sound Energy is distributed uniformly throughout the hall. ii) The Volume of the hall (V) is constant. iii) There is an overlapping of sound waves form Interference. iv) The absorption coefficient is independent of Intensity of sound wave. v) The rate of emission of sound energy from the source is constant.

29 (OR) b) What is ultrasonics? Explain the magnetostriction method of producing ultrasonic waves and hence describe its advantages over the piezoelectric method.

31 F I 2 L 1 C 1 F 2 I L 1 C 1 1 E 2l ' '

32 15. a) i) Explain What do you mean by laser emission? What are the conditions to achieve it? Stimulated Emission: The atom in the excited state can also return to the ground state by external triggering or inducement of photon thereby emitting a photon of energy equal to the energy of the incident photon, known as stimulated emission. Thus results in two photons of same energy, phase difference and of same directionality as shown.

33 Conditions to achieve laser emission: i) Population inversion should be achieved. ii) Stimulated emission should be predominant over spontaneous emission. ii) Give the principle of CO2 laser and explain its working by an energy level diagram. Principle, construction and the working of CO 2 Laser Fig Different modes of vibration The carbon dioxide laser (CO 2 laser) was one of the earliest lasers developed (invented by Indian born American Scientist C. K. N. Patel of Bell Labs in 1964), and is still one of the most useful. Carbon dioxide lasers are the highest power continuous wave lasers that are currently available. Construction: The schematic view of typical CO 2 laser is as shown in figure 5.6. It is basically a discharge tube having a bore of cross section of about 1.5 mm and a length of about 5 m and diameter 260 mm. The discharge tube is filled with a mixture of carbon dioxide, nitrogen and helium gases in 1:2:3 ratios. In order to get a high power output, the ends of this tube are fitted with NaCl Brewster windows so that the laser light generated will be polarized. The optical resonator is formed with two concave mirrors one fully reflecting and other partially reflecting.

34 Figure - CO 2 Laser Working: The diagram (Figure 5.7) representing some of the energy states of the CO 2 molecule. The vibrational and rotational modes of the CO 2 cannot be excited themselves by photons. When a voltage is placed across the gas, electrons collide with the N2 molecules and excite them to their lowest vibrational levels. The energy of this state is very close to the 001 and 002 levels in CO 2 (the n=1 state of N2 excites the 001state of CO 2 and the n=2 state of N 2 excites the 002 state of CO 2 ). In turn, this energy is transferred to the CO 2 molecules and results in populating their upper levels. This occurs because the restoring force constant (the k, spring constant) of N 2 is almost identical as that of the CO 2 molecule. The infrared output of the laser is the result of transitions between rotational states of the CO 2 molecule of the first asymmetric vibrational mode (001) to rotational states of both the first symmetric stretch mode (100) and the second bending mode (020), as seen in figure 5.6. During these transitions two different wavelengths are emitted such as: 10.4 μm and 9.4 μm. The 100 and 020 vibrational levels depopulate quickly by the presence of helium. Helium is not only present to maintain the plasma, but also as a depopulation mechanism. When in one of these lower energy levels, a collision between CO 2 and He atoms results in a transfer of the energy to the He atom. The infrared transitions are relatively slower than this depopulation, thus a population inversion is the result.

35 Nitrogen Carbon dioxide Figure - Energy states of CO 2 and N 2 molecules Advantages The construction is simple. The output of this laser is continuous. It has an extremely high efficiency. It has very high output power. The output power can be increased by increasing the length of the gas tube. Disadvantages The contamination of CO and O 2 will have some effect on the laser action. The operating temperature plays an important role in determining the output power of the laser. Corrosion problem may occur in the reflecting plates. (or) b) Explain the propagation of light through optical fibres and explain the different types of optical fibres. Principle and propagation of light The light launched inside the core at one end of the fiber propagates to the other end due to total internal reflection at the core and cladding interface.

$Total internal reflection at the fiber wall can occur only if two conditions are Satisfied. 1. The refractive index of the core material n1 must be higher than that of the cladding n2 surrounding it.$

36 Total internal reflection at the fiber wall can occur only if two conditions are Satisfied. 1. The refractive index of the core material n1 must be higher than that of the cladding n2 surrounding it. 2. At the core cladding interface, the angle of incidence ( between the ray and normal to the interface) must be greater than the critical angle defined as Sin c n 2 n 1 refractive index n2 n1 > n2. Let the light ray travel from core of refractive index n1 to cladding of

37 Case i) - i < c the ray is refracted into cladding surface. (OA light ray) light) light) Case ii) - i = c the ray travels along the interface of core and cladding. (OB Case iii) - i > c the ray is totally internally reflected into the same medium. (OC Consider an optical fiber through which the light is being sent. The end at which light enters is called launching end. Let the refractive indices of the core and cladding be n1 and n2 respectively; n1> n2. Let the refractive index of the medium from which the light is launched be n0. The ray strikes the core cladding interface at an angle θ. If θ is greater than the critical angle θc, the ray undergoes total internal reflection at the interface. Let us now find out up to what maximum value of i at A total internal reflection at B is possible. Principle and Propagation of light through optical fibre medium

LASERS. Dr D. Arun Kumar Assistant Professor Department of Physical Sciences Bannari Amman Institute of Technology Sathyamangalam

LASERS. Dr D. Arun Kumar Assistant Professor Department of Physical Sciences Bannari Amman Institute of Technology Sathyamangalam LASERS Dr D. Arun Kumar Assistant Professor Department of Physical Sciences Bannari Amman Institute of Technology Sathyamangalam General Objective To understand the principle, characteristics and types