Physics Unit -3 QB PDF

PHYSICS QUESTION BANK-UNIT-3 1.What is luminescence? Mention the various methods by which luminescence can be achieved. (3 or 6 marks) Luminescence is the emission of light from a substance that is not heated to a high temperature, and can be achieved through a number of methods, including: Chem...

PHYSICS QUESTION BANK-UNIT-3 1.What is luminescence? Mention the various methods by which luminescence can be achieved. (3 or 6 marks) Luminescence is the emission of light from a substance that is not heated to a high temperature, and can be achieved through a number of methods, including: Chemiluminescence: Light is emitted as a result of a chemical reaction Bioluminescence: Light is emitted as a result of an enzymatic reaction Photoluminescence: Light is emitted after the absorption of photons Electroluminescence: Light is emitted after an electric current passes through a substance Radioluminescence: Light is emitted after bombardment with ionizing radiation 2.What is a LASER? Explain its properties. (3 or 6 marks)(IMP) LASER: The word LASER stands for Light Amplification by the Stimulated Emission of Radiation. Laser is a source producing an intense, monochromatic, and highly parallel beam of coherent light. Properties of Laser: 1. High intensity - Laser beams are highly intense as a large number of photons are concentrated in a small region. 2. Monochromatic - Highly monochromatic radiation (i.e. single wavelength) 3. Coherence - Perfectly coherent as the emitted light waves has the same phase with one another. 4. High degree of directionality - Travels in a single direction as the photons are traveling along the optical axis of the system. 3.Distinguish between spontaneous emission and stimulated emission (3-6 marks) (IMP) 4.Explain how Einstein’s theory predicts existence of stimulated emission. (3 or 6 marks) (IMP) 5.Define the terms ‘Metastable state’ and ‘Population inversion”. (3 or 6 marks) 6. Mention the basic conditions for laser action. How can they be materialised? (3 or 6 marks) (i) An inverted population must be in the excited state, i.e. more atoms than in the ground state. (ii) The excited state out of all the states should be a metastable state. (iii) The emitted photons must stimulate further emission. OR (i) Rate of stimulated emission should be more than the rate of spontaneous emission (ii) Rate of stimulated emission should be more than the rate of absorption To materialize these conditions, energy is supplied to lower energy states to excite the atom from lower energy state to higher energy state. 7. What are the basic components required for the construction of a laser? Explain the method of production of laser. (4 or 10 marks) ▪ The Active Medium contains atoms which can emit light by stimulated emission.Solid (Crystal) ,Gas ,Liquid (Dye),Semiconductor (Diode) ▪ The Excitation source (pumping mechanism) is a source of energy to excite the atoms to the proper energy state.(Optical ,Electrical ,Chemical) ▪ The Optical Resonator or laser cavity reflects the laser beam through the active medium for amplification.(HR Mirror and Output Coupler) 8. Discuss the relative advantages and disadvantages of three level and four level lasers. (4 or 10 marks) Three-level and four-level lasers have several advantages and disadvantages, including: Output: Three-level lasers typically produce pulsed output, while four-level lasers can produce continuous output. Threshold frequency: Four-level lasers have a lower threshold frequency than three-level lasers. Efficiency: Four-level lasers are highly efficient. Pumping rate: Four-level lasers require a lower pumping rate. Population inversion: In a four-level laser, the condition of population inversion can be maintained continuously. In a three-level laser, the lower energy level rapidly becomes highly populated, preventing further lasing until the atoms relax. Terminal level: In a three-level laser, the terminal level is ground level, which transfers more than half of the atoms. 9. What are the different methods of achieving population inversion? (3 or 6 marks) 1. Optical Pumping : By using high intensity light or flash tubes for excitation. Ex: Ruby laser, Nd:YAG laser 2. Electrical Pumping: By applying very high potential between the plates of discharge tube, gas gets discharged, leads to atom – atom collision and pumping. Ex: He-Ne laser, CO2 laser, Argon laser. 3. Chemical Pumping :Exothermic chemical reactions liberate energy. This energy is used in pumping the atoms. Ex: Dye lasers. 10.Describe the construction and working of a semiconductor laser. (4 or 10 marks)(IMP) Construction: GaAs laser diode is a single crystal of GaAs and consists of heavily doped n and p sections. The n- section is formed by doping with Tellurium and p- section is formed by doping with Zinc. [The doping concentration is very high is of the order of 1017 to 1019 dopant atoms/cm3]. A pair of parallel planes of the crystal are cleaved or polished at right angle to the p-n layer. These planes play the role of reflecting mirror. The other two remaining sides perpendicular to the junction are roughened to suppress reflection of the photons. The end surfaces of the p and n sections parallel to the plane of the junction are provided with electrodes for the application of forward bias voltage. Working: GaAs laser diode is subjected to a forward bias as shown in figure. Due to the forward bias, the holes are injected into the p-side of the junction and electrons are injected into the n-side. During the flow when a hole meets an electron recombination takes place resulting in the emission of a photon. In a GaAs laser by heavy doping, a large number of electrons are available in an n- type material and a large number of holes are available in a p-type material.When the current in the diode is low, the concentration of the electrons at the bottom of conduction band will be still lesser than that in the valence band, and the recombination result in only spontaneous emissions. But as the current is increased, a threshold for lasing will be attained beyond which a population inversion can be achieved between the conduction band and valence band and an active region is formulated very near the junction. At this stage a photon released by a spontaneous emission may trigger stimulated emission over a large number of recombination, leading to the buildup of laser radiation of high power. Since the energy gap of GaAs is 1.4 eV, the wavelength of the emitted light is λ = hc/Eg = 8400 Å 11.Explain the construction and working of a Nd-YAG laser, with a neat diagram (8 marks)(VERY IMP) Construction: Neodymium-doped Yttrium Aluminium Garnet (Nd: YAG) laser is a solid-state laser in which Nd:YAG is used as a laser medium. It is a four-level laser system. These lasers operate in both pulsed and continuous mode. Nd stands for neodymium and YAG for Yttrium aluminium garnet (Y3Al5O12). [It is developed by J.E. Geusic, H.M. Marcos and L.G. Van Vitert in 1964]. The rod of Y3Al5O12 is doped 1% with triply ionized (trivalent) neodymium. Nd3+ ions will replace the Y3+ ions in the crystal. The YAG crystal is transparent and colourless. When doped with approximately 1% Nd (1.38 x 1020 Nd ions per cm3), the crystal takes a light blue colour. Maximum length of the rod is about 10 cm and diameter is 12 mm. Active medium: Nd3+ ions act as active medium or active centers. YAG is just the host. Pumping source: The pumping of Nd3+ ions to upper levels is done by krypton arc lamp or Xenon lamp. Thus, the optical pumping is used to achieve population inversion Optical resonator system: The ends of the Nd: YAG rod are polished and silvered or Nd: YAG rod placed between two mirrors so as to act as the optical resonator system. Working: Nd:YAG is a four level laser system. The optical pumping is used in this laser The pumping of neodynium (Nd3+) ions to upper state (E4) is done by using krypton arc lamp. The wavelength of light of wavelength 720 nm to 800 nm excites the ground state (E1) Nd3+ ions to E4 states (lifetime ~ ns). From E4 states, they make a non-radiative transition to E3 state. E3 is a metastable state (lifetime ~ ms) so population inversion is achieved between the levels E3 and E2. After this, the process of stimulated emission will occur. Thus, the laser emission will occur in between the levels E3 and E2 with the process of stimulated emission. So E3 is the upper laser level and E2 is the lower laser level. Then Nd3+ ions come back to the ground state E1. Laser emission will have wavelength of 1064 nm so occur in the infrared range of spectrum. 12. Briefly mention the applications of lasers. (3 or 6 marks) One of the major applications is in pure science to investigate the interaction of matter with intense electromagnetic radiation. The directionality and coherence of the laser beam are useful in the measurement of distances based on interferometric methods. In the field of communication, laser has been used with proper modulation for information transmission. Because of the high intensity and energy associated with laser beams, they can be utilized for applications such as welding, cutting and ablation of materials. Laser has also been used for medical applications like treatment of dental decay, destruction of tumors, treatment of skin diseases and eye surgery. Holography is another important application which helps in recording the amplitude as well as the phase of light reflected from objects thereby preserving the three- dimensional information. Used in various fields like Medicine Research Supermarkets Entertainment, Industry, Military, Communication, Art, Information technology etc  13.What is an optical fiber? What is the principle on the basis of which optical transmission is achieved through a fiber? Explain. (3 or 6 marks) o An optical fiber is essentially a waveguide for light. o It is long, thin, cylindrical in shape and it consists of mainly two regions. o The central cylindrical region is the light guiding region known as the Core. o The coaxial region surrounding the core is known as Cladding. o The refractive index of the core material is higher than that of the cladding (n1>n2). The core and cladding are made of either glass or plastic. o The outermost region surrounding cladding is called the sheath or protective buffer coating or jacket Optical fibers work on the principle of total internal reflection. The refraction at the interface between two media is governed by Snell’s law where n1 and n2 are refractive indices of medium 1 and medium 2 θ1 - angle of incidence & θ2 - angle of refraction. When the angle of incidence is increased angle of refraction also increases For particular angle of incidence (θ1 = θc) refracted ray grazes the interface of the two media. This angle of incidence is called critical angle (θc). For critical incidence θ1 = θc and θ2= 90° , the critical angle of incidence θc is given by n1 sin θc = n2 sin 90° The ray will be totally internally reflected into the denser medium when the angle of incidence is greater than the critical angle known as total internal reflection. 14.Explain the terms numerical aperture, semi angle of the cone of acceptance and relative refractive index difference. (4 or 10 marks) Consider a ray of light AO enters into the core of an optical fiber at an angle θ0 to the fiber axis as shown in figure. This ray is refracted along OB at an angle θ1 in the core and further proceeds to fall at critical angle (90-θ1) at B on the core-cladding interface. Hence the ray is refracted at 90° to the normal to the interface, i.e. it grazes along BC. It is clear from the figure that, any ray that enters the core at an angle less than θ0, will have angle of incidence (90-θ1) at the interface greater than the critical angle and hence undergoes total internal reflection. On the other hand, any ray that enters at an angle greater than θ0 at O, will have angle of incidence less than the critical angle at the interface. Hence it refracts into the cladding and is lost from the core. Numerical Aperture (N.A.) is a measure of light-gathering or light-collecting ability of an optical fiber. It is defined as the sine of the angle of acceptance. Angle of acceptance (semi-angle of the cone of acceptance) is the maximum allowed angle that the incident ray can make with the fiber axis for transmission through the fiber by total internal reflection. The angle θ0 is called angle of acceptance Fractional index Change or Relative Refractive index difference (Δ): It is the ratio of the refractive index difference between core and cladding to the refractive index of the core of an optical fiber. 15. Derive an expression for numerical aperture of an optical fiber in terms of R.I. of core and cladding. (4 or 10 marks)(VERY IMP) 16.What are the different modes of propagation possible in optical fibers. Explain in detail with necessary diagrams. (4 or 10 marks) There are various modes by which light waves can be made to propagate in an optical fiber. The mode of propagation depends on the construction of the optical fiber. Modes of propagation refers to the number of paths for the light rays along which the waves are in phase inside the fiber. Multimode means several paths are available, This type of fiber supports multiple transverse guided modes for a given optical frequency and polarization. It's commonly used for short range. Single mode means single ray propagation ,this type of fiber is designed to carry only the transverse mode of light. It's often used for long distances and performs better than multimode fibers with lower attenuation. 17.Write any four differences between single mode and multi-mode fibers. (3 or 6 marks) 18.What is attenuation in an optical fiber? Mention the possible reasons for the same. (3 or 6 marks) Attenuation is the loss of optical power suffered by the optical signal as it propagates through the fiber. The main reason for the loss of light intensity over the length of the fiber is due to (1)Light Absorption :In this case loss of signal occurs due to absorption of photons Photons are absorbed by a) Impurities ▪ Metal ions – iron, chromium, cobalt and copper ▪ OH (hydroxy) ions b) Intrinsic absorption by the glass material itself. (2) Scattering losses: Photons scatter because of sharp change in localized R.I. Variation of R.I occurs due to localized structural inhomogeneities during fiber fabrication process. Rayleigh scattering takes place due to scatter spot is less than light wavelength. (3) Radiation Losses: The radiation losses are mainly due to bending of fibbers and occurs in two ways. a) Macroscopic bending b) Microscopic bending 19.Describe how the optical fiber can be used in communication. Mention its advantages. (3 or 6 marks) Optical fiber communication is the transmission of information by propagation of optical signal through optical fibers over the required distance , which involves deriving optical signal from electrical signal at the transmitting end, and conversion of optical signal back to electrical signal at the receiving end. It is known as point to point communication Convert to light An electronic signal is converted into light using a laser or light-emitting diode (LED). Transmit light The light travels through the core of the fiber, bouncing off the walls and undergoing multiple reflections and refractions. Detect light A photodiode or other light-sensitive device detects the light at the end of the fiber. Convert back to electronic signal The light signal is converted back into an electronic signal. Amplify and process The signal is amplified, processed, and transmitted to its final destination Advantages of Optical Fibers: Fiber optic communications- Telecommunications ,Local Area Networks, Cable TV,CCTV Optical Fiber Sensors Applications in medicine - Endoscope Applications in industry or  Less Expensive, Small size, lighter weight and more compact than copper cables. They have higher signal carrying capacity (bandwidth up to 2 Gbps, or more), because of which the cost/meter/channel for the fiber would be lesser than that for metallic cable. Lower attenuation and less signal degradation - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) Signals are transmitted in the form of light signals, so higher speeds can be achieved High Security – as optical signals are transmitted through the fiber, there is no energy radiation and hence it is impossible to tap information. Optical fibers have longer life (20-30 years) than copper cables (12-15 years). 20.Describe an optical fiber with working principle. Write any two advantages of an optical fiber communication system over conventional communication system. (4 or 10 marks) For working principle refer question number 19 The advantages of an optical fiber communication system over conventional communication system. Optical fiber communication has advantages such as high-speed data transmission, data security, and data reliability. Optical fiber cables have higher bandwidth than copper conductor cables. 21.Describe the different types of optical fiber along with the sketches of geometry, refractive index profile and ray propagation. (4 or 10 marks) (1) Single mode fibers (SMF) A step-index fiber has a central core with a uniform refractive index and outside cladding surrounding the core also has a uniform refractive index; however, the refractive index of the cladding is less than that of the central core. (Fig. 1). The R.I. profile is a step-index profile as the refractive index changes abruptly at the interface between the core and the cladding. The diameter of the core is very small about 8- 10 μm and external diameter of cladding is 60-70 μm. In a single mode fiber, the core is so small that there is a single path for the light to propagate in the fiber. Only laser can be used as a source of light. (2) Step Index Multi mode fibers : A step-index Multi mode fibers has a central core with a uniform refractive index and outside cladding also has a uniform refractive index; however, the refractive index of the cladding is less than that of the central core. (Fig. 1). The refractive index changes abruptly at the interface between the core and the cladding and hence the R.I. profile is a step-index profile. The diameter of the core is 50-200 µm and external diameter of cladding is 100-250 µm. The fiber supports many modes for propagation. Either a LASER or LED can be used as source of light.  (3)Graded index Multi mode fibers: In a graded index multimode fiber, the refractive index of the core is maximum at the center of the core and decreases gradually and becomes equal to that of the cladding at the interface between core and cladding (Fig.1). As the refractive index decreases gradually away from the center of the core, the R. I. profile is a graded index profile. Diameter of the core is 50-200 µm, Cladding diameter is 100-250 µm. Hence The fiber supports many modes for propagation. Either a LASER or LED can be used as source of light. NOTE:Ruby laser,He-Ne laser(problems only)-theory is not necessary BY, SHRAVYA N BHAT(ISE-I)