First Year (FE)
- What is attenuation in an optical fibre and how is the attenuation coefficient calculated? In one line Attenuation is the gradual loss of optical power as light travels along a fibre, and the attenuation coefficient gives this loss... 1
- What factors cause attenuation or signal loss in an optical fibre? In one line Attenuation in fibres is caused mainly by absorption, scattering and bending losses, plus losses at joints and connectors. Expla... 1
- What are the main blocks of a fibre optic communication system and what does each do? In one line A fibre optic communication system converts an electrical signal into light at the transmitter, sends it through an optical fibr... 1
- Why is optical fibre communication better than copper cable communication? In one line Optical fibres carry far more information over longer distances with lower loss and no electrical interference, while being ligh... 1
- What are the conditions for bright and dark fringes in reflected light from a thin film? In one line In reflected light from a thin film, brightness occurs when 2μ tcos r = (2n-1)λ/2 and darkness when 2μ tcos r = nλ . Explained s... 1
- How does interference occur in a thin film of uniform thickness, and what is the path difference? In one line Light reflected from the top and bottom surfaces of a thin film comes from the same wave (division of amplitude), so the two ref... 1
- How are interference fringes formed in a wedge-shaped thin film and what is the fringe width? In one line A wedge-shaped film has thickness increasing steadily from the edge, so each fringe follows a line of constant thickness, giving... 1
- How are Newton's rings formed and why is the central spot dark in reflected light? In one line Newton's rings are concentric circular fringes formed by interference in the air film between a plano-convex lens and a flat gla... 1
- How are Newton's rings used to find the wavelength of monochromatic light? In one line By measuring the diameters of two dark rings that are p rings apart and knowing the lens radius R , the wavelength is found from... 1
- How are Newton's rings used to find the refractive index of a transparent liquid? In one line Filling the gap between the lens and plate with a liquid shrinks the rings by a factor √(μ) , so the refractive index is the rat... 1
- How does an anti-reflecting coating work and what thickness and refractive index should it have? In one line An anti-reflecting coating is a thin transparent film of quarter-wave optical thickness whose two reflected beams cancel by dest... 1
- What is population inversion and why is it necessary for laser action? In one line Population inversion is the state in which more atoms are in a higher energy level than in a lower one, and it is needed so that... 1
- What is pumping in a laser and what are the different methods of pumping? In one line Pumping is the process of supplying energy to the active medium to lift atoms to higher energy levels and create population inve... 1
- What do active medium and active centre mean in a laser? In one line The active medium is the material in which population inversion and light amplification take place, and the active centres are t... 1
- What is the difference between spontaneous emission and stimulated emission of radiation? In one line In spontaneous emission an excited atom drops to a lower level on its own and emits a photon in a random direction and phase, wh... 1
- What is the resonant (optical) cavity in a laser and what job does it do? In one line The resonant cavity is a pair of mirrors around the active medium that sends light back and forth through it many times, buildin... 1
- What are coherence length and coherence time, and how are they related? In one line Coherence time is the time for which a light wave keeps a fixed, predictable phase, and coherence length is the distance the wav... 1
- What are the main characteristics of laser light that make it different from ordinary light? In one line Laser light is highly monochromatic, coherent, directional and intense, unlike light from a bulb, which is mixed in wavelength,... 1
- How is a He-Ne laser constructed and how does it work? In one line A He-Ne laser is a gas laser in which an electric discharge excites helium atoms, which pass their energy to neon atoms by colli... 1
- How is a fibre laser constructed and how does it produce laser light? In one line A fibre laser uses an optical fibre whose core is doped with rare-earth ions as the active medium, pumped by diode lasers, with... 1
- How does LiDAR use a laser to measure distance and map surroundings? In one line LiDAR (Light Detection and Ranging) fires short laser pulses at objects and measures how long the reflected light takes to retur... 1
- How does a laser barcode reader read the product code at a shop counter? In one line A barcode reader sweeps a laser beam across the black and white bars, and a photodetector turns the pattern of reflected light i... 1
- How are lasers used in metal work such as cutting, welding and drilling? In one line A focused laser beam delivers a very high power density to a tiny spot, which heats metal fast enough to melt or vaporise it, so... 1
- A coil having a resistance of 10 Ω and an inductance of 40 mH is connected to a 200 V, 50 Hz supply. Calculate the impedance of the coil, current, power factor and power consumed. X_L = 2π fL = 2π×50×0.04 = 12.566\ Ω . Impedance: Z = √(10² + 12.566²) = √(257.91) = 16.06 Ω (= 10 + j12.566 = 16.06∠51.49° Ω). Current: I =... 1