Applied Physics
- 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
- Explain mobility of electrons. The resistivity of Cu is 1.72 x 10-8 ohm-m. Calculate the mobility of electrons in Cu given that number of electrons per unit volume is 10.41 x 1028 m-3. Mobility (μ): When an electric field E is applied, free electrons acquire a drift velocity v_d proportional to E. Mobility is the drift velo... 1
- Derive an expression for Fermi level for an intrinsic semiconductor. In an intrinsic semiconductor, the electron concentration in the conduction band is n=N_c e^(-(E_c-E_F)/kT) and the hole concentration in th... 1
- State Ampere’s Circuital law. Derive Maxwell’s fourth equation in differential form. Ampere's circuital law: The line integral of the magnetic field intensity vec H around any closed path equals the net current enclosed by th... 1
- The refractive indices of core and cladding materials of a step index fibre are 1.48 and 1.45 respectively. Calculate (1) Numerical Aperture, (2) acceptance angle, (3) the critical angle at the core cladding interface, (4) fractional refractive indic… Given: n₁=1.48 , n₂=1.45 , n₀=1 (air). (1) Numerical aperture: NA=√(n₁²-n₂²)=√(2.1904-2.1025)=√(0.0879)=0.2965 (2) Acceptance angle: θ_a=sin... 1
- Obtain the condition for maxima and minima of the light reflected from a thin transparent film of uniform thickness. Set-up: A thin transparent film of uniform thickness t and refractive index μ is in air. A ray AB is incident at angle i . Part of it is ref... 1
- With a neat energy level diagram, describe the construction and working of He-Ne Laser. He–Ne laser is a four-level gas laser that gives a continuous output at 6328 Å (red). Construction: discharge tube (He:Ne ≈ 10:1, ~1 mm Hg)... 1
- An electron has a speed of 400 m/sec with uncertainty of 0.01%. Find the accuracy in its position. Given: v=400 m/s, so Δ v=0.01% of 400 =0.01/100×400=0.04 m/s. m=9.1×10⁻³¹ kg. Uncertainty principle: Δ x Δ p≥h/4π Δ x=h/4π mΔ v=frac6.63×10⁻... 1
- Calculate the wavelength of de-Broglie waves associated with mass 1 kg moving with a speed of 103 m/sec. Given: m=1 kg, v=10³ m/s, h=6.63×10⁻³⁴ J s. Formula: λ=h/mv λ=frac6.63×10⁻³⁴1×10³=6.63×10⁻³⁷\ m λ=6.63×10⁻³⁷ m This is far too small to dete... 1
- Write Fermi- Dirac distribution function and explain the terms in it. The Fermi–Dirac distribution function gives the probability that an available energy state E is occupied by an electron at absolute temperat... 1
- Show that the divergence of the curl of a vector is zero. Let vec A=A_xhat i+A_yhat j+A_zhat k . nabla×vec A=Big(∂ A_z/∂ y-∂ A_y/∂ zBig)hat i+Big(∂ A_x/∂ z-∂ A_z/∂ xBig)hat j+Big(∂ A_y/∂ x-∂ A_x/∂ y... 1