$1.$Define numerical aperture $($NA$)$ in the context of optical fibers. Derive the formula for NA$.$

$2.$ A fiber optic cable with a core refractive index of $1\mathrm{.}53$ and cladding refractive index of $1\mathrm{.}45$ is used for long$-$distance communication. Calculate the maximum angle at whichlight can enter the fiber.

$3.$A fiber optic cable has a core diameter of $10$ um and a cladding diameter of $100$ um$.$ If the refractive indices of core and cladding are $1\mathrm{.}52$ and $1\mathrm{.}48$ respectively, calculate the numerical aperture and explain its significance for data transmission.

$4.$ Describe the three methods of heat transfer and give examples of each.

$5.$ What are the different modes of propagation in optical fibers? Explain the difference between single$-$mode and multimode propagation.

$6.$ What are the different theories based on the properties of light? Explain the wave theory of light.

$7.$Contrast the operational principles and applications of LED and LASER

$8.$Differentiate between step$-$index fibers and graded$-$index fibers. Discuss their structural characteristics and how they affect light propagation.

$9.$ What is a thermocouple thermometer? Explain its working principle.

$10.$Define thermometry. List out Points to consider for the Selection of thermometer. Give difference between heat and temperature.

$11.$Explain the working of a resistance thermometer and its applications.

$12.$What is thermal conductivity? Derive the expression for the rate of heat transfer by conduction in a material.

$13.$Define magnetic susceptibility and permeability. How are they related to the magnetization of a material?

$14.$Give classification of magnetic materials. Discuss their properties and applications.

$15.$Differentiate types of magnetic material in tabular format.

$16.$Define Bohr magnetron. How is it related to the magnetic moment of an electron and the fundamental unit of magnetism?

$17.$Define following terms:Magnetic MomentMagnetic DipoleMagnetic Field StrengthMagnetic Flux DensityMagnetic Permeability

$18.$What is a PN junction diode? Discuss the differences between forward bias and reverse bias in a PN junction diode with suitable diagram.