Physics Question Paper 2019 Set-2

General Instructions:

1. All questions are compulsory. There are 27 questions in all.
2. This question paper has four sections: Section A, Section B, Section C and Section D.
3. Section A contains five questions of one mark each, Section B contains seven questions of two marks each, Section C contains twelve questions of three marks each, and Section D contains three questions of five marks each.

1. There is no overall choice. However, internal choices have been provided in two

questions of one mark, two questions of two marks, four questions of three marks and three questions of five marks weightage. You have to attempt only one of the choices in such questions.

1. You may use the following values of physical constants wherever necessary.

<math>c = 3 \times 10^8 \,\text{m/s}</math>

<math>h = 6.63 \times 10^{-34} Js</math>

<math>e = 1.6 \times 10^{-19} C</math>

<math>\mu_0 = 4\pi \times 10^{-7} \text{ T m A}^{-1}</math>

<math>\varepsilon_0 = 8.854 \times 10^{-12} \,\mathrm{C}^2 \,\mathrm{N}^{-1} \,\mathrm{m}^{-2}</math>

<math>4\pi\epsilon_0 = 9 \times 10^9 \text{ N m}^2 \text{ C}^{-2}</math>

<math>m_e = 9.1 \times 10^{-31} \text{ kg}</math>

mass of neutron = <math>1.675 \times 10^{-27}</math> kg

mass of proton = <math>1.673 \times 10^{-27}</math> kg

Avogadro's number = <math>6.023 \times 10^{23}</math> per gram mole

Boltzmann constant = <math>1.38 \times 10^{-23} \text{ JK}^{-1}</math>

Question 1

Define the term "Threshold frequency", in the context of photoelectric emission. OR Define the term "Intensity" in the photon picture of electromagnetic radiation.

SOLUTION:

For a given Photosensitive material, there is a certain minimum cut off frequency at which Photoelectric emission is possible is called Threshold frequency, i.e. At this frequency just emission of photoelectrons happens without giving them any kind of additional energy. Higher the work function of the material, greater is the Threshold frequency.

See the figure below of two different metals having different Threshold Frequency. OR The intensity I, is defined as the total amount of energy falling on a given surface/Region per unit time 't' and per unit area 'A'.

If the total energy emitted <math>= nh\nu</math>

Then, <math>I = \frac{nh\nu}{\Lambda t}</math>

Question 2

When unpolarized light is incident on the interface separating the rarer medium and the denser medium, Brewster angle is found to be 60°. Determine the refractive index of the denser medium.

SOLUTION:

The relation between refractive index and Brewster's angle is, <math>\mu</math>=tan<math>\theta</math>_B (Assuming rarer medium as Air)

<math>\mu = \tan \theta_{\rm B}</math> | Where, <math>\theta_{\rm B} = \text{Brewster's angle}</math>

<math>\mu = \tan 60^{\circ} = \sqrt{3}</math>

So, the refractive index of the denser medium is <math>\sqrt{3}</math>

Question 3

In skywave mode of propagation, why is the frequency range of transmitting signals restricted to less than 30 MHz?