Physics Question Paper 2020 Delhi - Set-1

General Instructions:

Read the following instructions very carefully and strictly follow them:

1. This question paper comprises four sections – A, B, C and D.
2. There are 37 questions in the question paper. All questions are compulsory.
3. Section A: Q. no. 1 to 20 are very short-answer type questions carrying 1 mark each.
4. Section B: Q. no. 21 to 27 are short-answer type questions carrying 2 marks each.
5. Section C : Q. no. 28 to 34 are long-answer type questions carrying 3 marks each.
6. Section D : Q. no. 35 to 37 are also long answer type questions carrying 5 marks each.

1. There is no overall choice in the question paper. However, an internal choice has

been provided in two questions of one mark, two questions of two marks, one question of three marks and all the three questions of five marks. You have to attempt only one of the choices in such questions.

1. However, separate instructions are given with each section and question, wherever necessary.
2. Use of calculators and log tables is not permitted.
3. 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>\epsilon_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>

Mass of electron (m_e) = <math>9.1 \times 10^{-31}</math> kg 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 : The relationship between Brewester angle '<math>\theta</math>' and the speed of light 'v' in the denser medium is -

1. <math>v \tan \theta = c</math>
2. c tan <math>\theta = v</math>
3. <math>v \sin \theta = c</math>
4. <math>c \sin \theta = v</math>

SOLUTION: Let the absolute refractive index of the given medium be <math>\mu</math> and the speed of light in vacuum be c.

From Brewster's law:

<math>\tan \theta = \mu</math>

The refractive index can also be written as:

<math>\mu = \frac{c}{v}</math>

<math>\Rightarrow \tan \theta = \frac{c}{v}</math> <math>\Rightarrow c = v \tan \theta</math>

Hence, the correct answer is option (a).

Question 2: Photo diodes are used to detect

1. radio waves
2. gamma rays
3. IR rays
4. optical signals

SOLUTION: Photodiodes are used to detect the visible light, out of the given options optical signals is the most appropriate.

Hence, the correct answer is option (d).

Question 3: The selectivity of a series LCR a.c. circuit is large, when

1. L is large and R is large
2. L is small and R is small
3. L is large and R is small
4. <math>L = R</math>

SOLUTION: Selectivity of a circuit depends on the quality of resonance. The quality factor is given by:

<math>Q = \frac{\omega_0 L}{R}</math>

High value of quality factor make sure that the resonance curve is sharp. Sharper the resonance curve is more selective is the LCR circuit. Thus, the selectivity of the LCR circuit is large when L is large and R is small.