# Physics Delhi Set 1 2021-2022 Science (English Medium) Class 12 Question Paper Solution

Physics [Delhi Set 1]
Date & Time: 20th May 2022, 10:30 am
Duration: 2h

General Instructions:

1. This Question paper contains 12 questions. All questions are compulsory.
2.  This question paper is divided into THREE Sections, Sections A, B, and C.
3. Section - A: Question number 1 to 3 are of 2 marks each.
4. Section - B: Question number 4 to 11 are of 3 marks each.
5. Section - C: Question number 12 is a case-based question of 5 marks.
6. There is no overall choice in the question paper. However internal choice has been provided in some of the questions. Attempt any one of the alternatives in such questions.
7. Use of log tables is permitted, if necessary, but use of a calculator is not permitted.
c = 3 × 108 m/s
h = 6.63 × 10-34 Js
e = 1.6 × 10-19C
μ= 4π × 10-7T m A-1
ε= 8.854 × 10-12C2N-1m-2

1/(4piε_0) = 9 xx 10^9 Nm^2C^-2

Mass of electron = (me) = 9.1 × 10-31 kg

Mass of Neutron = 1.675 × 10-27kg

Mass of proton = 1.673 × 10-27kg

Avogadro's number = 6.023 × 1023 per gram mole

Boltzmann constant = 1.38 × 10-23 Jk-1

SECTION - A
 1

With the help of a circuit diagram, explain briefly how a p-n junction diode works as a half-wave rectifier.

Concept: Application of Junction Diode as a Rectifier
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
 2
 2.a

Suppose you are given a chance to repeat the alpha-particle scattering experiment using a thin sheet of solid hydrogen in place of the gold foil. (Hydrogen is a solid at temperatures below 14 K.) What results do you expect?

Concept: Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom
Chapter: [0.13] Atoms
OR
 2.b

Why it is the frequency and not the intensity of the light source that determines whether the emission of photoelectrons will occur or not? Explain.

Concept: Experimental Study of Photoelectric Effect
Chapter: [0.11] Dual Nature of Radiation and Matter
 3

Why a photo-diode is operated in reverse bias whereas the current in the forward bias is much larger than that in the reverse bias? Explain. Mention its two uses.

Concept: Special Purpose P-n Junction Diodes
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
SECTION - B
 4

Draw a graph showing the variation of the number of particles scattered (N) with the scattering angle θ in the Geiger-Marsden experiment. Why only a small fraction of the particles are scattered at θ > 90°?

Concept: Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom
Chapter: [0.13] Atoms

Write two important limitations of Rutherford nuclear model of the atom.

Concept: Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom
Chapter: [0.13] Atoms
 5
 5.a

Draw V-I characteristics of a p-n Junction diode.

Concept: Semiconductor Diode
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
 5.b

Differentiate between the threshold voltage and the breakdown voltage for a diode.

Concept: Semiconductor Diode
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
 5.c

Write the property of a junction diode which makes it suitable for rectification of ac voltages.

Concept: Application of Junction Diode as a Rectifier
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
 6

Consider the fission of ""_92^238"U" by fast neutrons. In one fission event, no neutrons are emitted and the final end products, after the beta decay of the primary fragments, are ""_58^140"Ce" and ""_44^99"Ru". Calculate Q for this fission process. The relevant atomic and particle masses are

"m"(""_92^238"U") = 238.05079 u

"m"(""_58^140"Ce") = 139.90543 u

"m"(""_44^99"Ru") = 98.90594 u

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [0.12] Nuclei
 7

How can you differentiate whether a pattern is produced by a single slit or double slit?

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [0.1] Wave Optics

Derive the expression for the angular position of (i) bright and (ii) dark fringes produced in a single slit diffraction.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [0.1] Wave Optics
 8
 8.a
 8.a.1

What is meant by a power of a lens? Define its SI unit.

Concept: Power of a Lens
Chapter: [0.09] Ray Optics and Optical Instruments
 8.a.2

A plano-convex lens is made of glass with a refractive index of 1.5. The radius of curvature of the convex surface is 25 cm.

1. Calculate the focal length of the lens.
2. If an object is placed 50 cm in front of the lens, find the nature and position of the image formed.
Concept: Lensmaker's Formula
Chapter: [0.09] Ray Optics and Optical Instruments
OR
 8.b

A slit of width 0.6 mm is illuminated by a beam of light consisting of two wavelengths 600 nm and 480 nm. The diffraction pattern is observed on a screen 1.0 m from the slit. Find:

1. The distance of the second bright fringe from the central maximum pertaining to the light of 600 nm.
2. The least distance from the central maximum at which bright fringes due to both wavelengths coincide.
Concept: Interference of Light Waves and Young’s Experiment
Chapter: [0.1] Wave Optics
 9
1. Calculate the energy and momentum of a photon in a monochromatic beam of wavelength 331.5 nm.
2. How fast should a hydrogen atom travel in order to have the same momentum as that of the photon in part (a)?
Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [0.11] Dual Nature of Radiation and Matter
 10

A ray of light passes through a prism of refractive index sqrt2 as shown in the figure. Find: 1. The angle of incidence (∠r2) at face AC.
2. The angle of minimum deviation for this prism.
Concept: Total Internal Reflection
Chapter: [0.09] Ray Optics and Optical Instruments
 11
 11.a
 11.a.1

Arrange the following electromagnetic radiation in the ascending order of their frequencies:

X-rays, microwaves, gamma rays, radio waves

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves
 11.a.2 | Attempt any two:
 11.a.2.1

Write two uses of the following radiation.

X-rays

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves
 11.a.2.2

Give two uses of Microwaves.

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
 11.a.2.3

Write two uses of the following radiation.

Gamma rays

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves
 11.a.2.4

Give two uses of radio waves.

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves
OR
 11.b
 11.b.1

With the help of a ray diagram explain the working of a reflecting telescope.

Concept: Optical Instruments - Telescope
Chapter: [0.09] Ray Optics and Optical Instruments
 11.b.2

Write two important advantages of reflecting telescope over a refracting telescope.

Concept: Optical Instruments - Telescope
Chapter: [0.09] Ray Optics and Optical Instruments
SECTION - C
 12
 12.a
 A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

A ray of light travels from a medium into the water at an angle of incidence of 18°. The refractive index of the medium is more than that of water and the critical angle for the interface between the two media is 20°. Which one of the following figures best represents the correct path of the ray of light?    Concept: Reflection and Refraction of Plane Wave at Plane Surface Using Huygens' Principle - Refraction at a Rarer Medium
Chapter: [0.1] Wave Optics
 12.b
 A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

A point source of light is placed at the bottom of a tank filled with water, of refractive index µ, to a depth d. The area of the surface of water through which light from the source can emerge is:

(π"d"^2)/(2(µ^2 - 1))

(π"d"^2)/((µ^2 - 1))

(π"d"^2)/(sqrt2 sqrt(µ^2 - 1))

(2π"d"^2)/((µ^2 - 1))

Concept: Speed of Light
Chapter: [0.1] Wave Optics
 12.c
 A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

For which of the following media, with respect to air, the value of critical angle is maximum?

Crown glass

Flint glass

Water

Diamond

Concept: Reflection and Refraction of Plane Wave at Plane Surface Using Huygens' Principle - Refraction at a Rarer Medium
Chapter: [0.1] Wave Optics
 12.d
 A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

The critical angle for a pair of two media A and B of refractive indices 2.0 and 1.0 respectively is:

30°

45°

60°

Concept: Speed of Light
Chapter: [0.1] Wave Optics
 12.e
 A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

The critical angle of the pair of a medium and air is 30°. The speed of light in the medium is:

1 × 108 ms−1

1.5 × 108 ms−1

2.2 × 108 ms−1

2.8 × 108 ms−1

Concept: Speed of Light
Chapter: [0.1] Wave Optics

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