CBSE Question Bank in

II

Physics

CLASS

12 Features Strictly Based on the Latest CBSE Term-wise Syllabus Chapter Summary Very Short Answer Type Questions

Short Answer Type Questions Long Answer Type Questions Case Study Based MCQs

Comprehensive

CBSE Question Bank in

Physics

Term–II (For Class-XII)

Comprehensive CBSE Question Bank in

Physics Term–II (For Class-XII) (According to the Latest CBSE Examination Pattern)

By

NARINDER KUMAR M.Sc. PES(I) Formerly, Senior Lecturer Department of Physics S.D. Govt. College, Ludhiana Punjab

 

laxmi Publications (P) Ltd (An iso 9001:2015 company)

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Contents Unit V: Electromagnetic Waves 1. Electromagnetic Waves (NCERT Textbook Chapter 8) ...................................... 1–35

Unit VI: Optics 2. Ray Optics and Optical Instruments (NCERT Textbook Chapter 9) ............. 36–118 3. Wave Optics (NCERT Textbook Chapter 10) ................................................ 119–163

Unit VII: Dual Nature of Radiation and Matter 4. Dual Nature of Radiation and Matter (NCERT Textbook Chapter 11) ....... 164–227

Unit VIII: Atoms and Nuclei 5. Atoms (NCERT Textbook Chapter 12) ........................................................... 228–264 6. Nuclei (NCERT Textbook Chapter 13) ........................................................... 265–282

Unit IX: Electronic Devices 7. Semiconductor Electronics (NCERT Textbook Chapter 14) ......................... 283–308

Syllabus Class XII (Code N. 042) (2021-22) Time: 2 Hours

Term–II (Theory)

Max Marks: 35 No. of Periods Marks

Unit–V

Electromagnetic Waves Chapter–8: Electromagnetic Waves

Unit-VI

02 17

Optics Chapter–9: Ray Optics and Optical Instruments

18

Chapter–10: Wave Optics Unit-VII

Dual Nature of Radiation and Matter Chapter–11: Dual Nature of Radiation and Matter

Unit-VIII

07 11

Atoms and Nuclei Chapter–12: Atoms

11

Chapter 13: Nuclei Unit-IX

Electronic Devices Chapter–14: Semiconductor-Electronics: Materials, Devices and Simple Circuits

07

07

Total

45

35

Unit V: Electromagnetic waves

2 Periods

Chapter–8: Electromagnetic Waves Electromagnetic waves, their characteristics, their Transverse nature (qualitative ideas only). Electromagnetic spectrum (radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays) including elementary facts about their uses. Unit VI: Optics

18 Periods

Chapter–9: Ray Optics and Optical Instruments Ray Optics: Refraction of light, total internal reflection and its applications, optical fibers, refraction at spherical surfaces, lenses, thin lens formula, lensmaker’s formula, magnification, power of a lens, combination of thin lenses in contact, refraction of light through a prism. Optical instruments: Microscopes and astronomical telescopes (reflecting and refracting) and their magnifying powers.

Chapter–10: Wave Optics Wave optics: Wave front and Huygen’s principle, reflection and refraction of plane wave at a plane surface using wave fronts. Proof of laws of reflection and refraction using Huygen’s principle. Interference, Young’s double slit experiment and expression for fringe width, coherent sources and sustained interference of light, diffraction due to a single slit, width of central maximum. Unit VII: Dual Nature of Radiation and Matter

7 Periods

Chapter–11: Dual Nature of Radiation and Matter Dual nature of radiation, Photoelectric effect, Hertz and Lenard’s observations; Einstein’s photoelectric equation-particle nature of light. Experimental study of photoelectric effect Matter waves-wave nature of particles, de-Broglie relation Unit VIII: Atoms and Nuclei

11 Periods

Chapter–12: Atoms Alpha-particle scattering experiment; Rutherford’s model of atom; Bohr model, energy levels, hydrogen spectrum. Chapter–13: Nuclei Composition and size of nucleus Nuclear force Mass-energy relation, mass defect, nuclear fission, nuclear fusion. Unit IX: Electronic Devices

7 Periods

Chapter–14: Semiconductor Electronics Materials, Devices and Simple Circuits Energy bands in conductors, semiconductors and insulators (qualitative ideas only) Semiconductor diode-I-V characteristics in forward and reverse bias, diode as a rectifier; Special purpose p-n junction diodes: LED, photodiode, solar cell.

Unit V: Electromagnetic Waves

Chapter 1: Electromagnetic Waves (NCERT Textbook Chapter 8)

SUMMARY OF THE CHAPTER  Maxwell introduced the concept of displacement current in 1865. The phenomenon of electricity and magnetism were brought together into a coherent and unified theory.  Maxwell proved that not only a current flowing through a conductor but a time varying electric field in vacuum or free space also produces a magnetic field. Thus, a changing electric field gives rise to current that flows through a region so long as the electric field is changing there.  The displacement current is the current that comes into play in a region where the electric field and hence the electric flux is changing with time.  Maxwell defined displacement current as ID =  0

d E dt

where E is electric flux and 0 is electric permittivity of free space/vacuum.  The continuity of current was established using the equation,

ò c

  d   B . dl = 0 (I + ID) = 0  I  0 E  dt  

 Based on Maxwell’s equation, following observations are made: (a) The electromagnetic waves can propagate through the space with the speed of light, i.e., 3 × 108 ms–1. (b) the source of electromagnetic (EM) waves is an accelerated charge. (c) The EM waves have a transverse nature. (d) Light is an EM wave.  According to Maxwell, ‘the electromagnetic waves are those waves which have sinusoidal variation of electric and magnetic field vectors at right angles to each other as well as at right angles to the direction of wave propagation.’ Both these fields vary with time and space and have the same frequency. In the figure, the 



electric field vector ( E ) and magnetic field vector ( B ) are vibrating along Y and Z-directions and propagation of EM wave is shown along X-direction. 1

2

Physics—XII

 Maxwell found that the EM wave should travel in free space (or vacuum) with a speed given by, c=

E

B

E

O

1

X

...(i)

0 0

Z

where 0 and 0 are permeability and permittivity of the free space respectively. We know,

Envelope having electric intensity vector

Y

B

B E Envelope having magnetic induction vector

0 = 4 × 10–7 WbA–1 m–1, 0 = 8.85 × 10–12 C2 N–1 m–2

Putting these values in (i), we have c = 3.0 × 108 ms–1  The important examples of EM waves are: radio waves, microwaves, infrared rays, light waves, ultraviolet rays, X-rays and -rays.  Hertz demonstrated experimentally the production of EM wave in 1888. The wavelength of EM waves produced with the help of Hertz’s experiment was about 6 m. The SI unit of frequency was named hertz (Hz) after his name.  Following important facts must be learnt about EM waves: (i) They do not need any medium for their propagation. (ii) They are produced by acceleration or oscillating charge. (iii) (iv) (v) (vi) (vii) (viii)





The sinusoidal variations in E and B occur simultaneously. They are transverse in nature. Their velocity in any dielectric medium is less than c, i.e., 3 × 108 ms–1. Their energy is equally divided between electric and magnetic field vectors. They are not deflected by electric and magnetic fields. The electric field vector, called light vector, is responsible for the optical effects of an EM waves. 



(ix) The cross product ( E  B ) gives the direction of EM wave. (x) They have linear momentum and energy. Therefore, they exert radiation pressure on the object on which they are incident.  The intensity of EM waves may be defined as the energy crossing per second per unit area normal to the direction of propagation of EM wave.  When some portion of EM wave of energy U is propagating with speed c, then the U c  The EM spectrum may be defined as the orderly distribution of EM radiation according to their wavelength or frequency. It has much wider range with wavelength variation ~ 10–14 m to 6 × 106 m.  The visible spectrum is only a part of the complete EM spectrum that is visible to human eye.

linear momentum (p) of EM wave is given by p =

3

Electromagnetic Waves

 Following table gives the salient features of the different rays, their wavelength, frequency and source, which form the EM spectrum.  Infrared rays are used: (a) in solar water heaters and cookers (b) to treat muscular strain (c) to provide electrical energy in solar powered satellite (d) in green houses (e) in weather forecasting through infrared photography (f) to ensure dehydration of fruits and vegetables.

Important Results 1. Velocity of EM waves through vacuum is given by c= where

0 = 8.85 ×

10–12

1  0 0 2 C N–1m–2

and

0 = 4 × 10–7 TA–1 m

Table: Electromagnetic waves S.No.

Name

1.

Gamma rays

6 × 10–14 to 1 × 10–11

5 × 1022 to 3 × 1019

Nuclear origin

2.

X-rays

1 × 10–11 to 3 × 10–8

3 × 1019 to 1 × 1016

Sudden deceleration of the high energy electron

3.

Ultra-violet

6 × 10–10 to 4 × 10–7

5 × 1017 to 8 × 1014

Excitation of atom, spark and arc lamp

4.

Visible light

4 × 10–7 to 8 × 10–7

8 × 1014 to 4 × 1014

Excitation of the valency electron

5.

Infrared rays

8 × 10–7 to 3 × 10–5

4 × 1014 to 1 × 1013

Excitation of atoms and molecules

6.

Heat radiations

10–5 to 10–1

3 × 1013 to 3 × 109

Hot bodies

7.

Microwaves

10–3 to 0.3

3 × 1011 to 1 × 109

Oscillating current in specially designed vacuum tube

8.

Ultra high frequency

1 × 10–1 to 1

3 × 109 to 3 × 108

Oscillating circuit

9.

Very high radio frequency

1 to 10

3 × 108 to 3 × 107

Oscillating circuit

Radio frequencies

10 to 104

3 × 107 to 3 × 104

Oscillation circuit

10.

Wavelength Range (m)

Frequency Range (Hz)

Source

4

Physics—XII

2. Maxwell’s equations are given by: (i)

ò





E . dS =

S

(iii)

ò





q 0

E . dl = -

d dt

ò



B . dS = 0

(iv)

ò

æd ö    ÷ B . dl = 0I + 00 ççç ò E . dS ÷÷÷  çç dt ÷ø è S

S

3. Energy of a photon, E = h =



ò



B . dS



(ii)

hc 

U , where U is energy of EM wave. c 4. E0 = B0 c, where E0 is amplitude or maximum value of electric field, and B0 is maximum value (or amplitude) of magnetic field.

Also, momentum of photon, p =

5. uE =

E 1 1 0 E2 = 0E02, where E = 0 2 4 2

6. uB =

1 B2 B2 = 0 , 20 40

where B =

B0 2

7. Total average density = uE + uB = 2uE = 2uB =

B2 1 0 E0 2 = 0 20 2

8. The units taken for E, B, uE and uB are NC–1, T, Jm–3 and Jm–3 respectively.

VERY SHORT ANSWER TYPE QUESTIONS (1 Mark) 1. Light of uniform intensity shines perpendicularly on a totally absorbing surface, fully illuminating the surface. If the area of the surface is decreased, do (a) the radiation pressure and (b) the radiation force on the surface increase, decrease, or stay the same? Ans. (a) same ; (b) decrease. 2. What is the frequency range of speech or music? Ans. 20 Hz to 20,000 Hz. 3. Why ultraviolet rays are used in some medical applications and also in sterilisation processes? Ans. When some micro-organisms absorb ultraviolet radiation, they can be destroyed as a result of the chemical reactions produced by the ionisation and dissociation of molecules. 4. What is the approximate wavelength of X-rays? Ans. Wavelength of X-rays ranges from approximately 0.001 nm to 1 nm (10–12 m to 10–9 m). 5. Which part of the electromagnetic spectrum does the wavelength 10–10 m corresponds to? Ans. X-rays.

5

Electromagnetic Waves

6. Arrange the following electromagnetic radiations in the ascending order of their wavelength. Microwaves, -rays, radiowaves, ultraviolet light. Ans. -rays, ultraviolet light, microwaves, radiowaves. 7. Give one medical use of UV rays. Ans. These are used for sterilising surgical instruments. 8. Give one use of infrared rays. Ans. Infrared ray photographs are used for weather forecasting. 9. The wavelength of electromagnetic radiation is doubled. What will happen to the energy of the photon? Ans. The energy will be halved because frequency will be halved. 10. “In electromagnetic waves, the infrared region lies between the radiowave and microwave regions”. Is this statement correct? Ans. The given statement is not correct. 11. What is the relation between amplitudes of electric and magnetic fields in free space for an electromagnetic wave? Ans. Speed of light, c =

E0 B0

12. Arrange the following radiations in the descending order of wavelength: -rays, infrared rays, red light. Ans. Infrared rays, red light, -rays. 13. Which of the following has the lowest frequency? Microwaves, UV rays and X-rays. Ans. Microwaves. 14. What is the nature of the waves used in Radar? What is their wavelength range? Ans. Microwaves are used in Radar. Their wavelength ranges from 10–3 m to 0.3 m. 15. Which of the following belong to electromagnetic spectrum? -rays, -rays, -rays, cathode rays, X-rays, ultraviolet rays, microwaves, ultrasonic waves, radiowaves, infrared rays. Arrange these in order of increasing frequency. Ans. From the given list, those which belong to the electromagnetic spectrum are given below. These are arranged in increasing order of frequency. Radiowaves, microwaves, infrared rays, ultraviolet rays, X-rays and -rays. 16. What is the principle of production of electromagnetic waves? Ans. Accelerated charges produce electric and magnetic fields which vary both in space and time. 17. Arrange the following radiations in the descending order of wavelengths: -rays, infrared rays, red light, yellow light, radiowaves. Ans. Radiowaves, Infrared rays, Red light, Yellow light, -rays. 18. What is electromagnetic spectrum? Ans. It is an orderly arrangement of radiations according to wavelength.

6

Physics—XII

19. Both radiowaves and gamma rays are transverse in nature, electromagnetic in character and have the same speed in vacuum. In what respects are they different? Ans. S.No.

-rays

Radiowaves

1.

These have atomic origin.

These have nuclear origin.

2.

These have small penetrating power.

These have large penetrating power.

20. Rewrite the following in descending order of wavelength: Infrared rays, radiowaves, -rays, microwaves. Ans. Radiowaves, microwaves, infrared rays and -rays. 21. Write the following radiations in a descending order of frequencies. Red light, X-rays, microwaves, radiowaves. Ans. X-rays, red light, microwaves and radiowaves. 22. What is the appropriate frequency range of the visible part of the electromagnetic spectrum? Ans. 4 × 1014 Hz to 8 × 1014 Hz. 23. What is the approximate wavelength range of visible spectrum? Ans. 3900 Å to 7600 Å. 24. What is the ratio of speeds of infrared rays and ultraviolet rays in vacuum? Ans. 1. 25. Name the scientist who first (i) predicted the existence of electromagnetic waves. (ii) experimentally demonstrated the existence of electromagnetic waves. Ans. (i) J.C. Maxwell (ii) H. Hertz. 26. Microwaves are used in Radar’. Why? Ans. Due to their smaller wavelength, microwaves can be transmitted as beam signals. 27. What is the ratio of velocity of light rays of wavelengths 4000 Å and 8000 Å in vacuum? Ans. Ratio is 1. The velocity of light in vaccum in independent of wavelength. 28. Write an expression for the speed of electromagnetic waves in free space. Ans. c =

29. Ans. 30. Ans.

1 0 0

where 0 is absolute permeability of free space and 0 is absolute permittivity of free space. Which physical quantity, if any, has the same value for waves belonging to the different parts of the electromagnetic spectrum? Speed in vacuum. Name the electromagnetic waves that have frequencies greater than those of ultraviolet light but less than those of gamma rays. X-rays.

Electromagnetic Waves

7

31. What are radio waves? Ans. Radio waves are electromagnetic waves whose frequency ranges from 500 kHz to nearly 1000 MHz. These are produced by accelerated motion of charges in conducting wires. 32. Identify the part of the electromagnetic spectrum to which the following wavelengths belong: (i) 10–3 nm (ii) 10–3 m (iii) 1 m. Ans. (i) X-rays (ii) Microwaves (iii) Infrared rays. 33. Name the electromagnetic radiation to which waves of wavelength in the range of 10–2 m belong. Give one use of this part of EM spectrum. Ans. Micro-wave—Cooking food containing water. 34. Write the following radiations in ascending order in respect of their frequencies: X-rays, microwaves, UV-rays and radio waves. Ans. Radio waves, Microwaves, Ultraviolet rays and X-rays. 35. Name the EM waves used for studying crystal structure of solids. What is its frequency range? Ans. X-ray. Frequency range 1017 to 1020 Hz. 36. Name the part of electromagnetic spectrum which is suitable for (i) radar systems used in aircraft navigation (ii) treatment of cancer tumours. Ans. (i) Microwaves. (ii) Gamma rays. 37. Name the characteristic of electromagnetic wave that (i) increases (ii) remains constant in the electromagnetic spectrum as one moves from radiowave region towards ultraviolet region. Ans. (i) Frequency (ii) Velocity. 38. From the following, identify the electromagnetic waves having the (i) Maximum (ii) Minimum frequency. (i) Radio waves (ii) Gamma-rays (iii) Visible light (iv) Microwaves (v) Ultraviolet rays, and (vi) Infrared rays. [CBSE Sample Paper II 2010] Ans. (i) Maximum–-rays (ii) Minimum–Radiowaves 39. State the condition under which a microwave oven heats up a food item containing water molecules most efficiently. [CBSE Sample Paper II 2010] Ans. The frequency of the microwaves should match the resonant frequency of the water molecules in the food. 40. Which of the following has the shortest wavelength: Microwaves, Ultraviolet rays, X-rays Ans. X-rays. [Note that the wavelength of X-rays is nearly 1 Å.] 41. Which part of electromagnetic spectrum is used in radar systems? Ans. Microwaves. 42. Arrange the following in descending order of wavelength: X-rays, Radio waves, Blue light, Infrared light. [CBSE 2010] Ans. Radio waves, Infrared light, Blue light, X-rays.

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