Quantum career institute

Sonipat

ASSIGNMENT
Class 12 - Physics

1. The frequency of e.m. wave which is best suited to observe a particle of radius 3 × 10-6 m, is of the order of [1]

a) 1013 b) 1015

c) 1012 d) 1014

2. Electromagnetic waves are transverse in nature. It is evident by: [1]

a) reflection b) interference

c) polarization d) diffraction

3. These are three wavelengths 107m, 10-10m, 10-7m. Find their respective names. [1]

a) Visible rays, γ -rays, X-rays b) X-Rays, Visible rays, Radiowaves

c) Radiowaves, X-rays, Visible rays d) X-rays, γ -rays, Visible rays

4. If, λ
v, λx and λ represent the wavelengths of visible light, x-rays and microwaves respectively in the free
m [1]
space then,

a) λ m > λv > λx b) λv > λx > λm

c) λ m > λx > λv d) λv > λm > λx

5. Plane electromagnetic wave travels in vacuum along z-direction. If the frequency of the wave is 30 MHz, its [1]
wavelength is

a) 10 m b) 13 m

c) 11 m d) 12 m
6. Using the formula λ T = 0.29 cmK, the characteristic temperature range obtained for λ
m m
−7
= 5 × 10 m is [1]

a) 5800 K b) 7000 K

c) 6500 K d) 7500 K
7. Dimensions of 1/μ 0 ε0 , where symbols have their usual meanings, are: [1]

a) [L2 T-2] b) [L2 T2]

c) [L-1 T] d) [L T-1]

8. What is the energy of an ultraviolet photon with a wavelength of 200 nm? [1]

a) 9.339 ×10 −10

J b) 9.739 ×10 −10
J

c) 9.94 ×10 −19

J d) 9.539 ×10 −10
J
9. Which of the following radiations has the least wavelength? [1]

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 a) α-rays b) β-rays

c) X-rays d) γ -rays
10. The structure of solids is investigated by using [1]

a) infrared radiation b) X-rays

c) cosmic rays d) γ -rays

11. The correct arrangement of colours in the descending order of their wavelength is: [1]

a) Yellow, Violet, Green, Orange b) Yellow, Green, Orange, Violet

c) Orange, Yellow, Green, Violet d) Violet, Green, Yellow, Orange

12. Light from the star Sirius takes 8.61 years to reach the earth. What is the distance from earth to Sirius in [1]
kilometers?

a) 7.85 ×10 13
km b) 8.05 ×10 13
km

c) 7.95 ×10 13
km d) 8.15 ×10 13
km
13. It is necessary to use satellites for long distance TV transmission because [1]

a) Television signals are attenuated by b) Satellites transmit the signals all over the
ionosphere earth

c) Television signals are absorbed by d) Television signals are not properly reflected
ionosphere by the ionosphere
14. The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is Bo = 510 nT. [1]
Amplitude of the electric field part of the wave is

a) 163 N/C b) 158 N/C

c) 153 N/C d) 173 N/C

15. The condition under which a microwave oven heats up a food item containing water molecules most efficiently [1]
is

a) the frequency of the microwaves must b) the frequency of the microwaves has no
match the resonant frequency of the water relation with the natural frequency of water
molecules molecules

c) infrared waves produce heating in a d) microwaves are heat waves, so always

microwave oven produce heating

16. i. How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of [2]
microwaves to match the resonant frequency of water molecules?
ii. Write two important uses of infrared waves.
17. Why does a galvanometer when connected in series with a capacitor shows a momentary deflection when it is [2]
being charged or discharged?
How does this observation lead to modifying the Ampere's circuital law? Hence write the generalized expression
of Ampere's law.
18. A capacitor made of two parallel plates each of plate area A and separation d is being charged by an external ac [2]
source. Show that the displacement current inside the capacitor is the same as the current charging the capacitor.
19. Name the laws associated with the following equations : [3]

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 −
→
q
i. ∮ E⃗ ⋅ dS =
ε0

→ −→
ii. ∮ E⃗ ⋅ dl = −
d ⃗
∮ B ⋅ dS
dt
−→ −→
iii. ⃗
∮ B ⋅ dS = μ0 ε0
d

dt
⃗
∮ E ⋅ dS

20. Which constituent radiation of the electromagnetic spectrum is used [3]

i. in radar,
ii. to photograph internal parts of a human body, and
iii. for taking photographs of the sky during the night and foggy conditions?
Give one reason for your answer in each case.
21. Name the e.m. waves having the wavelength ranges : [3]

a. 10 nm to 10-3 nm

b. 10-7 m to 10-9 m
c. 0.1 m to 1 mm
How are these waves generated? Write their two uses.
22. Answer the following questions: [5]
i. Show, by giving a simple example, how EM waves carry energy and momentum.
ii. How are microwaves produced? Why is it necessary in microwaves ovens to select the frequency of
microwaves to match the resonant frequency of water molecules?
iii. Write two important uses of infrared waves.
23. How are electromagnetic waves produced by oscillating charges? Draw a sketch of linearly polarised [5]
electromagnetic waves propagating in the Z-direction. Indicate the directions of the oscillating electric and
magnetic fields.
24. Given below are some famous numbers associated with electromagnetic radiations in different contexts in [5]
physics. State the part of the electromagnetic spectrum to which each belongs.
a. 21 cm (wavelength emitted by atomic hydrogen in interstellar space).
b. 1057 MHz (frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift).
c. 2.7 K [temperature associated with the isotropic radiation filling all space-thought to be a relic of the big-
bang origin of the universe].
o o

d. 5890 A - 5896 A [double lines of sodium]

e. 14.4 keV [energy of a particular transition in 57Fe nucleus associated with a famous high resolution
spectroscopic method (Mössbauer spectroscopy)].
25. Electromagnetic waves with wavelength [5]
i. λ is used in satellite communication.
1

ii. λ is used to kill germs in water purifies.

2

iii. λ is used to detect leakage of oil in underground pipelines.

3

iv. λ is used to improve visibility in runways during fog and mist conditions.
4

a. Identify and name the part of electromagnetic spectrum to which these radiations belong.
b. Arrange these wavelengths in ascending order of their magnitude.
c. Write one more application of each.
o

26. A source S1 is producing, 1015 photons per second of wavelength 5000 A. Another source S2 is producing 1.02 [1]

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 o

1015 photons per second of wavelength 5100 A. Then,

power of S2
×
power of S
is equal to
1

a) 1.02 b) 1.00

c) 0.98 d) 1.04
27. If K1 and K2 are maximum kinetic energies of photoelectrons emitted when lights of wavelenghts λ and λ 1 2 [1]
respectively incident on a metallic surface and λ1 = 3λ2 , then
K2
a) K2 = 2K1 b) K1 > ( 3
)
K2
c) K1 < ( 3
) d) K1 = 2 K2
o

28. A 5-watt source emits monochromatic light of wavelength 5000 A. When placed 0.5 m away, it liberates [1]

photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the
number of photoelectrons liberated will be reduced by a factor of

a) 8 b) 4

c) 2 d) 16
29. If h is Planck's constant, then the momentum of photon moving with frequency ν is represented by [1]

a) hc

ν
b) hν

c) hc
d) h

cν
λ

30. The velocity of a body of mass m, having de Broglie wavelength λ , is given by the relation [1]

a) v = λh

m
b) v =
h

λm

c) v = λ
d) v =
λm

hm h

31. If the kinetic energy of a particle is increased by 16 times, the percentage change in the de Broglie wavelength of [1]
the particle is

a) 75% b) 60%

c) 25% d) 50%
32. Electrons of mass m with de-Broglie wavelength λ fall on the target in an X-ray tube. The cutoff wavelength [1]
( λ0 ) of the emitted X-ray is

a) λ 0
=
2h

mc
b) λ0 = λ

2 2 3 2

c) λ 0 =
2m c λ

2
d) λ0 =
2mcλ

h h

33. When ultraviolet rays incident on metal plate then photoelectric effect does not occur. It occurs by incidence of [1]

a) X-rays b) Light waves

c) Radio waves d) Infrared rays

34. Photons of energies 1 eV and 2 eV are successively incident on a metallic surface of work function 0.5 eV. The [1]
ratio of kinetic energy of most energetic photoelectrons in the two cases will be

a) 1 : 2 b) 1 : 1

c) 1 : 4 d) 1 : 3
35. If the frequency of incident light on a certain metal is 8.2 ×10 Hz having threshold frequency of 3.3 ×10
14 14
Hz [1]
, then cut off potential is

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 a) 5.1 V b) 4.0 V

c) 2.2 V d) 2.03 V

36. i. How does one explain the emission of electrons from a photosensitive surface with the help of Einstein's [2]
photoelectric equation?
ii. The work function of the following metals is given: Na = 2.75 eV, K = 2.3 eV, Mo = 4.17 eV and Ni = 5.15
eV. Which of these metals will not cause photoelectric emission for radiation of wavelength 3300 A from a
laser source placed 1 m away from these metals? What happens if the laser source is brought nearer and
placed 50 cm away.
37. An electron and a photon each have a wavelength 1nm. Find [2]
a. their momenta
b. the energy of the photon
c. the kinetic energy of electron
38. An electron and a photon each have de-Broglie wavelength of 1.00 nm. [2]
i. Write the ratio of their linear momenta,
ii. Compare the energy of the photon with the kinetic energy of the electron.
39. Consider a metal exposed to the light of wavelength 600 nm. The maximum energy of the electron doubles when [2]
the light of wavelength 400 nm is used. Find the work function in eV.
40. Calculate the de-Broglie wavelength associated with an α-particle accelerated through a potential difference of [2]
200 V. Given mp = 1.67 × 10-27 kg.
41. a. What is photoelectric effect? [3]
b. Using the photon picture of light, show how Einstein's photoelectric equation can be established.
c. What is the photo-electric effect? Write Einstein's photoelectric equation. Explain how it enables us to
understand the
i. linear dependence, of the maximum kinetic energy of the emitted electrons, on the frequency of the
incident radiation.
ii. existence of a threshold frequency for a given photoemitter.
iii. independence of the maximum energy of emitted photo-electrons from the intensity of incident light.
42. The extent of localisation of a particle is determined roughly by its de Broglie wavelength. If an electron is [3]

localized within the nucleus (of size about 10-14 m) of an atom, what is its energy? Compare this energy with the
typical binding energies (of the order of a few MeV) in a nucleus, and hence argue why electrons cannot reside
in a nucleus.
43. a. Monochromatic light is incident on a surface separating two media. The frequency of the light after [3]
refraction remains unaffected but its wavelength changes. Why?

b. The frequency of an electromagnetic radiation is 1.0 × 1011 Hz. Identify the radiation and mention its two
uses.

44. Consider a thin target (10-2m square, 10-3m thickness) of sodium, which produces a photocurrent of 100μ A [3]

when a light of intensity 100W/m2 (λ = 660nm) falls on it. Find the probability that a photoelectron is produced

when a photon strikes a sodium atom. [Take density of Na = 0.97 kg/m3].

45. The energy flux of sunlight reaching the surface of the earth is 1.388 × 103 W/m2. How many photons (nearly) [3]

per square metre are incident on the Earth per second? Assume that the photons in the sunlight have an average

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 wavelength of 550 nm.
46. The following graph shows the variation of photocurrent for a photosensitive metal: [5]

a. Identify the variable X on the horizontal axis.

b. What does point A on the horizontal axis represent?
c. Draw this graph for three different values of frequencies of incident radiation v1, v2 and v3 (v1 > v2 > v3) or
same intensity
d. Draw this graph for three different values of intensities of incident radiation I1, I2 and I3 (I1 > I2 > I3) having

the same frequency.

47. Derive the expression for the de Broglie wavelength of an electron moving under a potential difference of V [5]
volt. Describe Davisson and Germer experiment to establish the wave nature of electrons. Draw a labelled
diagram of the apparatus used.
o

48. Ultraviolet light of wavelength 2271 A from a 100 W mercury source irradiates a photocell made of [5]
molybdenum metal. If the stopping potential is -1.3 V, estimate the work function of the metal. How would the
o

photocell respond to high intensity (∼ 105 W/m2) red light of wavelength 6328 A produced by a He-Ne laser?
49. a. A deutron and an alpha particle are accelerated with the same accelerating potential. Which one of the two [5]
has:
i. greater value of de-Broglie wavelength, associated with it, and
ii. less kinetic energy? Explain.
b. A proton and a deuteron are accelerated through the same accelerating potential. Which one of the two has:
i. greater value of de-Broglie wavelength associated with it, and
ii. less momentum? Give reasons to justify your answer.
o

50. When a surface is irradiated with light of λ = 4950A , a photocurrent appears which vanishes if a retarding [5]

potential greater than 0.6 V is applied across the photo tube. When a different source of light is used, it is found
that the critical retarding potential is changed to 1.1 V. What is the work function of the surface and the
wavelength of the second source? If the photoelectrons (after emission from the source ) are subjected to a
magnetic field of 10 tesla, what changes will be observed in the above two retarding potentials?
51. The difference in variation of resistance with temperature in a metal and semiconductor is due to [1]

a) type of bonding b) variation of scattering with temperature

c) variation in number of charge carries with d) crystal structure

temperature
52. In a middle of the depletion layer of a reverse biased p-n junction, the [1]

a) electric field is maximum b) potential is zero

c) potential is maximum d) electric field is zero

53. In an n-type semiconductor, the donor energy level lies [1]

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 a) in the conduction band b) just below the conduction band

c) just above the valance band d) at the center of the energy gap
54. In a p-type semiconductor, the majority carriers of current are [1]

a) electrons b) neutrons

c) holes d) protons
55. Which of the following, when added as an impurity, into the silicon, produces n-type semiconductor? [1]

a) phosphorous b) aluminium

c) magnesium d) both aluminium and magnesium

56. In a reverse-biased p-n junction, when the applied bias voltage is equal to the breakdown voltage, then [1]

a) current remains constant while voltage b) voltage remains constant while current
increases sharply increases sharply

c) current and voltage decrease d) current and voltage increase

57. A donor impurity results in [1]

a) increase of resistance of the semiconductor b) production of n-semiconductor

c) energy bands just above the filled valency d) production of p-semiconductor

58. The conductivity of p-type semiconductor is due to [1]

a) positive ions b) negative ions

c) electrons d) holes
59. In the middle of the depletion layer of reverse biased p-n junction, the: [1]

a) potential is maximum b) potential is zero

c) electric field is zero d) electric field is maximum

60. The depletion layer in the p-n junction is caused due to [1]

a) drift of electrons b) migration of impurity ions

c) diffusion of carrier ions d) drift of holes

61. Write the characteristics of a p-n junction which make it suitable for rectification. [2]
62. What happens when a forward bias is applied to the p-n junction? [2]
63. In the following diagrams indicate which of the diodes are forward biased and which are reverse biased. [2]

i.

ii.

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 iii.

iv.

64. Name the type of bias that results in very high resistance of a p-n junction diode. In the given circuit, a voltmeter [2]
V is connected across bulb B. What changes would occur in bulb B and voltmeter V, if the resistor R is
increased in value? Give reason for your answer.
65. What are donor and acceptor energy levels? [2]
66. With the help of a suitable diagram, describe briefly the two important processes involved in the formation of a [3]
p-n junction. Define the terms depletion region and potential barrier.
67. Define the terms potential barrier and depletion region for a p-n junction diode. State how the thickness of the [3]
depletion region will change when p-n junction diode is
i. forward biased and
ii. reverse biased
68. Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy [3]
band diagrams.
69. Using the concept of electron and hole current, derive an expression for the electrical conductivity of a [3]
semiconductor.
70. If each diode in the given Figure has a forward bias resistance of 125Ω and infinite resistance in reverse bias, [3]
what will be the values of the current I1, I2, I3 and I4?

71. i. Explain with the help of a suitable diagram, the two processes which occur during the formations of a p-n [5]
junction diode. Hence define the terms
a. depletion region and
b. potential barrier.
ii. Draw a circuit diagram of a p-n junction diode under forward bias and explain its working.

72. a. Distinguish between metals, insulators and semiconductors on the basis of their energy bands. [5]
b. Why are photodiodes used preferably in reverse bias condition? A photodiode is fabricated from a
semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 6000 nm? Justify.
73. State the principle of working of p-n diode as a rectifier. Explain, with help of a circuit diagram, the use of p-n [5]
diode as a full wave rectifier.
74. Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain, AV, of the [5]

amplifier is given by

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Lokesh Sharma
 β RL
ac
AV = −
ri

where β ac
is the current gain, RL is the load resistance and ri is the input resistance of the transistor. What is the
significance of the negative sign in the expression for the voltage gain?
75. In a p-n junction diode, the current I can be expressed as I = I0 exp(
eV
− 1) [5]
2kB T

where I0 is called the reverse saturation current, V is the voltage across the diode and is positive for forwarding
bias and negative for reverse bias, and I is the current through the diode, kB is the Boltzmann constant (8.6 ×
10–5 eV/K) and T is the absolute temperature. If for a given diode I0 = 5 × 10–12 A and T = 300 K, then

a. What will be the forward current at a forward voltage of 0.6 V?

b. What will be the increase in the current if the voltage across the diode is increased to 0.7 V?
c. What is the dynamic resistance?
d. What will be the current if reverse bias voltage changes from 1 V to 2 V?

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