SHASTRARTH ACADEMY

bhatapara

PRACTICE PAPER
Class 12 - Physics
Time Allowed: 2 hours Maximum Marks: 108

1. State two properties of electromagnetic waves. How can we show that em waves carry momentum? [1]
2. Write any two characteristics of an electromagnetic wave. Why are microwaves used in radar systems? [1]
3. What do you understand by the statement, Electromagnetic waves transport momentum? [1]
4. A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band? [1]

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5. The wavelength of an electromagnetic wave is 10-10 m. Identify the wave and mention its one use. [1]

6. The magnetic field of an electromagnetic wave oscillates parallel to y-axis and is given by By = B0y sin(kz - ωt). [2]

a. In what direction does the wave travel?

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b. Parallel to which axis does the associated electric field oscillate?
7. An e.m. wave is travelling in a medium with a velocity v = ^
vi . The electric field oscillations, of this e.m. wave, [2]
are along the y-axis.
a
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a. Identify the direction in which the magnetic field oscillations are taking place, of the e.m. wave.
b. How are the magnitudes of the electric field and magnetic fields in the electromagnetic wave related to each
other?
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8. Explain briefly how electromagnetic waves are produced by an oscillating charge? How is the frequency of the [2]
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electromagnetic waves produced related to that of the oscillating charge?

9. Two students A and B prepare the following table about the electromagnetic waves. Rewrite this table in its [2]
corrected form :
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Direction of Peak Value of

Student
Electric field Magnetic field Propagation Electric field Magnetic field
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A Along X-axis Along X-axis Along Y-axis E E = cB

B Along Y-axis Along Z-axis Along X-axis E = cB B

10. a. Identify the part of the electromagnetic spectrum used in (i) radar and (ii) eye surgery. Write their frequency [2]
range.
b. Prove that the average energy density of the oscillating electric field is equal to that of the oscillating
magnetic field.

11. a. Long distance radio broadcasts use short-wave bands. Why? [2]
b. The experimental demonstration of electromagnetic waves is possible only in the low frequency region (the
radio wave region). Explain.
12. How does a charge, q oscillating at a certain frequency produce electromagnetic waves? Sketch a schematic [2]
diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z- direction.
13. i. If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what [2]

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 it is now? Explain.
ii. An EM wave exerts pressure on the surface on which it is incident. Justify.
14. How do you convince yourself that electromagnetic waves carry energy and momentum? [2]
15. 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.

16. a. Depict a plane electromagnetic wave propagating along the x-axis. Write the expressions for its oscillating [3]
electric and magnetic fields.
b. Write three characteristics of electromagnetic waves.
17. Write one property and one use each of infrared rays, ultraviolet rays and radiowaves. [3]
18. Electromagnetic waves of wavelengths λ 1, λ2 and λ are used in radar systems, in water purifiers and in remote
3 [3]
switches of TV, respectively.

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a. Identify the electromagnetic waves, and
b. Write one source of each of them.
19. Answer the following questions: [3]

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i. Name the EM waves which are used for the treatment of certain forms of cancer. Write their frequency range.
ii. The thin ozone layer on top of the stratosphere is crucial for human survival. Why?
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iii. Why is the amount of momentum transferred by the EM waves incident on the surface so small?
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20. Show that electromagnetic waves are transverse in nature. [3]
21. Identify the following electromagnetic radiations as per the frequencies given below. Write one application of [3]
each.
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a. 1020 Hz
b. 109 Hz
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c. 1011 Hz.
22. State clearly how a microwave oven works to heat up a food item containing water molecules. Why are [3]
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microwaves found useful for the raw systems in aircraft navigation?

23. Answer the following questions : [3]
a. Show, by giving a simple example, how e.m. waves carry energy and momentum.
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b. How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of
microwaves to match the resonant frequency of water molecules?
24. How can we express mathematically a plane electromagnetic wave propagating along X-axis? Also, represent it [3]
graphically.
25. i. Write the expression for the speed of light in a material medium of relative permittivity ∈ and relative
r
[3]
magnetic permeability μ .r

ii. Write the wavelength range and name of the electromagnetic waves which are used in
a. radar systems for aircraft navigation, and
b. Earth satellites to observe the growth of the crops.
26. Answer the following questions: [5]
i. Show, by giving a simple example, how EM waves carry energy and momentum.

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 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.
27. Assertion (A): In electromagnetic waves, electric field and magnetic field lines are perpendicular to each other. [1]
Reason (R): Electric fields and magnetic fields are self-sustaining.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

28. Assertion (A): The basic difference between various types of electromagnetic waves lies in their wavelength or [1]
frequencies.
Reason (R): Electromagnetic waves travel through vacuum with the same speed.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the

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explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

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29. Assertion (A): Sound waves are not electromagnetic waves. [1]
Reason (R): Sound waves require a material medium for propagation.

a) Both A and R are true and R is the correcta b) Both A and R are true but R is not the
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explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

30. Assertion (A): The speed of electromagnetic waves in free space is maximum for gamma rays and minimum for [1]
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radio waves.
Reason (R): For waves with different wavelengths this just means that the speed will be equal to c.
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a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.
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c) A is true but R is false. d) A is false but R is true.

31. Assertion (A): The velocity of electromagnetic waves depends on electric and magnetic properties of the [1]
medium.
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Reason (R): Velocity of electromagnetic waves in free space is constant.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

32. Name the electromagnetic radiation which can be produced by a klystron or a magnetron valve. [1]
33. Long distance radio broadcasts use short wave bands. Why? [1]
34. What is the range of wavelength of television broadcast? [1]
35. From the following identify the electromagnetic waves having the (i) Maximum, (ii) Minimum frequency. [1]
i. Radiowaves
ii. Gamma-rays
iii. Visible light
iv. Microwaves

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 v. Ultraviolet rays, and
vi. Infrared rays.
36. How are X-rays produced? [1]
37. Name the electromagnetic waves with their frequency range, produced in [2]
a. some radioactive decay
b. sparks during electric welding
c. TV remote
38. To which part of the electromagnetic spectrum does a wave of frequency [2]

i. 5 × 1019 Hz
ii. 3 × 1013 Hz

iii. 5 × 1011 Hz belong?

39. Find the wavelength of electromagnetic waves of frequency 5 × 10 Hz in free space. Give its two applications. [2]
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40. Electromagnetic radiations with wavelength : [2]

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i. λ are used to kill germs in water purifiers.
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ii. λ are used in TV communication systems.

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2

iii. λ play an important role in maintaining the earth's warmth.

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Name the part of electromagnetic spectrum to which these radiations belong. Arrange these wavelengths in
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decreasing order of their magnitude.
41. Which of the following belong to the electromagnetic spectrum : α-rays, β-rays, cathode rays, X-rays, [2]
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ultraviolet rays, microwaves, ultrasonic waves, radiowaves, infrared rays? Arrange them in the order of
increasing frequency.
42. a. Which of the following, if any, can act as a source of electromagnetic waves? [3]
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i. A charge moving with a constant velocity.

ii. A charge moving in a circular orbit.
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iii. A charge at rest.

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Give reason.

b. Identify the part of the electromagnetic spectrum, to which waves of frequency (i) 1020 Hz, (ii) 109 Hz
belong.
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43. Name the parts of the electromagnetic spectrum which is [3]

i. suitable for RADAR systems in aircraft navigations.
ii. used to treat muscular strain.
iii. used as a diagnostic tool in medicine.
Write in brief, how these waves can be produced.
44. Answer the following questions: [3]
i. Name the EM waves which are suitable for RADAR systems used in aircraft navigation. Write the range of
frequency of these waves.
ii. If the earth did not have atmosphere, would its average surface temperature be higher or lower than what it is
now? Explain.
iii. An EM wave exerts pressure on the surface on which it is incident. Justify.
45. Assertion (A): When cooking in microwave ovens, metal containers are used. [1]

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 Reason (R): The energy of microwaves can be easily transferred to the food through metal.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A correct explanation of A.

c) A is true but R is false d) A is false but R is true.

46. Assertion (A): Skywave signals are used for long-distance radio communication. These signals are in general, [1]
less stable than ground wave signals.
Reason (R): The state of the ionosphere varies from hour to hour, day to day, and season to season.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false d) A is false but R is true.

47. Assertion (A): The microwaves are better carriers of signals than radio waves. [1]

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Reason (R): The electromagnetic waves does not required any material medium for propagation.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

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c) A is true but R is false. d) A is false but R is true.
48. Assertion (A): X-rays travel at the speed of light. [1]
Reason (R): X-rays are electromagnetic rays. a
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a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A correct explanation of A.
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c) A is true but R is false d) A is false but R is true.

49. Read the text carefully and answer the questions: [5]
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In an electromagnetic wave both the electric and magnetic fields are perpendicular to the direction of
propagation, that is why electromagnetic waves are transverse in nature. Electromagnetic waves carry energy as
they travel through space and this energy is shared equally by the electric and magnetic fields. Energy density of
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an electromagnetic waves is the energy in unit volume of the space through which the wave travels.
(a) The electromagnetic waves propagated perpendicular to both E⃗ and B⃗ . The electromagnetic waves travel
in the direction of
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a) E⃗ ⋅ B⃗ b) ⃗
B ⋅E
⃗

c) E⃗ × B⃗ d) ⃗
B × E
⃗

(b) Fundamental particle in an electromagnetic wave is

a) photon b) phonon

c) electron d) proton
(c) Electromagnetic waves are transverse in nature is evident by

a) diffraction b) interference

c) polarisation d) reflection
(d) For a wave propagating in a medium, Name the property that is independent of the others.

a) frequency b) wavelength

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 c) velocity d) all these depend on each other
(e) The electric and magnetic fields of an electromagnetic waves are

a) in opposite phase and parallel to each b) in phase and parallel to each other.
other

c) in phase and perpendicular to each other d) in opposite phase and perpendicular to

each other
50. Read the text carefully and answer the questions: [5]
Microwave oven: The spectrum of electromagnetic radiation contains a part known as microwaves. These
waves have frequency and energy smaller than visible light and wavelength larger than it. What is the principle
of a microwave oven and how does it work? Our objective is to cook food or warm it up. All food items such as
fruit, vegetables, meat, cereals, etc., contain water as a constituent. Now, what does it mean when we say that a
certain object has become warmer? When the temperature of a body rises, the energy of the random motion of

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atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. The frequency

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of rotation of water molecules is about 2.45 gigahertz (GHz). If water receives microwaves of this frequency, its
molecules absorb this radiation, which is equivalent to heating up water. These molecules share this energy with

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neighbouring food molecules, heating up the food. One should use porcelain vessels and non-metal containers in
a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also
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melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and
rotate with much smaller frequencies, and thus cannot absorb microwaves. Hence, they do not get eaten up.
Thus, the basic principle of a microwave oven is to generate microwave radiation of appropriate frequency in the
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working space of the oven where we keep food. This way energy is not wasted in heating up the vessel. In the
conventional heating method, the vessel on the burner gets heated first and then the food inside gets heated
because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly
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delivered to water molecules which is shared by the entire food.

(a) As compared to visible light microwave has frequency and energy
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a) Frequency is less but energy is more b) less than visible light

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c) more than visible light d) equal to visible light

(b) When the temperature of a body rises
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a) the energy of the random motion of b) the energy of the random motion of
atoms and molecules decreases. atoms and molecules remains same.

c) the energy of the random motion of d) the random motion of atoms and
atoms and molecules increases molecules becomes streamlined.
(c) The frequency of rotation of water molecules is about

a) 2.45 THz b) 2.45 kHz

c) 2.45 MHz d) 2.45 GHz

(d) Why should one use porcelain vessels and non-metal containers in a microwave oven?

a) Because it will prevent the food items to b) Because it will get too much hot
become hot

c) Because of the danger of getting a d) Because it may crack due to high

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 shock from accumulated electric frequency
charges
(e) In the microwave oven

a) Energy is directly delivered to the food b) The vessel gets heated first and then the
grains. water molecules collect heat from the
body of the vessel

c) Energy is directly delivered to water d) The vessel gets heated first, and then
molecules which is shared by the entire the food grains inside
food

Question No. 51 to 55 are based on the given text. Read the text carefully and answer the questions: [5]
Electrons oscillating in a circuit give rise to radiowaves. A transmitting antenna radiates most effectively the

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radiowaves of wavelength equal to the size of the antenna. The infrared waves incident on a substance set into
oscillation all its electrons, atoms and molecules. This increases the internal energy and hence the temperature of the
substance.

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51. If vg, vX and vm are the speeds of gamma rays, X-rays and microwaves respectively in vacuum, then

a) v b)

c) v
g > vX > vm
a d)
vg < vX < vm
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g > vX > vm vg = vX = vm

52. Which of the following will deflect in electric field?

a) ultraviolet rays b) γ -rays

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c) X-rays d) cathode rays

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53. γ -rays are detected by

a) point contact diodes b) ionization chamber

c) thermopiles d) photocells
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54. The frequency of electromagnetic wave, which best suited to observe a particle of radius 3 × 10-4 cm is the order of

a) 1014 Hz b) 1012 Hz
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c) 1013 Hz d) 1015 Hz

55. We consider the radiation emitted by the human body. Which one of the following statements is true?
i. The radiation emitted is in the infrared region.
ii. The radiation is emitted only during the day.
iii. The radiation is emitted during the summers and absorbed during the winters.
iv. The radiation emitted lies in the ultraviolet region and hence it is not visible.

a) Option (iv) b) Option (ii)

c) Option (iii) d) Option (i)

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