Question Set A

CBSE 12th Physics Electromagnetic Waves Important Questions And Answers Study Material - QB365

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Questions:

Multiple Choice Question

32 x 1 = 32

An electromagnetic wave is produced by

(a)

a stationary charge

(b)

a chargeless particle

(c)

an accelerating charge

(d)

a charge moving with constant velocity

The electromagnetic radiations used to kill germs in water purifiers are called

(a)

infrared waves

(b)

X-rays

(c)

gamma rays

(d)

ultraviolet rays

The phase difference between electric field E and magnetic field B in an electromagnetic wave propagating along Z-axis is

(a)

zero

(b)

\(\pi\)

(c)

\(\frac{\pi}{2}\)

(d)

\(\frac{\pi}{4}\)

Radio waves are produced by

(a)

accelerated motion of electrons in oscillating circuits.

(b)

sudden deceleration of fast moving electrons by metal target

(c)

heating of certain substances at particular temperature

(d)

de excitation of electron from higher energy orbital to lower one.

Speed of electromagnetic wave related to electric field and magnetic field vector in vacuum

(a)

\( c=\frac{E_{0}}{B_{0}} \)

(b)

\( c=\frac{B_{0}}{E_{0}} \)

(c)

\(c=E_{0} B_{\mathrm{o}} \)

(d)

\(c=\frac{1}{\sqrt{E_{0} B_{0}}} \)

The oscillating magnetic field in a plane electromagnetic wave is given as \(B_{y}=\left(8 \times 10^{-6}\right) \sin \left[2 \times 10^{11} t+300 \pi x\right] \mathrm{T}\) wavelength of the cm wave is

(a)

0.80 cm

(b)

1 x 10³ m

(c)

2 x 10^-2 cm

(d)

0.67 cm

Waves in decreasing order of their wavelength are

(a)

X-rays, infrared rays, visible rays, radio waves

(b)

radio waves, visible rays, infrared rays, X-rays

(c)

radio waves, infrared rays, visible rays, X-rays.

(d)

radio waves, ultraviolet rays, visible rays, X-rays

If ε₀ and μ₀ are the electric permittivity and magnetic permeability of free space and ε and μ are the corresponding quantities in the medium, the index of refraction of the medium in terms of above parameter is

(a)

\(\frac{\varepsilon \mu}{\varepsilon_{0} \mu_{0}}\)

(b)

\(\left(\frac{\varepsilon \mu}{\varepsilon_{0} \mu_{0}}\right)^{1 / 2}\)

(c)

\(\left(\frac{\varepsilon_{0} \mu_{0}}{\varepsilon \mu}\right)\)

(d)

\(\left(\frac{\varepsilon_{0} \mu_{0}}{\varepsilon \mu}\right)^{1 / 2}\)

Which of the following maxwell equation have a source of \(\overset { \rightarrow }{ E } \) and \(\overset { \rightarrow }{ B } \) ?

(a)

\(\oint { \overset { \rightarrow }{ E } .\overset { \rightarrow }{ ds } } =\frac { q }{ { \epsilon }_{ 0 } } \)

(b)

\(\oint { \overset { \rightarrow }{ B } .\overset { \rightarrow }{ dl } } ={ \mu }_{ 0 }l+{ \mu }_{ 0 }{ \epsilon }_{ 0 }\frac { d }{ dt } \oint { \overset { \rightarrow }{ E } .\overset { \rightarrow }{ ds } } \)

(c)

\(\oint { \overset { \rightarrow }{ E } .\overset { \rightarrow }{ dl } } =-\frac { d }{ dt } \oint { \overset { \rightarrow }{ B } .\overset { \rightarrow }{ ds } } \)

(d)

\(\oint { \overset { \rightarrow }{ B } .\overset { \rightarrow }{ ds } } =0\)

10.

If \({ u }_{ E },{ u }_{ m }\) are the energy density of electromagnetic wave due to electric and magnetic field vectors, \({ E }_{ rms },{ B }_{ rms }\) are the rms value of electric and magnetic field vectors in an electromagnetic wave. then the total energy density of a sinusoidal electromagnetic wave is

(a)

\({ u }_{ E }\)

(b)

\({ u }_{ E }{ +u }_{ m }\)

(c)

\(\frac { 1 }{ 2 } { \epsilon }_{ 0 }{ E }_{ rms }^{ 2 }+\frac { { E }_{ rms }^{ 2 } }{ { 2\mu }_{ 0 } } \)

(d)

\(\frac { 1 }{ 2 } { \epsilon }_{ 0 }{ E }_{ 0 }^{ 2 }+\frac { 1 }{ 2 } \frac { { E }_{ 0 }^{ 2 } }{ { \mu }_{ 0 } } \)

11.

Displacement current goes through the gap between the plates of a capacitor when the charge of the capacitor

(a)

Is zero

(b)

decrease

(c)

does not change

(d)

increase

12.

The unity of the electric and magnetic waves was found by maxwell from
(i) Gauss's law in electrostatics
(ii) Gauss's law in magnetism
(iii) Faraday's law of electromagnetic induction
(iv) Ampere's law with displacement current

(a)

(i) and (iii) only

(b)

(iii) and (iv) only

(c)

all

(d)

velocity of light also

13.

An electromagnetic wave going through vacuum is denoted by \(E={ E }_{ 0 } \ sin \ (kz-\omega t)\). Which of the following is/are independent of wavelength?

(a)

(b)

\(\omega \)

(c)

\(k/\omega \)

(d)

\(k\omega \)

14.

The condition of which a microwave oven heats up a food item containing water molecules most efficiently is:

(a)

Infrared waves produce heating in a microwave oven

(b)

The frequency of microwave must match the resonant frequency of the water molecules

(c)

The frequency of microwave has no relation with natural frequency of water molecules

(d)

microwaves are heat waves, so they always produce heating

15.

The magnetic field in a plane electromagnetic wave is given by \({ B }_{ Y }=2\times { 10 }^{ -7 }sin \ (0.2\times { 10 }^{ 3 }x+1.5\times { 10 }^{ 11 }t)\) . This electromagnetic wave is

(a)

A visible lightMi

(b)

An infrared

(c)

Microwaves

(d)

Radiowaves

16.

Consider the following types of electromagnetic radiation: radio waves, infrared, visible light. which of the following statements are correct?
(i) only radio waves can be used to transmit audio information
(ii) only infrared radiation is emitted by the very hot object.
(iii) only visible light can be detected by humans.

(a)

only (i) is correct

(b)

only (ii) is correct

(c)

only (iii) is correct

(d)

None of the above is correct

17.

The correct option if in vacuum the speed of gamma rays, x-rays and microwaves are \({ v }_{ g }, \ { v }_{ x }and \ { v }_{ m }\)

(a)

\({ v }_{ g }<{ v }_{ x }<{ v }_{ m }\)

(b)

\({ v }_{ g }<{ v }_{ x }>{ v }_{ m }\)

(c)

\({ v }_{ g }>{ v }_{ x }>{ v }_{ m }\)

(d)

\({ v }_{ g }={ v }_{ x }={ v }_{ m }\)

18.

Which of the following statements about electromagnetic waves is/are correct
(1) X-rays in vacuum travel faster than light waves in vacuum
(2) The energy of x-rays photon is greater than that of a light photon
(3) light can be polarised by x-rays cannot

(a)

1 and 2

(b)

2 and 3

(c)

1,2 and 3

(d)

2 only

19.

During the propagation of electromagnetic waves in a medium

(a)

electric energy density is equal to the magnetic energy density

(b)

both electric and magnetic energy density are zero

(c)

electric energy density is doubled off the magnetic energy density

(d)

electric energy density is half of the magnetic energy density

20.

A flood light is covered with a filter that transmits red light. The electric field of the emerging beam is represented by a sinusoidal wave.
\({ E }_{ x }=36 \ sin\quad (1.20\times { 10 }^{ 7 }z=3.6\times { 10 }^{ 15 }t) \ V/m\)
the average intensity of the beam is watt/\({ (metre) }^{ 2 }\) will be:

(a)

6.88

(b)

3.44

(c)

1.72

(d)

0.86

21.

If \({ \mu }_{ 0 }\) be the permeability and \({ k }_{ 0 }\) the dielectric constant of a medium. its refractive index in given by

(a)

\(\frac { 1 }{ \sqrt { { \mu }_{ 0 }{ k }_{ 0 } } } \)

(b)

\(\frac { 1 }{ { \mu }_{ 0 }{ k }_{ 0 } } \)

(c)

\(\sqrt { { \mu }_{ 0 }{ k }_{ 0 } } \)

(d)

\({ \mu }_{ 0 }{ k }_{ 0 }\)

22.

A charged particle oscillates about its mean equilibrium position with a frequency of 10⁹ Hz. What is the frequency of electromagnetic waves produced by the oscillator?

(a)

will have frequency of 10⁹ Hz

(b)

will have frequency of 2 x 10⁹ Hz

(c)

will have a wavelength of 0.3 m

(d)

fall in the region of radiowaves

23.

In an electromagnetic wave, the phase difference between electric field and magnetic field is

(a)

(b)

\(\pi/4\)

(c)

\(\pi/2\)

(d)

\(\pi\)

24.

The correct arrangement of course in the descending order of their wavelength is

(a)

yellow, violet, green, orange

(b)

orange, yellow, green, violet

(c)

violet, green, yellow, orange

(d)

yellow, green, orange, violet

25.

A light has a wavelength 6000 A. The energy of light is

(a)

5 eV

(b)

2.07 eV

(c)

1.07 eV

(d)

0.207 eV

26.

One cannot see through fog, because

(a)

fog absorbs the light

(b)

light suffers total reflection at droplets

(c)

refractive index of the fog is infinity

(d)

light is scattered by droplets

27.

Maxwell's equations related to study of electromagnetic waves describe the fundamental laws of

(a)

electricity only

(b)

magnetism only

(c)

mechanics only

(d)

both (a) and (b)

28.

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of 2.5 x 10¹⁰ Hz and amplitude 480 V/m.The amplitude of oscillating magnetic field will be

(a)

1.52 x 10^-8 Wb/m²

(b)

1.52 x 10^-7 Wb/m²

(c)

1.6 x 10^-6 Wb/m²

(d)

1.6 x 10^-7 Wb/m²

29.

The source of electromagnetic waves can be a charge

(a)

moving with a constant velocity

(b)

moving in a circular orbit

(c)

at rest

(d)

falling in an electric field.

30.

If \(\overset { \rightarrow }{ E } \) and \(\overset { \rightarrow }{ B } \) represent electric and magnetic field vectors of the electromagnetic wave the direction of propagation of electromagnetic wave is along

(a)

\(\overset { \rightarrow }{ E } \)

(b)

\(\overset { \rightarrow }{ B } \)

(c)

\(\overset { \rightarrow }{ B } \times \overset { \rightarrow }{ E } \)

(d)

\(\overset { \rightarrow }{ E } \times \overset { \rightarrow }{ B } \)

31.

Light with an energy flux of \(20 \ W/cm^{ 2 }\) falls on a non-reflecting surface at normal incidence. If the surface has an area of \(30 cm^{ 2 }\), the total momentum delivered (for complete absorption) during \(30\) minutes is:

(a)

\(36\times 10^{ -5 } \ Kg \ m/s\)

(b)

\(36\times 10^{ -4 } \ Kg \ m/s\)

(c)

\(108\times 10^{ 4 } \ Kg \ m/s\)

(d)

\(1.08\times 10^{ 7 } \ Kg \ m/s\)

32.

A linearly polarized electromagnetic wave given as \(E={ E }_{ 0 }\overset { \wedge }{ i } cos \ (kz-wt)\) incident wall at \(z=a\) . Assuming that the material of the wall os optically inactive, the reflected wave will be given as

(a)

\(\overset { \rightarrow }{ { E }_{ r } } ={ E }_{ 0 }\overset { \wedge }{ i } cos(kz-wt)\quad \)

(b)

\(\overset { \rightarrow }{ { E }_{ r } } ={ E }_{ 0 }\overset { \wedge }{ i } cos(kz+wt)\quad \)

(c)

\(\overset { \rightarrow }{ { E }_{ r } } ={ -E }_{ 0 }\overset { \wedge }{ i } cos(kz+wt)\quad \)

(d)

\(\overset { \rightarrow }{ { E }_{ r } } ={ -E }_{ 0 }\overset { \wedge }{ i } sin(kz+wt)\quad \)

Assertion and reason

9 x 1 = 9

33.

Assertion : Gamma rays are electromagnetic waves having the smallest wavelength.
Reason : Gamma rays are having the lowest frequency.
Codes:
A) Both A and R are true and R is the correct explanation of A
B) Both A and R are true but R is NOT the correct explanation of A
C) A is true but R is false
D) A is false and R is true

34.

Assertion : Electromagnetic waves carry energy and momentum.
Reason : Electromagnetic waves can be polarised.
Codes:
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

35.

Assertion (A) : An oscillating charge produces an electric field in space, which produces an oscillating magnetic field, which in turn, is a source of electric field and so on.
Reason (R) : The oscillating electric and magnetic fields thus regenerate each other, so to speak, as the wave propagates through the space.
(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
(b) Both Assertion and Reason are true but Reason in not the correct explanation of Assertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.

36.

Assertion (A) : While applying Ampere's circuital law to given surfaces with same perimeter, the left hand side \(\oint B \cdot d l=\mu_0 i(t)\) has not changed but the right hand side is zero.

Reason (R) : No current passes through the surface.
(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
(b) Both Assertion and Reason are true but Reason in not the correct explanation of Assertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.

37.

Assertion (A) Electrons are ejected from the surface of zinc when it is irradiated by yellow light.
Reason (R) Energy associated with a photon of yellow light is more than the work function of Zinc.
(a) If both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
(b) If both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
(c) If Assertion (A) is true and Reason (R) is false.
(d) If both Assertion (A) and Reason (R) are false.

38.

Assertion (A) : In an electromagnetic wave, magnitude of magnetic field vector is much smaller than the magnitude of electric field vector.
Reason (R) : Energy of electromagnetic waves is shared equally by the electric and magnetic fields
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

39.

Assertion (A) : X-rays in vacuum travel faster than light waves in vacuum.
Reason (R) : The energy of X-rays photon is less than that of light photon.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

40.

Assertion (A) : The microwaves are better carriers of signals than radio waves.
Reason (R) : The electromagnetic waves do not required any material medium for propagation.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

41.

Assertion (A) : The electric field and magnetic field have equal average values in linearly polarised plane em wave.
Reason (R) : The electric energy and magnetic energy have equal average values in linearly polarised plane em wave.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

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Answers Key:

Multiple Choice Question

32 x 1 = 32

(d)
ultraviolet rays

(a)
zero

(a)
accelerated motion of electrons in oscillating circuits.

(a)
\( c=\frac{E_{0}}{B_{0}} \)

(d)
0.67 cm

(b)
\(\left(\frac{\varepsilon \mu}{\varepsilon_{0} \mu_{0}}\right)^{1 / 2}\)

(a)
\(\oint { \overset { \rightarrow }{ E } .\overset { \rightarrow }{ ds } } =\frac { q }{ { \epsilon }_{ 0 } } \)

10.

(b)
\({ u }_{ E }{ +u }_{ m }\)

11.

(b)
decrease

12.

13.

14.

(b)
The frequency of microwave must match the resonant frequency of the water molecules

15.

16.

(d)
None of the above is correct

17.

(d)
\({ v }_{ g }={ v }_{ x }={ v }_{ m }\)

18.

(d)
2 only

19.

(a)
electric energy density is equal to the magnetic energy density

20.

21.

22.

(a)
will have frequency of 10⁹ Hz

23.

(a)
0

24.

(b)
orange, yellow, green, violet

25.

(b)
2.07 eV

26.

(d)
light is scattered by droplets

27.

(d)
both (a) and (b)

28.

29.

(b)
moving in a circular orbit

30.

(a)
\(\overset { \rightarrow }{ E } \)

31.

(b)
\(36\times 10^{ -4 } \ Kg \ m/s\)

32.

(b)
\(\overset { \rightarrow }{ { E }_{ r } } ={ E }_{ 0 }\overset { \wedge }{ i } cos(kz+wt)\quad \)

Assertion and reason

9 x 1 = 9

33.

C) A is true but R is false

34.

(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
Explanation:
Consider a plane perpendicular to the direction of propagation of the electromagnetic wave. If electric charges are present in this plane, they will be set and sustained in motion by the electric and magnetic fields of the electromagnetic wave. The charge thus acquired energy and momentum from the wave. This illustrate the fact that an electromagnetic wave like other waves carries energy and momentum.

35.

(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
An oscillating charge produces an electric field in space, which produces an oscillating magnetic field, which in turn, is a source of electric field.

36.

(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
On applying Ampere's circuital law to such sarfaces with the same perimeter, we find that the left hand side of equation \(\oint B \cdot d l=\mu_0 i(t)\)has not changed but the right hand side is zero and not \(\mu_0\)i. Since, no carrent passes through the surface.

37.

(c) Assertion (A) is true and Reason (R) is false. Electrons are ejected from the surface of zinc metal when it is irradiated by yellow light of suitable frequency fall on them. Work function of zinc metal is 3.74 eV.
Energy associated with a photon of yellow light is 2.14 eV.
So, energy of photon is less than the work function of zinc.

38.

(b): At every instant the ratio of the magnitudes of the electrip field to the magnetic field of an em wave is given by E/B = c.
From this equation, the magnitude of electric vector is much greater than the magnitude of magnetic vector. Also electromagnetic waves, carry energy which is equally shared by electric and magnetic field.

39.

(d): All electromagnetic waves including X-rays travels with same velocity in vacuum. The energy of X-rays is greater than energy of the light because energy is inversely proportional to wavelength (E = hc/\(\lambda\)) and wavelength of X-rays are smaller than light waves.

40.

(b): Microwaves are the electromagnetic waves of wavelength of the order of a few millimetres, which is less than those of T.V. signals. On account of smaller wavelength, the microwaves can be transmitted as beam signals in a particular direction and are much better than radiowaves because microwaves do not spread or bend around the corners of any obstacle coming in their way. Therefore microwaves are better carriers of signals than radiowaves.

41.

(b): In case of a linearly polarised plane electromagnetic wave, the average values of electric field and magnetic field are equal and average values of electric energy and magnetic energy are also equal.