Electromagnetic Waves — DPP 3

20 NEET questions from Electromagnetic Waves, with answers and explanations. Attempt it as a timed test to get scoring, accuracy breakdown and mistake tracking.

Attempt this test free 20 questions · 30 minutes · +80 marks
1
An electromagnetic wave of frequency 3 MHz passes from vacuum into a dielectric medium with permittivity ε = 4. Then (Radiation):
  1. A Wavelength and frequency both remain unchanged
  2. B Wavelength is doubled and the frequency remains unchanged
  3. C Wavelength is doubled and the frequency becomes half
  4. D Wavelength is halved and the frequency remains unchanged
Show answer & explanation

Correct answer: D

v = 3 MHz, ε = 4
∵ In another medium the frequency remains the same so as to conserve the energy but the wavelength changes as speed also gets changed in other medium such that
$\begin{array}{l}E=\frac{h c}{\lambda}=\frac{h c^{\prime}}{\lambda^{\prime}} \\ \because c^{\prime}=\frac{1}{\sqrt{\mu_{0} \varepsilon_{r}}}=\frac{1}{\sqrt{\mu_{0} \varepsilon \varepsilon_{0}}}=\frac{c}{\sqrt{\varepsilon}},\end{array}$
Hence λ also decreases by factor of $\frac{1}{\sqrt{\varepsilon}}$ i.e., $\lambda^{\prime}=\frac{\lambda}{\sqrt{\varepsilon}}$
∵ λ becomes half & frequency remains unchanged.
Therefore, option (D) is the correct answer.
2
Microwaves are used in RADAR because of its-
  1. A Small wave length
  2. B Large wavelength
  3. C High speed
  4. D Large penetration power
Show answer

Correct answer: A

3
An electromagnetic wave radiates outwards from a dipole antenna, with the amplitude of its electric field vector E$_{0}$. The electric field which transports significant energy from the source falls off as:
  1. A $\frac { 1 } { \mathrm { r } ^ { 3 } }$
  2. B $\frac { 1 } { \mathrm { r } ^ { 2 } }$
  3. C $\frac { 1 } { \mathrm { r } }$
  4. D r
Show answer & explanation

Correct answer: C

$\because E_{0} \propto \frac{1}{r}$
amplitude of electric field vector, for a dipole antennas.
Therefore, option (C) is the correct answer.
4
A lamp emits monochromatic green light uniformly in all directions. The lamp is 3% efficient in converting electrical power to electromagnetic waves and consumes 100 W of power. The amplitude of electric field associated with the electromagnetic radiation at a distance of 5 m from the lamp will be-
  1. A 1.34 V/m
  2. B 2.68 V/m
  3. C 4.02 V/m
  4. D 5.36 V/m
Show answer

Correct answer: B

5
Which of the following is incorrect about a plane electromagnetic wave?
  1. A The electric field and magnetic field have equal average values
  2. B The electric energy and the magnetic energy have equal average values
  3. C The electric field and magnetic field both oscillate in same phase
  4. D The electric field and magnetic field oscillate in opposite phase
Show answer & explanation

Correct answer: D

$\vec{E}$ and $\vec{B}$ are in the same phase but oscillate in different planes which are perpendicular to each other.
6
40 cm is the wavelength corresponding to the spectrum of:
  1. A Infrared rays
  2. B Ultraviolet rays
  3. C Microwaves
  4. D X-rays
Show answer

Correct answer: C

7
The magnetic field in a plane electromagnetic wave is given by, $B = 3.01 \times 10 ^ { - 7 } \sin \left( 6.28 \times 10 ^ { 2 } x + 2.2 \right. \left. \times 10 ^ { 10 } t \right) \mathrm { T }$. [where $x$ in m and $t$ in second]. The wavelength of the given wave is
  1. A 1 cm
  2. B 628 cm
  3. C 1.129 cm
  4. D 314 cm
Show answer & explanation

Correct answer: A

Electromagnetic Waves – NEET-13346
8
Which of the following rays has highest wavelength?
  1. A X-rays
  2. B Radiowaves
  3. C UV-rays
  4. D Microwaves
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Correct answer: B

Radiowave has longest wavelengths than all waves in whole E.M. spectrum.
9
The transmitting antenna of a radio-station is mounted vertically. At a point 10 km due north of the transmitter the peak electric field is $10 ^ { - 3 } \mathrm { volt } / metre$. The magnitude of the radiated magnetic field is-
  1. A $3.33 \times 10 ^ { - 10 }$ Tesla
  2. B $3.33 \times 10 ^ { - 12 }$ Tesla
  3. C $10 ^ { - 3 }$ Tesla
  4. D $3 \times 10 ^ { 5 }$ Tesla
Show answer

Correct answer: B

10
For any E.M. wave if E = 100 V/m and $B = 3.33 \times 10 ^ { - 7 } T$. Then the rate of energy flow per unit area is-
  1. A $3.33 \times 10 ^ { - 5 } \mathrm {~J} / \mathrm { m } ^ { 2 }$
  2. B $26.5 \mathrm { VA } / \mathrm { m } ^ { 2 }$
  3. C $3 \times 10 ^ { 8 } \mathrm {~J} / \mathrm { m } ^ { 2 }$
  4. D $2 \mathrm {~A} / \mathrm { m } ^ { 2 }$
Show answer

Correct answer: B

11
The electric field part of an electromagnetic wave in vacuum is $\vec { E } = 3.1 \mathrm {~N} \mathrm { C } ^ { - 1 } \cos \left[ \left( 1.8 \mathrm { radm } ^ { - 1 } \right) \mathrm { y } + \left( 5.4 \times 10 ^ { 8 } \mathrm { rads } ^ { - 1 } \right) \mathrm { t } \right]$. The wavelength of this part of electromagnetic wave is:
  1. A 1.5 m
  2. B 2 m
  3. C 2.5 m
  4. D 3.5 m
Show answer & explanation

Correct answer: B

Electromagnetic Waves – NEET-15409 Therefore, option (D) is the correct answer.
12
Maxwell's equation $\oint \overrightarrow { \mathrm { E } } \cdot \overrightarrow { \mathrm { d } } = - \frac { \mathrm { d } \phi _ { \mathrm { B } } } { \mathrm { dt } }$ is a statement of
  1. A Ampere's law
  2. B Faraday's law of induction
  3. C Gauss's law of electricity
  4. D Gauss's law of magnetism
Show answer

Correct answer: B

13
The energy stored in a 90 cm length of laser beam operating at 10 mW -
  1. A $3 \times 10 ^ { 8 } \mathrm {~J}$
  2. B $3 \times 10 ^ { 9 } \mathrm {~J}$
  3. C $3 \times 10 ^ { 11 } \mathrm {~J}$
  4. D $3 \times 10 ^ { - 11 } \mathrm {~J}$
Show answer

Correct answer: D

14
In the above question, the frequency corresponding to the given part of electromagnetic wave is:
  1. A $5.4 \times 10 ^ { 8 } \mathrm {~Hz}$
  2. B $8.6 \times 10 ^ { 7 } \mathrm {~Hz}$
  3. C $3.2 \times 10 ^ { 8 } \mathrm {~Hz}$
  4. D $4.8 \times 10 ^ { 7 } \mathrm {~Hz}$
Show answer & explanation

Correct answer: B

In above solution:
$\omega=5.4 \times 10^{8}=2 \pi \mathrm{v}, v=\frac{5.4 \times 10^{8}}{2 \pi}=8.6 \times 10^{7} \mathrm{~Hz}$
Therefore, option (B) is the correct answer.
15
If electric field $E = E _ { 0 } \sin ( \omega t - k z ) \hat { i }$ and magnetic field $B = B _ { 0 } \sin ( \omega t - k z ) \hat { j }$, then electromagnetic wave travels along
  1. A +z direction
  2. B -y direction
  3. C +x direction
  4. D +y direction
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Correct answer: A

(ωt - kz) indicates wave travels in +ve z direction.
16
A parallel plate capacitor is charged to 60 μC. Due to a radioactive source, the plate loses charge at the rate of $1.8 \times 10 ^ { - 8 } \mathrm { C } \mathrm { s } ^ { - 1 }$. The magnitude of displacement current is
  1. A $3.6 \times 10 ^ { - 8 } \mathrm { C } \mathrm { s } ^ { - 1 }$
  2. B $1.8 \times 10 ^ { - 8 } \mathrm { C } \mathrm { s } ^ { - 1 }$
  3. C $4.1 \times 10 ^ { - 11 } \mathrm { C } \mathrm { s } ^ { - 1 }$
  4. D $5.7 \times 10 ^ { - 12 } \mathrm { C } \mathrm { s } ^ { - 1 }$
Show answer

Correct answer: B

17
The S.I unit of displacement current is-
  1. A Henry
  2. B Coulomb
  3. C Ampere
  4. D Farad
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Correct answer: C

18
The maxwell’s equation: $\oint \overrightarrow{\mathrm{B}} \cdot \overrightarrow{\mathrm{d}} \overrightarrow{\mathrm{l}}=\mu_{0}\left(\mathrm{i}_{\mathrm{C}}+\varepsilon_{0} \cdot \frac{\mathrm{~d} \phi_{\mathrm{E}}}{\mathrm{dt}}\right)$ is a statement of
  1. A Faraday’s law of induction
  2. B Modified Ampere’s law
  3. C Gauss’s law of electricity
  4. D Gauss’s law of magnetism
Show answer

Correct answer: B

19
Choose the incorrect statement among the following for EM waves
  1. A Electromagnetic waves are produced by accelerated charge and oscillating charge
  2. B Electromagnetic waves travel in free space with speed equal to $3 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s }$
  3. C Electromagnetic waves require material medium for their propagation
  4. D The velocity of electromagnetic wave in a medium is decided by electric and magnetic properties of medium
Show answer & explanation

Correct answer: C

Electromagnetic waves do not require material medium for their propagation.
20
EM waves can transport
  1. A Momentum
  2. B Energy
  3. C Charge
  4. D Both (A) & (B)
Show answer & explanation

Correct answer: D

EM wave can transport both energy and momentum.

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