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The nature of electromagnetic waves - class-XII

Description: the nature of electromagnetic waves
Number of Questions: 30
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Tags: observing space: telescopes physics electromagnetic waves
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Choose the correct answer from the alternatives given.
Maxwell in his famouse questions of  eletromagnetism introduce the concept of:

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. ac current

  2. displacement current

  3. impedance

  4. reatance

Show answer
Correct Option: B
Explanation:

Maxwell in his famous question's of electromagnetism introduce the concept of displacement current

$J=J _E+J _C\ \quad=\epsilon _0\cfrac{dE}{dT}+\cfrac{dP}{dT}$

Which of the following has/have zero average value in a plane electromagnetic wave?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Both magnetic and electric fields

  2. Electric fields only

  3. Magnetic field only

  4. None of these

Show answer
Correct Option: A
Explanation:

Both electric and magnetic field have sinusoidal nature in a plane electromagnetic wave. As we know, the average value of a sinusoidal wave is zero, so both magnetic and electric fields have average values zero.

Which of the following ray is not electromagnetic wave?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. X-rays

  2. $\gamma$-rays

  3. $\beta$-rays

  4. Heat rays

Show answer
Correct Option: C
Explanation:

Cosmic rays, γ-rays, and X-rays are part of electromagnetic spectrum, while β-rays are emitted by radioactive elements. Hence β-rays is not electromagnetic waves.

Which of the following is not true for electromagnetic waves?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. They transport energy.

  2. They have momentum.

  3. They travel at different speeds in air depending on their frequency.

  4. They travel at different speeds in medium depending on their frequency.

Show answer
Correct Option: C
Explanation:

Electromagnetic wave have constant velocity in a particular medium and it is independant of frequency and it is equal to $V=\sqrt{\dfrac{1}{\mu _0 \epsilon _0}}=3 \times 10^8 \ m/s$ 

Option C

An electromagnetic wave of frequency $\upsilon = 3$ MHz passes from a vacuum into a dielectric medium with permittivity $ = 4$, Then

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Wavelength and frequency both become half.

  2. Wavelength is doubled and frequency remains unchanged.

  3. Wavelength and frequency both remains unchanged.

  4. Wavelength is halfved and frequency remains unchanged.

Show answer
Correct Option: D
Explanation:

Frequency remains constant during refraction.

$v _{med}=\dfrac{1}{\sqrt{\mu _0 \epsilon _0 *4}}$=$\dfrac{c}{2}$

$\dfrac {\lambda _{med}}{\lambda _{air}} = \dfrac{v _{med}}{v _{air}}$

= $\dfrac{\dfrac{c}{2}}{c}$ =$\dfrac{1}{2}$
​$\therefore$ wavelength is halved and frequency remains unchanged.

A plane electromagnetic wave of frequency 50 MHz travels in free space along the x-direction . At a particular point in space and time, $\overrightarrow { E } $ = 6.3 $\hat j$ V ${m}^{-1}$. At this point $\overrightarrow { B } $ is equal to

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $8.33\ \times { 10 }^{ -8 }\hat { k }\ T$

  2. $18.9\ \times { 10 }^{ -8 }\hat { k }\ T$

  3. $4.1\ \times { 10 }^{ -8 }\hat { k }\ T$

  4. $2.1\ \times { 10 }^{ -8 }\hat { k }\ T$

Show answer
Correct Option: D
Explanation:
Given: The Electric field intensity due to electromagnetic waves is, $E _0=6.3\ V/m$

To find: The direction and intensity of the magnetic field of Electromagnetic wave.

The magnetic field intensity due to the wave is given by
$B _0=\dfrac{E _0}{C}$

$\Rightarrow B _0=\dfrac{6.3}{3\times 10^8}=2.1\times 10^{-8}$ Tesla

Since the propagation of the wave is along $\hat i$ (X-direction) and the electric field vector is along $\hat j$, the magnetic field of the wave should lie perpendicular to both the direction of propagation and direction of electric field.

So, $\vec B$ should be along $\vec k$ such that $\vec E\times \vec B$ should give $\hat i$ ( propagation in X-direction).

$\Rightarrow \vec B=B _0\hat k$
$\vec B=2.1\times 10^{-8}\ T\ \hat k$

Which one of the following is a property of a monochromatic, plane electromagnetic wave in free space?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Electric and magnetic fields have a phase difference of ${\pi}/{2}$

  2. The energy contribution of both electric and magnetic fields are equal

  3. The direction of propagation is in the direction of $\overline { B } \times \overline { E } $

  4. The pressure exerted by the wave is the product of its speed and energy density.

Show answer
Correct Option: B
Explanation:

The energy contribution of both electric and magnetic fields are equal, Energy density is written as $E=\dfrac 12\epsilon _0E^2=\dfrac{B^2}{2 \mu _0}$

And the direction of propagation of wave is written as $\hat V= \hat E\times \hat B$
Option B

The ultra high frequency band of radiowaves in electromagnetic waves is used as in

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Television waves

  2. Cellular phone communication

  3. Commercial FM radio

  4. Both (b) and (c)

Show answer
Correct Option: B
Explanation:
Radio Waves
  • Radio waves are usually in the frequency range from $500\ kHz$ to $1000\ MHz$.
  • TV waves range from $54\ MHz$ to $890\ MHz$.
  • The FM (frequency modulated) radio band is from $88\ MHz$ to $108\ MHz$.
  • Cellular phones also use radio waves to transmit voice communication in an ultra-high frequency $(UHF)$ band.
As given in last point radio waves are used in cellular phone communication. 

Which of the following electromagnetic wave play an important role in maintaining the earths warmth or average temperature through the green house effect?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Visible rays

  2. Infrared rays

  3. Gamma rays

  4. Ultraviolet rays

Show answer
Correct Option: B
Explanation:

Infrared radiation plays an important role in maintaining the earth's warmth or average temperature through the greenhouse effect. It is the part of the spectrum that is detected by the human eyedetected by the human eye.

A plane electromagnetic wave travels in free space along X-direction. If the value of $\overrightarrow { B } $ (in tesla) at a particular point in space and time is $1.2 \times {10}^{-8} \hat {k}$, the value of $\overrightarrow { E } $ (in V ${m}^{-1}$) at that point is,

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $1.2\ \hat {j}$

  2. $3.6\ \hat {k}$

  3. $1.2\ \hat {k}$

  4. $3.6\ \hat {j}$

Show answer
Correct Option: D
Explanation:
Given: The magnetic field of the plane electromagnetic wave is $1.2×10^{-8} \hat k\ T$
The direction of propagation of the electromagnetic wave is along X-direction.

The magnitude of  $\vec E$ is given by:
$E\, = \, B\cdot c\\ \ \ \ \ = (1.2 \, \times \, 10^{-8} T)(3 \, \times \, 10^{-8} m \,  s^{-1})\\ \ \ \ \ = 3.6 \,  V/m$

Since the magnetic field is along $Z-$ direction and the wave propagates along $X -$ direction. Therefore $\vec E$  should be in a direction perpendicular to both $X$ and $Z$ axes.

Using vector algebra  should be along X-direction.

Since $(+\hat j) \times (\hat k )= \hat i$

$\vec E$ is along the $Y-$direction.
Thus,  $\vec E= 3.6\hat j\ Vm^{-1}$

The electromagnetic wave travel in free space with the velocity of 

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Sound

  2. Light

  3. Greater than that of light

  4. Greater than that of sound

Show answer
Correct Option: B
Explanation:

Electromagnetic waves travel at the speed of light ($3 \times 10^8\ m/s$) in vacuum.

The waves that are propagated by simultaneous periodic variations of electric and magnetic field intensity known as:

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Transverse wave

  2. Longitudinal wave

  3. Electromagnetic wave

  4. None of the above

Show answer
Correct Option: C
Explanation:

Maxwell predicted that an accelerated charge produces a sinusoidal time varying magnetic field, which in turn produces a sinusoidal time varying electric field. These two fields are mutually perpendicular to each other and sources of each other. They (fields) form an electromagnetic wave i.e. the wave that is propagated by simultaneous periodic variations of electric field and magnetic field intensity.

Select wrong statement from the following:
Electromagnetic waves

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. are transverse

  2. travel with same speed in all media

  3. travel with the speed of light

  4. are produced by accelerating charge

Show answer
Correct Option: B
Explanation:

Electromagnetic waves are transverse and do not need a material medium to travel. 
The speed of all electromagnetic waves is equal to the speed of light. In fact this let to the conclusion that light is an electromagnetic wave.

Accelerating charge particles produce electromagnetic waves.
However the speed of electromagnetic waves changes with media as 
$v=\dfrac{c}{n}$
where n is the refractive index of the given medium.

Electromagnetic waves do not transport

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. energy

  2. charge

  3. momentum

  4. information

Show answer
Correct Option: B
Explanation:

Waves carry energy.

Since it carries energy, it must have some momentum, which is given as
$p=\dfrac{h}{c}$
Energy is a form of information. Hence it is carried by a wave.

The wavelengths for the light of red and blue colours are roughly $7.8\times 10^{-7}m$ and $4.8\times 10^{-7}m$ respectively. Which colour has the greater speed in glass?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Red light

  2. Violet light

  3. Yellow light

  4. All colors have same speed

Show answer
Correct Option: A
Explanation:

When travelling through a medium, the frequency of light doesn't change. Hence, red light having the maximum wavelength will travel fastest through glass as speed of light is given by $v=f\lambda$ where; $f$ is the frequency and $\lambda$ is the wavelength of light travelling at speed $v$.

What is the speed of Light $c$?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $c=3\times 10^{-8} m s^{-1}$

  2. $c=3\times 10^8 km s^{-1}$

  3. $c=3\times 10^8 mm s^{-1}$

  4. $c=3\times 10^8 m s^{-1}$

Show answer
Correct Option: D
Explanation:

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly $299792458$ meters per second.
That is, $c=3\times { 10 }^{ 8 }m/s$.

The electromagnetic waves do not transport

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. energy

  2. charge

  3. momentum

  4. information

Show answer
Correct Option: B
Explanation:

$Answer:-$ B

EM waves carry energy (light ), momentum (can be seen by photoelectric experiment) and information (like radiowaves ) but does'nt carry charge. 

In which form does the Light propagates?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Electric waves

  2. Magnetic waves

  3. Electromagnetic waves

  4. Sound wave

Show answer
Correct Option: C
Explanation:

Electromagnetic waves are waves which can travel through the vacuum of outer space.light waves are examples of electromagnetic waves.
so,Light propagates in the form of electromagnetic waves.

As speed decreases, if we change medium of electromagnetic waves from air to water, frequency

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. also decreases.

  2. also increases.

  3. remains same.

  4. may increase or decrease.

Show answer
Correct Option: C
Explanation:

As speed decreases, if change medium of electromagnetic wave from air to water, frequency remains same.

As we go from one medium to another the frequency stays same, but the wavelength changes. Change in speed is proportional to change in wavelength.

A plane electromagnetic wave of angular frequency ${ mm }^{ 2 }$ propagates in a poorly conducting medium of conductivity $\sigma $ and relative permittivity $\epsilon $ Find the ratio of conduction current density and displacement current density in the medium.

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. ${ \epsilon \epsilon } _{ 0 }\omega /\sigma $

  2. $\sigma /{ \epsilon \epsilon } _{ \sigma }\omega $

  3. $\omega /{ \sigma \epsilon } _{ 0 }\omega $

  4. $\omega \sigma /{ \epsilon } _{ 0 }\epsilon $

Show answer
Correct Option: A

Which among the four options is not correct ion given situation ? A charged particle oscillates about its mean equilibrium position with a frequency of $10^9\, Hz.$ The electromagnetic waves produced 

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Will have frequency of $10^9\, Hz.$

  2. Will have frequency of $2\times 10^9\, Hz.$

  3. Will have wavelength of $0.3\,m.$

  4. Fall in the region of radio waves

Show answer
Correct Option: C

An electromagnetic wave with frequency $\omega$ and wavelength $\lambda$ travels in the $+y$ direction. Its magnetic field is along $-x$ axis. The vector equation for the associated electric field (of amplitude E_0)is :

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $ \vec { E } = E _0 \cos \left(\omega t -\dfrac{2\pi}{\lambda}y\right)\hat{x}$

  2. $\vec{ E } = -E _0 \cos \left(\omega t +\dfrac{2\pi}{\lambda}y\right)\hat{x}$

  3. $-\vec{ E } = E _0 \cos \left(\omega t +\dfrac{2\pi}{\lambda}y\right)\hat{z}$

  4. $\vec { E } = E _0 \cos \left(\omega t -\dfrac{2\pi}{\lambda}y\right)\hat{z}$

Show answer
Correct Option: A

Electro magnetic waves are transverse in nature as is evident by

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. polarization

  2. interference

  3. reflection

  4. deffraction

Show answer
Correct Option: A
Explanation:

Electromagnetic waves are transverse in nature as is evident by polarization interference and diffraction explain the wave nature of light or $EM$ waves polarization is the phenomenon by which we restrict the vibration of wave in a particular direction perpendicular to direction of $W$ are propagation. So, it explains the transverse nature of $EM$ waves.

Answer :- polarization.

The maximum kinetic energy of electron if wavelength of incident electromagnetic wave is $260 \,nm$ and cut-off wavelength is $380 \,nm$ given $hc = 1237 \,nm-eV$ is

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $1.5 \,eV$

  2. $6.4 \,eV$

  3. $10 \,eV$

  4. None of these

Show answer
Correct Option: A
Explanation:

We know 
$KE _{max} = \dfrac{hc}{\lambda} - \dfrac{hc}{\lambda _0} $
$= \dfrac{1237 \, nm. eV}{260 \, nm} - \dfrac{1237 \, nm . eV}{380 \, nm}$
$= 1.5 eV$

If $\vec{E} = E _0 \cos (kz) \cos (\omega t)\hat{i}$ then $\vec{B}$ for electromagnetic wave is:

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $\vec{B} = \dfrac{E _0}{C} \hat{k}$

  2. $\vec{B} = \dfrac{E _0}{C} \sin (kz) \sin (\omega t)\hat{i}$

  3. $\vec{B} = \dfrac{E _0}{C} \sin (kz) \cos (\omega t) \hat{j}$

  4. $\vec{B} = \dfrac{E _0}{C} \cos (kz) \sin (\omega t) \hat{j}$

Show answer
Correct Option: B
Explanation:

$\dfrac{dE}{dz} = -\dfrac{dB}{dt}$
If $\vec{E} = E _0 \, \cos (kz) \cos (\omega t)$ then
$\vec{B} = \dfrac{E _0}{C} \sin (kz) \sin (\omega t)$ will satisfy the equation

For the propagation of electromagnetic waves

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Medium is required

  2. no medium is required

  3. E and B are in mutually opposite phase

  4. E and b do not contriute

Show answer
Correct Option: B
Explanation:

Electromagnetic waves are a result of changing electric field creating magnetic field and the magnetic field does the same for electric field. Unlike mechanical waves, electromagnetic waves does not require material medium to transfer energy from one place to another.

On the basis of the following features identify correct option:
I. They require no material medium.
II. They are always transverse.
II. They are produced by accelerating charged particles

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Mechanical waves

  2. Electro-magnetic waves

  3. Both (1) and (2)

  4. Neither (1) and (2)

Show answer
Correct Option: B
Explanation:

Electromagnetic waves do not need any material medium for their propagation and they are always transverse in nature e.g. light waves.

A long spring is fixed at one end. A person holding the other end compresses the spring with a jerk. The compression travels along the length of the spring. Which kind of wave is travelled along the length of the spring?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Electromagnetic waves

  2. Transverse waves

  3. Longitudinal waves

  4. Both (A) and (C)

Show answer
Correct Option: C
Explanation:

If the spring is compressed, particles of the spring come close and there is compression.When spring is extended, particles rarefact from each other . Direction of particle motion is in the direction of wave propagation, Hence longitudinal waves  .


In which of the following media can non-mechanical waves travel?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. In vacuum as well as in a medium

  2. In vacuum but not in a medium

  3. In a medium but not in a vacuum

  4. Neither in a medium nor in vacuum

Show answer
Correct Option: A
Explanation:

Non mechanical waves are waves that do not require a medium for transfer of their energy to occur electromagnetic waves are the only type of non mechanical waves.

Non mechanical waves can travel both in vacuum or in any medium

Which of the following have zero average value in a plane electromagnetic wave?
(i) Magnetic field
(ii) Electric field
(iii) Electric energy
(iv) Magnetic energy.

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. (i) and (ii)

  2. (i) and (iv)

  3. (ii) and (iii)

  4. (iii) and (iv)

Show answer
Correct Option: A
Explanation:

Electric field has zero average value in electromagnetic waves

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