• Introduction to electromagnetic waves - class-XII
Back
0

Introduction to electromagnetic waves - class-XII

Description: introduction to electromagnetic waves
Number of Questions: 49
Created by:
Tags: physics electromagnetic waves observing space: telescopes option a: relativity electromagnetic waves and communication system
Start the Quiz
Attempted 0/49 Correct 0 Score 0
‹
›

Choose the correct answer from the alternatives given.
A plane electromagnetic wave of frequency $25 MHz$ travels in free space along $X$-direction. At a particular point in space and time, electric field $\vec E=6.3\ \hat j\ V/m$. What is $B$ at this point.

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $1.2 \, \times \, 10^{-6} \, T$

  2. $1.2 \, \times \, 10^{-8} \, T$

  3. $2.1 \, \times \, 10^{-6} \, T$

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

Show answer
Correct Option: D
Explanation:

Given: The frequency of the electromagnetic wave is $25\ MHz$.

The electric field at the particular point is $6.3\hat j\ V/m$

To find: The magnetic field at that point.

The magnetic field of the electromagnetic wave at a point is given by:
$B = \dfrac{E}{c}\= \dfrac{6.3}{3 \times 10^8}\ \Rightarrow2.1 \times 10^{-8} T$

So, option $(D)$ is correct.

The electric field of an electromagnetic wave traveling through the vacuum is given by the equation $E=E _0\ sin (Kx-\omega t).$ The quantity that is independent of wavelength is:

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $k\omega$

  2. $\dfrac{k}{\omega}$

  3. $k^2\omega$

  4. $\omega$

Show answer
Correct Option: B
Explanation:

To find: The quantity that is independent of the wavelength.


The angular frequency $\omega$ is given by:
$\omega \, = \, 2\pi \nu$
The frequency of a wave varies with the wavelength. So, angular frequency is dependent on wavelength.

The quantity $k$ is defined as the wavenumber and it is given by:
$k = \dfrac{2\pi}{\lambda}$
It shows that $k$ is dependent on wavelength.


The value of $\dfrac{k}{\omega}$ can be obtained as:
$\dfrac {k}{\omega} \, = \, \dfrac{2\pi / \lambda}{2\pi \nu}\\implies \, \dfrac{1}{\nu \lambda} \, = \, \dfrac{1}{c}\,\,\ \ \ \ \ \ \ \ \ \ \ \ \  (\because \, c \, = \, \nu \lambda)$
where c is the speed of electromagnetic wave in vacuum. It is a constant whose value is $3 \, \times \, 10^8 \, ms^{-1}$.

So, option $(B)$ is correct.

Maxwell in his famous equations of electromagnetism, introduced the concept of

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. ac current

  2. displacement current

  3. impedance

  4. reactance

Show answer
Correct Option: B
Explanation:

Maxwell's equations are:

1. $\nabla .E=\rho / \epsilon$
2. $\nabla.B=0$
3. $\nabla \times E= -\dfrac{dB}{dt}$
4. $\nabla \times B= \mu _0J+ \dfrac{1}{c^2} \dfrac{dE}{dt}$
so, considering the last eqn. written,
$\nabla \times B=\mu _0 J$ is the Ampere's eqn.
so, Maxwell modified the Ampere's eqn. and introduced the concept of displacement current.
So, displacement current =$\dfrac{1}{c^2} \dfrac{dE}{dt}$

Hence the correct option is $(B)$

$X-$ray falling on a material 

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Exerts a force on it

  2. Transfer energy to it

  3. Transfers momentum to it

  4. Transfers impules to it

Show answer
Correct Option: B
Explanation:

The emitted X-rays transfer energy to the material on which it is falling.

A parallel plate capacitor of plate separation 2 mm is connected in an electric circuit having source voltage 400. What is the value of the displacement current for $10^{-6}$ s, if plate area is 60 $cm^2$

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $1.062 \times 10^{-2} \ A$

  2. $2.062 \times 10^{-2} \ A$

  3. $3.062 \times 10^{-2} \ A$

  4. $5.062 \times 10^{-2} \ A$

Show answer
Correct Option: B

The displacement current flows in the dielectric of a capacitor when the potential difference across its plates

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. becomes zero

  2. has assumed a constant value

  3. is increasing with time

  4. is decreasing with time

Show answer
Correct Option: C
Explanation:

According to Maxwell's hypothesis, a displacement current will flow through a capacitor when the potential difference across its plates is varying. Thus a varying electric field will exist between the plates and this displacement current is same in magnitude to the current flowing in outer circuit.  When a D.C voltage applied across its plates, constant voltage appears across its plates and so there will be no displacement current flowing through the capacitor. Thus the displacement current will flow when the potential is increasing with time.

The displacement current was first populated by

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Maxwell

  2. Marconi

  3. Ampere

  4. Hertz

Show answer
Correct Option: A
Explanation:

In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field.

According to Maxwell's equation, the velocity of light in any medium is expressed as

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\displaystyle\frac{1}{\sqrt{\mu _0\varepsilon _o}}$

  2. $\displaystyle\frac{1}{\sqrt{\mu\varepsilon}}$

  3. $\displaystyle\sqrt{\frac{\mu}{\varepsilon}}$

  4. $\displaystyle\sqrt{\frac{\mu _0}{\varepsilon}}$

Show answer
Correct Option: B
Explanation:

Velocity of light in a medium,

$\displaystyle c=\frac{1}{\sqrt{\mu _0\varepsilon _o\mu _r\varepsilon _r}}=\frac{1}{\sqrt{\mu\varepsilon}}$

Maxwell's equation describe the fundamental laws of

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. electricity

  2. magnetism

  3. mechanics

  4. both (A) and (B)

Show answer
Correct Option: D
Explanation:

Maxwell's equation describe the fundamental laws of electricity and magnetism. His equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.

According to Maxwell's hypothesis, a changing electric field gives rise to

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. an electromagnetic force

  2. electric displacement current

  3. magnetic field

  4. pressure gradient

Show answer
Correct Option: C
Explanation:
$Answer:-$ C option
$\nabla \times B={ \mu  } _{ 0 }(J+{ \epsilon  } _{ 0 }\dfrac { dE }{ dt } )$
using this equation of maxwell we can say changing electric field $\dfrac{dE}{dt}$ induces magnetic field.

The electric field associated with an e.m. wave in vacuum is given by $\vec {E} = 40\cos (kz - 6\times 10^{8}t)\hat {i}$, where $E, z$ and $t$ in $volt/m$, meter and seconds respectively. The value of wave vector $k$ is

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $6m^{-1}$

  2. $3m^{-1}$

  3. $2m^{-1}$

  4. $0.5m^{-1}$

Show answer
Correct Option: C
Explanation:
Given: The electric field associated with  an electromagnetic wave in vacuum is given by $\vec E =40 \cos(kz−6\times 10^8t)\hat i$  , where E, z and t are in volt per meter, meter and second respectively.
To find the value of wave vector k
Solution: 
We know electromagnetic wave eqution is
$E=E _0\cos(kz-\omega t)$
And given equation is
$\vec E =40 \cos(kz−6\times 10^8t)\hat i$
By comparing these two, we get
$\omega=6\times10^8$ and 
$E _0=40\hat i$
we also know,
Speed of electromagnetic wave, $v=\dfrac \omega k$
where v is the speed of the light
Hence, $k=\dfrac \omega v\\\implies k=\dfrac {6\times 10^8}{3\times 10^8}\\\implies k=2m^{-1}$
is the required value

Wavelength of light in different media are proportional to:

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. speed of light in that medium

  2. Amplitude of light in that medium

  3. frequency of light in that mrdium

  4. Nove of above

Show answer
Correct Option: A

The Maxwell's equation : $\oint \vec { \mathrm { B } }$ . $\vec { \mathrm { d } 1 } = \mu _ { 0 } \left( \mathrm { i } + \varepsilon _ { 0 } \cdot \frac { \mathrm { d } \phi _ { \mathrm { E } } } { \mathrm { dt } } \right)$ is a statement of

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Faraday's law of induction

  2. Modified Ampere's law

  3. Gauss's law of electricity

  4. Gauss's law of magnetism

Show answer
Correct Option: A

What is the displacement current between the square plate of side 1 cm of a capacitor, if electric field between the, plates is changing at the rate of 3 x $10^6 V _m^{-1}S^{-1}$? 

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. 2.7 x $10^{-6}$ A

  2. 3.2 x $10^{-6}$ A

  3. 4.2 x $10^{-6}$ A

  4. 4.0 x $10^{-6}$ A

Show answer
Correct Option: B

In Maxwell's velocity distribution curve area under the graph 

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Increases when temperature is increased

  2. Deccreases when temperature is increased

  3. Remains same at all temperature

  4. Depends on the pressure of the gas

Show answer
Correct Option: C
Explanation:

Area under the Maxwell's velocity distribution curve gives the number of particles. Since number of particles remains the same at all the temperatures, so the area under the curve also remains the same at all temperature.

If a plane electromagnetic wave satisfies the equation $\dfrac{\partial ^2E _x}{\partial _z^2}= C^2 \dfrac{\partial^2E _x}{\partial^2},$the wave propagates in

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $x$-direction

  2. $z$-direction

  3. $y$-direction

  4. $xz$ plane at an angle of $45^0$ between the $x$ and $z$direction

Show answer
Correct Option: A

According to the electromagnetic wave theory, light consists of electric and magnetic fields which are __________.

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. parallel to each other

  2. perpendicular to each other

  3. inclined at an angle of ${45}^{o}$ to each other

  4. none of these

Show answer
Correct Option: B
Explanation:

Light consists of electric and magnetic field that are perpendicular ${ 90 }^{ 0 }$ to each other.
APPOACH by example
Electric field inside plates. The magnetic field this given rise to via the displacement current is along the perimeter of the circle parallel to capauatates plates.

So B and E are perpendicular in this case.

Which of the following conclusion can be drawn from the result $\oint \bar{B}\cdot d\bar{A}=0$

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Magnetic field is zero everywhere

  2. Magnetic monopole cannot exist

  3. Magnetic lines of force do not intersect each other

  4. A current produces magnetic field

Show answer
Correct Option: B
Explanation:

Flux of certain closed surface is zero and so it tells that net magnetic charge is equal to zero. This is possible when there are two equal and opposite poles.

Which of the following effects could not be explained by Maxwell's electromagnetic wave theory?

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. Photoelectric effect

  2. Compton effect

  3. Raman effect

  4. All of these

Show answer
Correct Option: D
Explanation:

  1. Photoelectric effect was discovered by heinrich Rudoy Hertz.
  2. Compton effect was discovered by Aethur Holl Compton.
  3. Raman effect was discovered by Sir Chandrasekhar Venbata Ram. 
      So, none of these effect was discovered by Maxwell.

A parallel plate capacitor having plate area A and plate separation $d$ is connected to a battery of emf $\varepsilon$ and internal resistance $R$ at $t=0$. Consider a plane surface of area $\dfrac{A}{2}$, parallel to the plates and situated symmetrically between them. Find the displacement current through this surface as a function of time?

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\dfrac {-\varepsilon}{2R} \ \ \ e^{\dfrac{-td}{\varepsilon AR}}$

  2. $\dfrac {2\varepsilon}{R} \ \ \ e^{\dfrac{-td}{\varepsilon AR}}$

  3. $\dfrac {5\varepsilon}{2R} \ \ \ e^{\dfrac{-td}{4 \varepsilon AR}}$

  4. $\dfrac {\varepsilon}{2R} \ \ \ e^{\dfrac{-td}{4\pi \varepsilon AR}}$

Show answer
Correct Option: A

According to Maxwell's hypothesis, changing of electric filed give rise to

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. magnetic field

  2. pressure gradient

  3. charge

  4. voltage

Show answer
Correct Option: A
Explanation:

According to Maxwell's hypothesis, changing of electric field gives rise to Magnetic field.

We know that $F=qE,$, where $F$ is force and $E$ is electric field.
We can relate magnetic field and force by $F=qvB$, where $v$ is velocity and $B$ is the magnetic field.
Therefore we can obtain magnetic field by changing electric field.
Therefore option $A$ is correct.

Unpolarized light falls first on polarizer $\left( P \right) $ and then on analyzer $\left( A \right) $. If the intensity of the transmitted light from the analyser is $\dfrac { 1 }{ 8 }$th of the incident unpolarized light. What will be the angle between optic axes of $P$ and $A$?

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. ${ 45 }^{ o }$

  2. ${ 30 }^{ o }$

  3. Zero

  4. ${ 60 }^{ o }$

Show answer
Correct Option: D
Explanation:

Given,
$I=\dfrac { { I } _{ 0 } }{ 2 } $              ....(i)
${ I }^{ ' }=I\cos ^{ 2 }{ \theta  } $                 $\left( \because { I }^{ ' }=\dfrac { { I } _{ 0 } }{ 8 }  \right) $
$\therefore \dfrac { { I } _{ 0 } }{ 8 } =\dfrac { { I } _{ 0 } }{ 2 } \cos ^{ 2 }{ \theta  } $
From the equation (i), we have
$\dfrac { 1 }{ 4 } =\cos ^{ 2 }{ \theta  } \Rightarrow \cos { \theta  } ={ 1 }/{ 2 }$
$\Rightarrow \cos { \theta  } =\cos { { 60 }^{ o } } $
$\Rightarrow \theta ={ 60 }^{ o }$

A plane electromagnetic wave with an intensity of $200 W/m^2$ is incident normal to a flat plate of radius 30 cm. If the plate absorbs $60%$ and reflect $40%$ of the incident radiation, what is the momentum transferred to it in 5 min?

physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

  1. $1.7 \times 10^{-3} kg ms^{-1}$

  2. $2.7 \times 10^{-4} kg ms^{-1}$

  3. $3.7 \times 10^{-4} kg ms^{-1}$

  4. $3.7 \times 10^{-3} kg ms^{-1}$

Show answer
Correct Option: A

Fill the blank space with the best suitable option. All electromagnetic waves have the same _________ while travelling in a vacuum.

physics electromagnetic waves and communication system maxwell's equations the nature of light introduction to electromagnetic waves

  1. amplitude

  2. frequency

  3. wavelength

  4. Intensity

  5. speed

Show answer
Correct Option: E
Explanation:

All electromagnetic waves (EM waves) travel with the speed of light in vacuum but these have different frequency, amplitude, wavelength and intensity.

Thus option E is correct.

This question has statement-1 and statement-2. Of the four choices given after the Statements, choose the one that best describes the two statements.
Statement 1: Short wave transmission is achieved due to the total internal reflection of the e-m wave from an appropriate height in the ionosphere.
Statement 2: Refractive index of a plasma is independent of the frequency of e-m waves.

physics electromagnetic waves and communication system maxwell's equations the nature of light introduction to electromagnetic waves

  1. Statement 1 is true, Statement 2 false.

  2. Statement 1 is false, Statement 2 is true

  3. Statement 1 is true, Statement 2 is true but statement 2 is not the correct of statement 1.

  4. Statement 1 is true, statement 2 is true and statement 2 is the correct explanation of statement 1.

Show answer
Correct Option: C

The waves which can not travel in vacuum are :

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. X-rays

  2. radio-waves

  3. infrasonic waves

  4. ultra-viloet rays

Show answer
Correct Option: C

Light can pass through

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Vacuum

  2. Glass

  3. Air

  4. All of the above

Show answer
Correct Option: D
Explanation:

Light can pass through vacuum,glass and air.

Identify the electromagnetic wave which is most often used in medicine for diagnostic imaging?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. infrared

  2. ultraviolet

  3. microwave

  4. radio

  5. gamma ray

Show answer
Correct Option: A
Explanation:

During diagnosis of the body heat signatures of the organs are vital. These heat waves lie in the infrared region of the spectrum.

Therefore, infrared waves are used in diagnostic imaging.

 Option A is, therefore, correct. 

Which of the following is responsible for passing the energy from one to the another to transmit the light waves.

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. atom

  2. neutron

  3. fiber

  4. object

  5. wavelength

Show answer
Correct Option: A
Explanation:

Answer A is correct 

Atom is responsible for transmission of light.
By collision of atoms they passes energy from one to another and hence light is transmitted. 

Identify the medium required for Electromagnetic waves to travel?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. none

  2. liquid

  3. solid

  4. gas

  5. fluid (liquid or gas)

Show answer
Correct Option: A
Explanation:

Electromagnetic wave does not require any material medium to propagate. These can also travel through vacuum. Also EM waves travel with the speed equal to that of light  i.e $3 \times 10^8$ m/s

Identify which of the following best describe the difference between electromagnetic (EM) waves and other types of waves?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. EM waves can travel without a medium.

  2. EM waves are higher in frequency than all other waves

  3. EM waves have shorter wave lengths than any other type of wave.

  4. EM waves transport matter and energy.

  5. EM waves require a medium to travel.

Show answer
Correct Option: A
Explanation:

EM waves do not require a material medium to travel, these can propagate even through vacuum while all other waves require a certain medium to travel.

Which of the following is an electromagnetic waves that cause sunburn?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. ultraviolet

  2. X-rays

  3. infrared

  4. microwaves

  5. gamma rays

Show answer
Correct Option: A
Explanation:

UV waves are shorter than visible light waves, so UV possesses more energy than regular light.

UV photons have the right energies to cause chemical changes. When UV light hits your skin, the DNA in your skin cells can undergo chemical change. This chemical change is what sunburn is. In extreme cases, this change in DNA can result in cancer.

In an electormagnetic wave, the phase difference between electric field $\vec { E }$ and magnetic field $ \vec { B } $ is :

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\dfrac { \pi }{ 4 } $

  2. $\dfrac { \pi }{ 2 } $

  3. $\pi $

  4. Zero

Show answer
Correct Option: D
Explanation:

The electric and magnetic field components of a linearly polarized electromagnetic wave oscillate in such a way that they peak at the same time and they become zero at the same time but they point to different directions in space, separated by an angle of $90^{\circ}$.


Since there is no time difference between the peaks of the electric and magnetic oscillations the phase difference between the electric and magnetic field vectors of a linearly polarized electromagnetic wave is zero.

Electromagnetic wave is deflected by

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Electric field

  2. Magnetic field

  3. Both ( 1 ) & ( 2 )

  4. Neither electric field nor magnetic field

Show answer
Correct Option: D
Explanation:

EM waves are formed by electric and magnetic fields but are not deflected by them, and also they can be deflected by gravitational field....

 Light (electromagnetic wave) has an electric and a magnetic field and should thus deflect a compass needle during daytime.

Visible light - Being an Electromagnetic wave is reflected by glass (take mirror).

For highly intense X-Rays & gamma rays, take that the mirror is stronger such that it could resist for at least 30 seconds.

hence, light wave neither electric field nor magnetic field

Wavelength of an electron having energy 10KeV is ...........$ A^0 $

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. 0.12

  2. 1.2

  3. 12

  4. 120

Show answer
Correct Option: C

An electromagnetic wave in vacuum has the electric and magnetic field $\overset { \rightarrow  }{ E } $ and $\overset { \rightarrow  }{ B } $  which are always perpendicular to each other. If the direction of polarization is given by $\overset { \rightarrow  }{ X }  $ and that of wave propagation by $\overset { \rightarrow  }{ k } $ then:

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ B } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $

  2. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ E } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ E\times } \overset { \rightarrow }{ B } $

  3. `$\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ B } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ E\times } \overset { \rightarrow }{ B } $

  4. $\overset { \rightarrow }{ X } \parallel \overset { \rightarrow }{ E } $ and $\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $

Show answer
Correct Option: C
Explanation:

In an electromagnetic wave, electrical and magnetic fields are perpendicular to each other. The wave propagates in a direction perpendicular to both electric and magnetic fields as given by  $\vec E\times \vec B$.

So, the direction of propagation of the wave will be perpendicular to the to the direction of oscillation of the fields.
$\overset { \rightarrow }{ k } \parallel \overset { \rightarrow }{ B\times } \overset { \rightarrow }{ E } $
And the direction of polarization must be perpendicular to the electric field and parallel to the magnetic field.
$\vec X||\vec E$.
The correct option is $(B)$.

Electro-magnetic wave can be:

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Transverse in nature

  2. longitudinal in nature

  3. Both

  4. Electrical in nature

Show answer
Correct Option: C

Intensity of electromagnetic wave will be

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $I=c{\mu} _{0}{B} _{0}^{2}/2$

  2. $I=c{\epsilon} _{0}{B} _{0}^{2}/2$

  3. $I={B} _{0}^{2}/c{\mu} _{0}$

  4. $I={E} _{0}^{2}/c{\epsilon} _{0}$

Show answer
Correct Option: B

The direction of propagation of electromagnetic wave is along.

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Electric field vector, $\vec{E}$

  2. Magnetic field vector, $\vec{B}$

  3. $\vec{E}\cdot \vec{B}$

  4. $\vec{E}\times \vec{B}$

  5. $\vec{B}\times \vec{E}$

Show answer
Correct Option: D
Explanation:
The direction of propagation of the electromagnetic wave is always perpendicular to the plane in which $\vec{E}$  and  $\vec{B}$ lies.
So, the direction of the propagation of the wave, $\vec{C}=E\times B$.
So, the correct option is $(D)$

The electromagnetic radiations used for taking photographs of objects in dark.

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $X$-rays

  2. Infra-red rays

  3. $\gamma$ rays

  4. $UV$ rays

Show answer
Correct Option: B
Explanation:

Answer is B.

Infrared (IR) is invisible radiant energy, electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nano-meters to 1 mm. 
Infrared is used in night vision equipment when there is insufficient visible light to see. Night vision devices operate through a process involving the conversion of visible light photons into electrons that are then amplified by a chemical and electrical process and then converted back into visible light. Infrared light sources can be used to enhance the available visible light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source.

Light appears to travel in a straight line, because.

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Its wavelength is very small

  2. Its velocity is large

  3. It is not absorbed by surroundings

  4. It is reflected by surroundings

Show answer
Correct Option: A
Explanation:

Light appears to travel in a straight line because diffraction (or deviation from the path) is least in light. Diffraction is least because -of small wavelength of light. So small wave length of light causes the light to travel almost in straight line.

Which of the following rays have the highest frequency?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Radiowaves

  2. Infrared rays

  3. Gamma rays

  4. X-rays

Show answer
Correct Option: C
Explanation:

Gamma rays have the highest frequency.

The speed at which the light travels in vacuum is -

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\displaystyle 3\times { 10 }^{ 8 }m/s$

  2. $\displaystyle 3\times { 10 }^{ 3 }m/s$

  3. $\displaystyle 3\times { 10 }^{ 4 }m/s$

  4. $\displaystyle 3\times { 10 }^{ 10 }m/s$

Show answer
Correct Option: A
Explanation:

The speed at which the light travels in vacuum is $3×10^8m/s$

In vacuum, electromagnetic waves travel at the speed of

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $3 \times {10}^{8} {m}/{s}$

  2. $3 \times {10}^{6} {m}/{s}$

  3. $3 \times {10}^{-8} {m}/{s}$

  4. $3 \times {10}^{18} {m}/{s}$

Show answer
Correct Option: A
Explanation:

In vacuum, electromagnetic waves travel at speed of $3\times { 10 }^{ 8 }$ m/s.

C = speed of EM waves
C = $\frac { 1 }{ \sqrt { { \mu  } _{ 0 }{ \varepsilon  } _{ 0 } }  } $
                ${ \mu  } _{ 0 }$ = permaebility of free space
                ${ \varepsilon  } _{ 0 }$ = permittivity of free space
putting the value of ${ \mu  } _{ 0 }$ and ${ \varepsilon  } _{ 0 }$ we get value of C approximately $3\times { 10 }^{ 8 }$ m/s.
                ${ \mu  } _{ 0 }$ = $1.257\times { 10 }^{ -6 }$ Henry/meter
                ${ \varepsilon  } _{ 0 }$ = $8.85\times { 10 }^{ -12 }$ Farad/meter

Reflection of a light wave at a fixed point results in a phase difference between incident and reflected wave of

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $\dfrac{3\pi}{2}$

  2. $2 \pi $ rad

  3. $\pi$ rad

  4. $\dfrac{\pi}{2}$ rad

Show answer
Correct Option: C
Explanation:

Reflection of light wave at fixed point result in phase difference b/w incident and reflected ray of $\pi$. The wave gets inverted. Inversion behavior is noticed when medium is connected to dense medium.

Choose the correct answer from the alternatives given.


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

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Both magnetic and electric fields

  2. electric field only

  3. Magnetic field only

  4. None of these

Show answer
Correct Option: A
Explanation:

A. Both magnetic and electric field.


Because both electric and magnetic field oscillate as sine and cosine wave, which over one period, average value is zero.

Which of the following statement is false for the properties of electromagnetic waves?

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. Both electric and magnetic field vectors attain the maxima and minima at same place and same time.

  2. The energy in electromagnetic wave is divided equally between electric and magnetic field vectors.

  3. Both electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of wave.

  4. These waves do not require any material medium for propagation.

Show answer
Correct Option: C
Explanation:

In electromagnetic waves,

1. The electric field and magnetic field varies continuously with time and have maxima and minima at same place and at same time.
2. Both electric and magnetic field have same energy.
3. both electric and magnetic field are perpendicular to each other and perpendicular to direction of propagation.
4. These waves don't require any material medium to propagate, they can propagate in vacuum as well.
So, the false statement will be the statement given in the option $(C)$
Hence, the correct option is $(C)$

The electric field of a plane electromagnetic wave is given by
$\vec{E} = E _0 \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \cos (kz + \omega t)$
At $t = 0$, a positively charged particle is at the point $(x, y , z) = \left(0, 0 , \dfrac{\pi}{k} \right)$. If its instantaneous velocity at $(t = 0)$ is $v _0 \hat{k}$, the force acting on it due to the wave is :

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. parallel to $\hat{k}$

  2. parallel to $\dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

  3. antiparallel to $\dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

  4. zero

Show answer
Correct Option: B
Explanation:

$\vec{E} = E _0 \left(\dfrac{i + j}{\sqrt{2}}\right) \cos (kz + wt)$


$\therefore$ unit vector along electric field, $\vec{E} = \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right)$


Direction of electromagnetic wave is in (-z) direction 

$\therefore \hat{C} = -\hat{k}$  wave direction.

Let $\hat{B} $ is the unit vector along the direction of magnetic field.

$\hat{B} = \hat{C} \times \hat{E} = -\hat{k} \times \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right) = - \left(\dfrac{\hat{k} \times i + \hat{k} \times j}{\sqrt{2}} \right)$

$\hat{B} = -\left(\dfrac{\hat{j} + (-i)}{\sqrt{2}} \right)  = \left(\dfrac{\hat{i} - \hat{j}}{\sqrt{2}} \right)$

$\vec{F _e} =$ electric force on the charge particle 

$\vec{F _e} = $ unit vector of electric force $= \dfrac{q \hat{E}}{|q\hat{E}|} = \hat{E}$

$\vec{F} _e = \dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

$\vec{F} _b = $ magnetic force $= q \vec{V} \times \vec{B} = q \left(V _0 \hat{k} \times \dfrac{i - j}{\sqrt{2}}\right)$

$\vec{F} _b = q V _0 \left[\dfrac{\hat{k} \times \hat{i} - \hat{k} \times \hat{j}}{\sqrt{2}} \right] = q V _0 \left[\dfrac{\hat{j} - (-\hat{i})}{\sqrt{2}}\right]$

$\vec{F} _b = q V _0 \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right)$

$\vec{F} _{Net} = \hat{F} _e + \hat{F} _b = \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} + \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} = \dfrac{2}{\sqrt{2}} (\hat{i} + \hat{j})$

$\therefore \hat{F} _{Net} = $ unit vector $= \dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

Option (B) is correct.

The Schrodinger equation for a free electron of mass m and energy E written in terms of the wave function $\psi $ is $\frac{d^2\psi}{dx^2}+\frac{8 \pi ^2mE}{h^2}\psi =0$. The dimensions of the coefficient $\psi$ of in the second term must be

physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

  1. $[M^1L^1]$

  2. $[L^2]$

  3. $[L^{-2}]$

  4. $[M^1L^{-1}T^1]$

Show answer
Correct Option: C
Explanation:

By dimensional analysis the dimensions of each term in an equation must be the same. In the first term the second derivative with respect to distance x indicates the dimensions of the coefficient $\psi$ of to be $[L^{-2}]$ and hence the answer.

- Hide questions
ADVERTISEMENT

Find more quizzes from top tags

general knowledge
3615 Quizzes
119858 Questions
 technology
307 Quizzes
36705 Questions
 sports
963 Quizzes
19148 Questions
 history
1187 Quizzes
13475 Questions
 physics
598 Quizzes
13441 Questions
 biology
1160 Quizzes
12174 Questions
 science & technology
551 Quizzes
11015 Questions
 chemistry
490 Quizzes
9892 Questions
 maths
518 Quizzes
9324 Questions
 softskills
0 Quizzes
7131 Questions