Electromagnetic Waves Notes and MCQs

Notes 13 Pages

Joint Entrance Examination (Main)

Physics

Contributed by

Yash Kuruvilla

198
SUMMARY
Page 1
199
Page 2
200
MCQ
For the answer of the following questions choose the correct alternative from among the given ones.
(1) Who produced the electromagnetic waves first ?
(A) Marconi (B) Maxwell (C) J.C. Bose (D) Hertz
(2) The dimensional formula of
o o
E is _______________
(A)
2 2
L T

(B)
2 2
L T

(C)
1 1
LT

(D)
1 1
L T

(3) A plane electromagnetic wave is incident on a material surface. The wave delivers
momentum P and energy E
(A) P=0, E

0 (B) P

0, E=0 (C) P

0, E

0 (D) P=0, E=0
(4) If
x m
V ,V and V

are the velocity of the v rays, x rays, micro waves respectively in
space, then
(A)
r x m
V V V  (B)
r x m
V V V  (C)
r x m
V V V  (D)
r x m
V V V 
(5) If ,
r x m
and
  
are the wave lengths of the r-rays, x rays and micro waves respec-
tively in space then
(A)
r x m
  
  (B)
r x m
  
  (C)
r x m
  
  (D)
r m x
  
 
(6) According to Maxwell, a changing electric field produces
(A) emf (B) Electric current
(C) magnetic field (D) radiation pressure
(7) An electromagnetic wave going through vaccum is described by E=
o
E sin(kx-
ω
t).
Which of the following is independent of the wavelength?
(A)
ω
(B) k/
ω
(C) k
ω
(D) k
(8) Which of the following have zero average value in a plane electromagnetic wave?
(A) Electric energy (B) Magnetic energy
(C) Electric field (D) None of these.
(9) If the relative permeability and dielectric constant of a given medium are equal to
r

nd K respectively, then the refractive index of the medium is equal
to__________
(A)
r
K
(B)
r o
E (C)
o o
E (D)
r
1/ k
(10) Astraonomers have found that electromagnetic waves of wavelength 21cm are
continuously reaching the Earth’s surface . Calculate the frequency of this radia-
tion. (c= 3

8
10
m/s)
(A) 14.28 GHz (B) 1.428 kHz (C) 1.428 MHz (D) 1.428 GHz
Page 3
201
(11) Electric field in an electromagnetic wave is given by E=50 sin
ω
(t-x/c) N
1
C

.
Intensity of this wave is ___________
-2
Wm
(A) 50 (B)
8
1.1 10 (C) 3.3 (D)
19
5.5 10


(12) The amplitude of the electric field in a parallel beam of light of intensity 2.0
-2
Wm
is _______
(A)
1
38.8NC

(B)
1
19.4NC

(C)
1
9.7NC

(D) None of these.
(13) Speed of electromagnetic wave is the same
(A) for all wavelengths (B) in all media
(C) for all intensities (D) for all frequencies
(14) The maximum electric field in a plane electromagnetic wave is
1
900NC

. The wave
is going in the x direction and the electric field is in the y direction. The maxi-
mum magnetic field in the wave is ____________T
(A)
8
3 10


(B)
6
3 10


(C)
6
27 10


(D)
10
27 10
(15) Electromagnetic waves are produced by
(A) a static charge (B) a moving charge
(C) an accelerating charge (D) chargeless particles
(16) Maxwells equations are derived from the laws of___________
(A) electricity (B) magnetism
(C) both electricity and magnetism (D) mechanics
(17) Which of the following electromagnetic waves has the longest wavelength?
(A) Radio waves (B) Infrared radiations
(C) x rays (D) visible rays
(18) Which of the following electromagnetic waves has the highest frequency?
(A) radio waves (B) micro waves (C) r rays (D) x rays
(19) Which of the following electromagnetic waves is used in telecommunication?
(A) radio waves (B) visible radiations
(C) ultraviolet rays (D) micro waves
(20) The maximum value of
E

in an electromagnetic waves in air is equal to
4
6.0 10


1
Vm

. The maximum value of
B

is ____________
(A)
5
1.8 10 T (B)
4
2.0 10
T (C)
12
2.0 10

 T (D)
13
1.8 10
T
(21) Dimensional formula of intensity of radiation is _____________
(A)
1 2 2
M L T

(B)
1 0 2
M L T

(C)
1 2 3
M L T

(D)
1 0 3
M L T

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202
(22) The frequency of an electromagnetic wave in free space is 3MHz. When it passes
through a medium of relative permeability
r
ε
=4.0 , then its frequency
(A) becomes half (B) become doubled
(C) remain same (D) become
2
times
(23) The frequency of electromagnetic wave having wavelength 25mm is _________ Hz
(A)
10
1.2 10
(B)
5
7.5 10
(C)
8
1.2 10
(D)
6
7.5 10
(24) Unit of energy density of electromagnetic wave is _________
(A)
3
Jm

(B)
2
Jm

(C)
2
wm

(D) None of these
(25) What is the ratio of velocities of light rays of wavelengths
0
4000
A and
0
8000
A in
vaccum ?
(A) 1:2 (B) 1:1
(C) 2:1 (D) cannot be determined
(26) Which of the following rays are not electromagnetic waves?
(A)

rays (B)

rays (C)

rays (D) heat rays
(27) A new system of unit is evolved in which the values of
0
 and
0
ε are 2 and 8
respectively. Then the speed of light in this system will be
(A) 0.25 (B) 0.5 (C) 0.75 (D) 1
(28) Our eyes respond to wavelength ranging from
(A) 400nm to 700nm (B)

to

(C) 1mm to 700nm (D)700nm to 800nm
(29) In microwave oven, we use electromagnetic oscillators which produce electro-
magnetic waves in the wavelength range
(A) 1mm to 10m (B) 0.7
m

to 1mm
(C) 0.1m to 1mm (D) 0.1
m

to 0.7
m

(30) What is the direction of
E B
 
in an electromagnetic wave?
(A) same as that of
E

(B) same as that of
B

(C) same as the direction of propagation of electromagnetic wave
(D) none of these
(31) The wavelength of x rays is of the order of
(A) 1cm (B) 1m (C) 1micron (D)1angstrom
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203
(32) A plane electromagnetic wave of frequency 25MHz travels in free space along the
x direction. At a particular point in space and time
E

= 6.3
ˆ
j
1
Vm

then
B

at this
point is__________
(A)
ˆ
2.1 10 T
8
i

 (B)
ˆ
2.1 10 T
8
k

 (C)
ˆ
1.89 10 T
9
k
(D)
ˆ
2.52 10 T
7
k


(33) The magnetic field in a plane electromagnetic wave is given B
y
=
3 11
2 10 sin (0.5 10 + 1.5 10 t)T
7
x

   . The expression for electric field is
(A)
3 11 1
60 sin(0.5 10 1.5 10 )
x
E x t Vm

   
(B)
3 11 1
60 sin(0.5 10 1.5 10 )
z
E x t Vm

   
(C)
11 3 1
60 sin(1.5 10 0.5 10 )
z
E x t Vm

   
(D)
15 11 3 1
60 10 sin(1.5 10 0.5 10 )
z
E x t Vm

    
(34) Light with an energy flux of
3 -3
w m or Wm
falls on a non-reflecting surface at
normal to surface. If the surface has an area of
2
20m
. The average force exerted
on the surface during 30 minutes is________
(A)
5
6.48 10 N
(B)
2
3.60 10 N
(C)
6
1.2 10 N


(D)
3
2.16 10 N


(35) Energy density of an electromagnetic wave of intensity
-2
0.02 Wm
is _______
(A)
11 3
6.67 10 Jm
 

(B)
6 3
6 10 Jm


(C)
10 3
1.5 10 Jm


(D) none of the above
(36) The waves used in communication are generally called
(A)

rays (B)

rays (C) microwaves (D) radiowaves
(37) For an electromagnetic wave, the phase difference between vectors
E

and
B

(far
away from the source)
(A) 0 (B)
2

(C)

(D)
3
2

(38) In an electromagnetic wave, if the amplitude of magnetic field is
10
3 10 T


, the
amplitude of the associated electric field will be________
(A)
2 1
9 10 Vm
 
 (B)
10 1
3 10 Vm
 

(C)
2 1
3 10 Vm
 

(D)
18 1
1 10 Vm
 

(39) The electric and magnetic field of an electromagnetic wave are
(A) in phase and perpendicular to each other
(B) in phase and parallel to each other
(C) in opposite phase and perpendicular to each other
(D) in opposite phase and parallel to each other
Page 6
204
(40) The direction of electric and magnetic vector for
an electromagnetic wave travelling along X-axis
are shown in fig. A and B then
(A) A is correct, B is wrong
(B) B is correct, A is wrong
(C) Both A and B are correct
(D) Both A and B are wrong
(41) The dimensions of
1

are
(A)
0 1 1
M LT

(B)
0 1 1
M L T

(C)
0 2 2
M L T

(D)
0 1 2
M LT

(42) The dimension of
r
k

is
(A)
0 1 1
M L T
 
(B)
0 1 1
M LT

(C)
0 2 2
M L T

(D)
0 0 0
M L T
(43) The frequency of light wave of wavelength
0
5000 A
is _______Hz
(A)
14
6 10
(B)
2
1.5 10


(C) 1.5 (D)
1
6 10
(44) Unit of
0
C

is same as that of
(A) current (B) resistance (C) electric charge (D) velocity
(45) The amplitude of the magnetic field part of an electromagnetic wave in vaccum is
Bm = 510 nT. Then the amplitude of the electric part of the wave is_________
(A)
11
1.53 10 /V m
(B) 1.53 V/m
(C)
2
1.53 10 /V m
(D)
8
1.53 10 /V m
(46) If the direction of magnetic field
B

at some instant is along +ve Z direction and
the electromagnetic wave is propagating along +ve X direction, then the direction
of electric field
E

at that instant is_________
(A) along -ve Y direction (B) along +ve Y direction
(C) along +ve X direction (B) along -ve X direction
(47) Relation between amplitudes of electric and Magnetic field is..............
(A)
0 0
E B (B)
0 0
E cB (C)
0
0
B
E
c

(D)
0
0
c
E
B

(48) If a source is transmitting electromagnetic wave of frequency
6
8.2 10 Hz
, then the
wavelength of the electromagnetic waves transmitted from the source will
be______
(A) 36.6m (B) 42.3m (C)40.5m (D)50.9m
E

C

B

E

C

B

Page 7
205
(49) The velocity of light in vaccum can be changed by changing____
(A) frequency (B)wavelength (C)amplitude (D) none of these
(50) An electromagnetic wave going through vaccum is described by E=Eo sin(kx-
ω
t)
then B=Bo sin(kx-
ω
t) then
(A) E
0
B
0
=
ω
k (B) E
0
k = B
0
ω
(C) E
0
ω
= B
0
k (D)none of these
(51) If the wavelength of light is
0
4000
A then the number of waves in 1mm length will
be___
(A)2.5 (B)2500 (C)250 (D)25000
(52) The SI unit of displacement current is_________
(A) coulomb (B)henry (C) ampere (D)faraday
(53) The electromagnetic waves do not transport_______
(A) energy (B) charge (C)momentum (D)information
(54) An electric charge oscillating with a frequency of 1kilo cycles/s can radiates
electromagnetic waves of wavelength
(A) 100km (B)200km (C) 300km (D)400km
(55) The frequency 1057MHz of radiation arising from two close energy levels in
hydrogen belongs to_____
(A) radio waves (B)infrared waves (C)micro waves (D)

rays
(56) Electromagnetic waves travelling in a medium which has relative permeability 1.3
and relative permeability 2.14 speed of electromagnetic waves in this medium
will be_____
(A)
8
3.6 10 /m s
(B)
8
1.8 10 /m s (C)
6
1.8 10 /m s
(D)
6
13.6 10 /m s
(57) A plane electromagnetic wave is incident on a material surface. If the wave deliv-
ers momentum p and energy E, then
(A) p=0,E=0 (B)
p 0, E 0
 
(C)
p 0, E = 0

(D) p = 0,
0E 
(58) Maxwell’s modified form of Ampere’s circuital law is____
(A)
B . dS

 

(B)
o
B . dS i



 

(C)
E
o
d
B . d +
dt
o
l i


 
 

 

(D)
q
o
d
1
B . d +
dt
o
l i




 

(59) The wavelength of x rays is of the order of
(A) 10
–3
m (B) 10
–5
m (C) 10
–10
m (D) 10
–12
m
(60) A point source of electromagnetic radiation has an average output power of 800W.
The maximum value of electric field at a distance of 4.0m from the source is
(A)
1
64.7Vm

(B)
1
57.8Vm

(C)
1
56.72Vm

(D)
1
54.77Vm

Page 8
206
(61) A plane electromagnetic wave
8 1
100 cos(6 10 4 )
z
E t x Vm

  
propagate in a medium
of refractive index
(A)1.5 (B)2.0 (C)2.4 (D)4.0
(62) A plane electromagnetic wave of wave intensity
-2
10ωm
strikes a small mirror of
area 20
2
cm
, held perpendicular to the approaching wave. The radiation force on
the mirror will be
(A)
.
11
6 6 N


(B)
11
1.33 10 N


(C)
10
1.33 10 N


(D)
10
6.6 10 N


(63) An observer is at 2m from an isotropic point source of light emitting 40w power.
The rm.s value of electric due to the source at the position of the observer
is_____
(A)
8 1
5.77 10 Vm
 

(B)
1
17.3Vm

(C)
8 1
57.7 10 Vm
 

(D)
1
1.73Vm

(64) Electromagnetic waves used in medicine to destroy cancer cells
(A) radio waves (B)infrared rays
(C)gamma rays (D)ultraviolet rays
(65) What is the name associated with the equation
E .
d
dt
dt


 
 
(A) Gauss law for electricity (B) Gauss law for magnetism
(C) ampere’s law (D)faraday’s law
(66) What oscillates in an electromagnetic wave?
(A)
E

and
B

(B)
B

(C)
E

(D)none of these
(67) Which of the following rays are not electromagnetic waves?
(A)

rays (B)

rays (C)

rays (D)heat rays
(68) The rms value of the electric field of the light coming from the sun is 720 N/c.
The average tatal energy density of the electromagnetic wave is
(A)
6 3
4.58 10 Jm
 

(B)
9 3
6.3 10 Jm
 

(C)
12 3
81.35 10 Jm
 

(D)
3 3
3.3 10 Jm
 

(69) What is the wave length of range of electromagnetic waves?
(A) 10
-8
m to 10
15
m (B) 10
-15
m to 10
8
m
(C) 10
-15
m to 10
15
m (D) 10
8
m to 10
15
m
(70) What is the wavelength range of visible light?
(A)
0
10 A to
0
100
A (B)
0
4000 A to
0
7000
A
(C)
0
8000 A to
0
10000
A (D)
0
10000 A to
0
15000
A
Page 9
207
(71) Unit of

oC is same as that of
(A)current (B)resistance (C)electric charge (D) velocity
(72) In electromagnetic spectrum, the visible light lie between
(A) radiowaves and microwaves
(B) ultraviolet rays and infrared rays
(C) ultraviolet rays and x rays
(D) infrared rays and microwaves
(73) Which of the following statements is not true in case of electromagnetic waves?
(A) they are light waves (B)theay are transverse waves
(C) propagates through space (D) they are longitudinal waves
(74) The oscillating electric and magnetic field vectors of an electromagnetic waves
far away from source are oriented along
(A) Mutually perpendicular direction and differ in phase by
0
90
(B) Mutually perpendicalar and in same phase
(C) In same direcfi on and in same phase
(D) In same divecfi on and differ in phaseby 90
o
(75) Which of the following option of electromagnetic waves is in order of
increasing frequency ?
(A) microwaves, ultraviolet rays, x rays
(B) gamma rays, ultraviolet rays, radiowaves
(C) radiowaves, visible light, infrared rays
(D) gamma rays, visible light, ultraviolet rays
(77) The sum delivers
3
10
Wm
-2
of electromagnetic flux to earth’s surface. The total
power that is incident on a roof of dimension 8m x 20m will be________
(A)
5
4 10 w
(B)
4
2.56 10 w
(C)
5
6.4 10 w
(D)
5
1.6 10 w
(77) Bolometer is used to detect ___________.
(A) infrared rays (B) ultraviolet rays (C) x rays (D)

rays
(78) Range of frequency of microwaves is about__________.
(A)530kHz to 1710kHz (B)54MHz to 890MHz
(C) 3GHz to 300GHz (D)
14
4 10 Hz
to
14
7 10 Hz
(79) SI unit of displacement current is
(A) coulomb (B)ampere (C)faraday (D)
Page 10

Electromagnetic Waves Notes and MCQs

Electromagnetic Waves Notes and MCQs

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