Multiple Choice Test For FPSC, PPSC, NTS, CSS
And Various Exams
Subject Physics Chapter # 33
ELECTROMAGNETIC WAVES
1. Select the correct statement:
A. ultraviolet light has a longer wavelength than infrared
B. blue light has a higher frequency than x rays
C. radio waves have higher frequency than gamma rays
D. gamma rays have higher frequency than infrared waves
E. electrons are a type of electromagnetic wave
ans: D
2. Consider: radio waves (r), visible light (v), infrared light (i), x-rays (x), and ultraviolet light
(u). In order of increasing frequency, they are:
A. r, v, i, x, u
B. r, i, v, u, x
C. i, r, v, u, x
D. i, v, r, u, x
E. r, i, v, x, u
ans: B
3. The order of increasing wavelength for blue (b), green (g), red (r), and yellow (y) light is:
A. r, y, g, b
B. r, g, y, b
C. g, y, b, r
D. b, g, y, r
E. b, y, g, r
ans: D
4. Of the following human eyes are most sensitive to:
A. red light
B. violet light
C. blue light
D. green light
E. none of these (they are equally sensitive to all colors)
ans: D
5. Which of the following is NOT true for electromagnetic waves?
A. they consist of changing electric and magnetic fields
B. they travel at different speeds in vacuum, depending on their frequency
C. they transport energy
D. they transport momentum
E. they can be reflected
ans: B
6. The product µ0 0 has the same units as:
A. (velocity) 2
B. (velocity) 1 / 2
C. 1 / velocity
D. 1 / velocity2
E. 1 / velocity1 / 2
ans: D
7. Maxwell’s equations predict that the speed of electromagnetic waves in free space is given by:
A. µ0 0
B. ( µ0 0 ) 1 / 2
C. 1 /µ0 0
D. 1 / ( µ0 0 ) 1 / 2
E. 1 / ( µ0 0 ) 2
ans: D
8. Maxwell’s equations predict that the speed of light in free space is
A. an increasing function of frequency
B. a decreasing function of frequency
C. independent of frequency
D. a function of the distance from the source
E. a function of the size of the source
ans: C
9. The speed of light in vacuum is about:
A. 1100 ft / s
B. 93 × 106 m / s
C. 6 × 1023 m / s
D. 3 × 1010 cm / s
E. 186 , 000 mph
ans: D
10. The Sun is about 1 . 5 × 1011 m away. The time for light to travel this distance is about:
A. 4 . 5 × 1018 s
B. 8 s
C. 8 min
D. 8 hr
E. 8 yr
ans: C
11. The time for a radar signal to travel to the Moon and back, a one-way distance of about
3 . 8 × 108 m, is:
A. 1 . 3s
B. 2 . 5s
C. 8 s
D. 8 min
E. 1 × 106 s
ans: B
12. Which of the following types of electromagnetic radiation travels at the greatest speed in
vacuum?
A. Radio waves
B. Visible light
C. X rays
D. Gamma rays
E. All of these travel at the same speed
ans: E
13. Radio waves differ from visible light waves in that radio waves:
A. travel slower
B. have a higher frequency
C. travel faster
D. have a lower frequency
E. require a material medium
ans: D
14. Visible light has a frequency of about:
A. 5 × 1018 Hz
B. 5 × 1016 Hz
C. 5 × 1014 Hz
D. 5 × 1012 Hz
E. 5 × 1010 Hz
ans: C
15. The theoretical upper limit for the frequency of electromagnetic waves is:
A. just slightly greater than that of red light
B. just slightly less than that of blue light
C. the greatest x-ray frequency
D. none of the above (there is no upper limit)
E. none of the above (but there is an upper limit)
ans: D
16. Radio waves of wavelength 3 cm have a frequency of:
A. 1 MHz
B. 9 MHz
C. 100 MHz
D. 10 , 000 MHz
E. 900 MHz
ans: D
17. Radio waves of wavelength 300 m have a frequency of:
A. 10-3 kHz
B. 500 kHz
C. 1 MHz
D. 9 MHz
E. 108 kHz
ans: C
18. If the electric field in a plane electromagnetic wave is given by Em sin[(3 ×106 m-1 ) x -?t ], the
value of is:
A. 0 . 01 rad / s
B. 10 rad / s
C. 100 rad / s
D. 9 × 1014 rad / s
E. 9 × 1016 rad / s
ans: D
19. An electromagnetic wave is generated by:
A. any moving charge
B. any accelerating charge
C. only a charge with changing acceleration
D. only a charge moving in a circle
E. only a charge moving in a straight line
ans: B
20. The separation of white light into colors by a prism is associated with:
A. total internal reflection
B. partial reflection from each surface
C. variation of index of refraction with wavelength
D. a decrease in the speed of light in the glass
E. selective absorption of various colors
ans: C
21. A ray of light in water (index n1 ) is incident on its surface (with air) at the critical angle for
total internal reflection. Some oil (index n2 ) is now floated on the water. The angle between
the ray in the oil and the normal is:
A. sin-1 (1 . 00)
B. sin-1 (1 / )n1
C. sin-1 (1 /n2 )
D. sin-1 ( n1 /n2 )
E. sin-1 ( n2 /n1 )
ans: C
22. In a plane electromagnetic wave vacuum, the ratio E/B of the amplitudes in SI units of the two fields is:
A. the speed of light
B. an increasing function of frequency
C. a decreasing function of frequency
D. v2
E. 1 / v2
ans: A
23. If the magnetic field in a plane electromagnetic wave is along the y axis and its component
is given by Bm sin( kx - t ), in SI units, then the electric field is along the z axis and its
component is given by:
A. ( cBm ) cos( kx - t )
B. - ( cBm /c ) cos( kx - t )
C. - ( cBm /c ) sin( kx - t )
D. Bm cos( kx - t )
E. ( cBm /c ) sin( kx - t )
ans: E
24. If the electric field in a plane electromagnetic wave is along the y axis and its component is given
by Em sin( kx + t ), in SI units, then the magnetic field is along the z axis and its component
is given by:
A. ( Em /c ) cos( kx + t )
B. - ( Em /c ) cos( kx + t )
C. - ( Em /c ) sin( kx + t )
D. Em cos( kx + t )
E. ( Em /c ) sin( kx + t )
ans: C
25. An electromagnetic wave is traveling in the positive x direction with its electric field along the
z axis and its magnetic field along the y axis. The fields are related by:
A. E/ x = µ0 0B/ x
B. E/ x = µ0 0B/ t
C. B/ x = µ0 0E/ x
D. B/ x = µ0 0E/ t
E. B/ x = -µ0 0E/ t
ans: E
26. If the amplitude of the electric field in a plane electromagnetic wave is 100 V / m then the
amplitude of the magnetic field is:
A. 3 . 3 × 10-7 T
B. 6 . 7 × 10-7 T
C. 0 . 27 T
D. 8 . 0 × 107 T
E. 3 . 0 × 109 T
ans: A
27. For an electromagnetic wave the direction of the vector E × B gives:
A. the direction of the electric field
B. the direction of the magnetic field
C. the direction of wave propagation
D. the direction of the electromagnetic force on a proton
E. the direction of the emf induced by the wave
ans: C
28. The dimensions of S =(1 /µ0 ) E × B are:
A. J/m2
B. J/s
C. W/s
D. W/m2
E. J/m3
ans: D
29. The time-averaged energy in a sinusoidal electromagnetic wave is:
A. overwhelmingly electrical
B. slightly more electrical than magnetic
C. equally divided between the electric and magnetic fields
D. slightly more magnetic than electrical
E. overwhelmingly magnetic
ans: C
30. At a certain point and a certain time the electric field of an electromagnetic wave is in the
negative z direction and the magnetic field is in the positive y direction. Which of the following
statements is true?
A. Energy is being transported in the positive x direction but half a cycle later, when the electric field is in the opposite direction, it will be transported in the negative x direction
B. Energy is being transported in the positive x direction and half a cycle later, when the electric field is in the opposite direction, it will still be transported in the positive x direction
C. Energy is being transported in the negative x direction but half a cycle later, when the electric field is in the opposite direction, it will be transported in the positive x direction
D. Energy is being transported in the negative x direction and half a cycle later, when the electric field is in the opposite direction, it will still be transported in the negative x direction
E. None of the above are true
ans: B
31. An electromagnetic wave is transporting energy in the negative y direction. At one point and
one instant the magnetic field is in the positive x direction. The electric field at that point and
instant is:
A. positive y direction
B. negative y direction
C. positive z direction
D. negative z direction
E. negative x direction
ans: D
32. A point source emits electromagnetic energy at a rate of 100 W. The intensity 10 m from the
source is:
A. 10 W / m2
B. 1 . 6W / m2
C. 1 W / m2
D. 0 . 024 W / m2
E. 0 . 080 W / m
ans: E
33. The light intensity 10 m from a point source is 1000 W / m . The intensity 100 m from the same
source is:
A. 1000 W / m2
B. 100 W / m2
C. 10 W / m2
D. 1 W / m2
E. 0 . 1W / m
ans: C
34. When the distance between a point source of light and a light meter is reduced from 6 . 0m to
2 . 0 m, the intensity of illumination at the meter will be the original value multiplied by:
A. 3
B. 9
C. 1 / 3
D. 1 / 9
E. 1
ans: A
35. The magnetic field in a sinusoidal light wave has an amplitude of 3 . 3 × 10-7 T. The intensity
of the wave is:
A. 1 . 7 × 10-4 W / m2
B. 13 W / m2
C. 27 W / m2
D. 1 . 0 × 105 W / m2
E. 4 . 0 × 1010 W / m
ans: B
36. A sinusoidal electromagnetic wave with an electric field amplitude of 100 V / m is incident nor-
mally on a surface with an area of 1 cm2 and is completely absorbed. The energy absorbed in
10 s is:
A. 1 . 3mJ
B. 13 mJ
C. 27 mJ
D. 130 mJ
E. 270 mJ
ans: B
37. Evidence that electromagnetic waves carry momentum is:
A. the tail of a comet points away from the Sun
B. electron flow through a wire generates heat
C. a charged particle in a magnetic field moves in a circular orbit
D. heat can be generated by rubbing two sticks together
E. the Doppler effect
ans: A
38. Light of uniform intensity shines perpendicularly on a totally absorbing surface, fully illumi-
nating the surface. If the area of the surface is decreased:
A. the radiation pressure increases and the radiation force increases
B. the radiation pressure increases and the radiation force decreases
C. the radiation pressure stays the same and the radiation force increases
D. the radiation pressure stays the same and the radiation force decreases
E. the radiation pressure decreases and the radiation force decreases
ans: D
39. Light with an intensity of 1 kW / m falls normally on a surface and is completely absorbed.
The radiation pressure is:
A. 1 kPa
B. 3 × 1011 Pa
C. 1 . 7 × 10-6 Pa
D. 3 . 3 × 10-6 Pa
E. 6 . 7 × 10-6 Pa
ans: D
40. Light with an intensity of 1 kW / m falls normally on a surface and is completely refrected. The
radiation pressure is:
A. 1 kPa
B. 3 × 1011 Pa
C. 1 . 7 × 10-6 Pa
D. 3 . 3 × 10-6 Pa
E. 6 . 7 × 10-6 Pa
ans: E
2
41. Light with an intensity of 1 kW / m falls normally on a surface with an area of 1 cm2 and is
completely absorbed. The force of the radiation on the surface is:
A. 1 . 0 × 10-4 N
B. 3 . 3 × 10-11 N
C. 1 . 7 × 10-10 N
D. 3 . 3 × 10-10 N
E. 6 . 7 × 10-10 N
ans: D
2
42. Light with an intensity of 1 kW / m falls normally on a surface with an area of 1 cm2 and is
completely reflected. The force of the radiation on the surface is:
A. 1 . 0 × 10-4 N
B. 3 . 3 × 10-11 N
C. 1 . 7 × 10-10 N
D. 3 . 3 × 10-10 N
E. 6 . 7 × 10-10 N
ans: E
43. A company claims to have developed material that absorbs light energy without a transfer of
momentum. Such material is:
A. impossible
B. possible, but very expensive
C. inexpensive and already in common use
D. in use by NASA but is not commercially available
E. a breakthrough in high technology
ans: A
44. Polarization experiments provide evidence that light is:
A. a longitudinal wave
B. a stream of particles
C. a transverse wave
D. some type of wave
E. nearly monochromatic
ans: C
45. A vertical automobile radio antenna is sensitive to electric fields that are polarized:
A. horizontally
B. in circles around the antenna
C. vertically
D. normal to the antenna in the forward direction
E. none of the above
ans: C
46. For linearly polarized light the plane of polarization is:
A. perpendicular to both the direction of polarization and the direction of propagation
B. perpendicular to the direction of polarization and parallel to the direction of propagation
C. parallel to the direction of polarization and perpendicular to the direction of propagation
D. parallel to both the direction of polarization and the direction of propagation
E. none of the above
ans: D
47. Light from any ordinary source (such as a flame) is usually:
A. unpolarized
B. plane polarized
C. circularly polarized
D. elliptically polarized
E. monochromatic
ans: A
48. The electric field in unpolarized light:
A. has no direction at any time
B. rotates rapidly
C. is always parallel to the direction of propagation
D. changes direction randomly and often
E. remains along the same line but reverses direction randomly and often
ans: D
49. If n water =1 . 50 and n glass =1 . 33, then total internal reflection at an interface between this
glass and water:
A. occurs whenever the light goes from glass to water
B. occurs whenever the light goes from water to glass
C. may occur when the light goes from glass to water
D. may occur when the light goes from water to glass
E. can never occur at this interface
ans: D
50. A clear sheet of polaroid is placed on top of a similar sheet so that their polarizing axes make an
angle of 30 with each other. The ratio of the intensity of emerging light to incident unpolarized
light is:
A. 1 : 4
B. 1 : 3
C. 1 : 2
D. 3 : 4
E. 3 : 8
ans: E
51. An unpolarized beam of light has intensity I0 . It is incident on two ideal polarizing sheets.
The angle between the axes of polarization of these sheets is ? . Find ? if the emerging light
has intensity I0 / 4:
A. sin-1 (1 / 2)
B. sin-1 (1 / v5)
C. cos-1 (1 / 2)
D. cos-1 (1 / v2)
E. tan-1 (1 / 4)
ans: D
52. In a stack of three polarizing sheets the first and third are crossed while the middle one has its
axis at 45? to the axes of the other two. The fraction of the intensity of an incident unpolarized
beam of light that is transmitted by the stack is:
A. 1 / 2
B. 1 / 3
C. 1 / 4
D. 1 / 8
E. 0
ans: D
53. Three polarizing sheets are placed in a stack with the polarizing directions of the first and third perpendicular to each other. What angle should the polarizing direction of the middle sheet make with the polarizing direction of the first sheet to obtain maximum transmitted intensity when unpolarized light is incident on the stack?
A. 0
B. 30?
C. 45?
D. 60?
E. 90?
ans: C
54. Three polarizing sheets are placed in a stack with the polarizing directions of the first and third
perpendicular to each other. What angle should the polarizing direction of the middle sheet
make with the polarizing direction of the first sheet to obtain zero transmitted intensity when
unpolarized light is incident on the stack?
A. 0
B. 30?
C. 45?
D. 60?
E. All angles allow light to pass through
ans: A
55. The relation incident = reflected , which applies as a ray of light strikes an interface between
two media, is known as:
A. Faraday’s law
B. Snell’s law
C. Ampere’s law
D. Cole’s law
E. none of these
ans: E
56. The relation n1 sin 1 = n2 sin 2 , which applies as a ray of light strikes an interface between
two media, is known as:
A. Gauss’ law
B. Snell’s law
C. Faraday’s law
D. Cole’s law
E. law of sines
ans: B
57. As used in the laws of reflection and refraction, the “normal” direction is:
A. any convenient direction
B. tangent to the interface
C. along the incident ray
D. perpendicular to the electric field vector of the light
E. perpendicular to the interface
ans: E
58. When an electromagnetic wave meets a reflecting surface, the direction taken by the reflected
wave is determined by:
A. the material of the reflecting surface
B. the angle of incidence
C. the index of the medium
D. the intensity of the wave
E. the wavelength
ans: B
59. The index of refraction of a substance is:
A. the speed of light in the substance
B. the angle of refraction
C. the angle of incidence
D. the speed of light in vacuum divided by the speed of light in the substance
E. measured in radians
ans: D
60. The units of index of refraction are:
A. m/s
B. s/m
C. radian
D. m/s2
E. none of these
ans: E
61. A ray of light passes obliquely through a plate of glass having parallel faces. The emerging ray:
A. is totally internally reflected
B. is bent more toward the normal than the incident ray
C. is bent further away from the normal than the incident ray
D. is parallel to the incident ray but displaced sideways
E. lies on the same straight line as the incident ray
ans: D
62. As light goes from one medium to another, it is bent away from the normal. Then:
A. the speed of the light has increased
B. dispersion must occur
C. the second medium has a higher index of refraction than the first
D. no change in speed has occurred
E. refraction has not occurred because refraction means a bending toward the normal
ans: A
63. A pole stands in a river, half in and half out of the water. Another pole of the same length
stands vertically on the shore at a place where the ground is level. The shadow cast by the
pole in the river on the river bottom is:
A. slightly longer than the shadow of the pole on land
B. much longer than the shadow of the pole on land
C. shorter than the shadow of the pole on land
D. shorter than the shadow of the pole on land if the Sun is high and longer if the sun is low
E. the same length as the shadow of the pole on land
ans: C
64. The index of refraction of benzene is 1 . 80. The critical angle for total internal reflection, at a
benzene-air interface, is about:
A. 56
B. 47
C. 34
D. 22
E. 18
ans: C
65. The index of refraction of a certain glass is 1 . 50. The sine of the critical angle for total internal
reflection at a glass-air interface is:
A. 0 . 50
B. 0 . 67
C. 0 . 75
D. 1 . 00
E. 1 . 50
ans: B
66. The critical angle for total internal reflection at a diamond-air interface is 25 . Suppose light
is incident at an angle of ? with the normal. Total internal reflection will occur if the incident
medium is:
A. air and =25
B. air and > 25
C. air and < 25
D. diamond and < 25
E. diamond and > 25
ans: E
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