Deck 22: Electromagnetic Waves

Question 1

What color would an EM wave with a frequency of 430 THz be?

A) blue
B) green
C) yellow
D) red
E) not in the visible range

Answer 1

red

Question 2

In what spectral range are black lights that cause materials to fluoresce in the visible?

A) x-ray
B) infrared
C) ultraviolet
D) radio
E) microwave

Answer 2

ultraviolet

Question 3

ε_o μ_o = ?

A) c
B) c²
C) c⁻¹
D) c⁻²
E) c^½

Answer 3

c⁻²

Question 4

What is a light-year in SI units?

A) 3.0 × 10¹⁵ m
B) 9.5 × 10¹⁵ m
C) 3.0 × 10¹¹ m
D) 1.1 × 10¹¹ m
E) 4.5 × 10³⁷ m

Question 5

An electromagnetic wave with a wavelength about the same as the diameter of an apple would be

A) an x-ray.
B) UV.
C) infrared.
D) a microwave.
E) a radio wave.

Question 6

Light of wavelength 600 nm in air enters a medium in which the index of refraction is 1.73. What is the frequency of this light in the medium?

A) 5.00 × 10¹⁴ Hz
B) 2.89 × 10¹⁴ Hz
C) 8.65 × 10¹⁴ Hz
D) 3.46 × 10¹⁴ Hz
E) 1.04 × 10¹⁴ Hz

Question 7

Wavelengths just shorter than visible light are classified as

A) gamma rays.
B) x-rays.
C) microwaves.
D) ultraviolet.
E) radio waves.

Question 8

The longest wavelengths of visible light are in the color

A) red.
B) orange.
C) green.
D) blue.
E) violet.

Question 9

How long does it take for a radio signal to travel to a spacecraft 1.5 × 10¹¹ m away?

A) 5.0 s
B) 50 s
C) 5.0 × 10² s
D) 5.0 × 10³ s
E) 1.0 × 10³ s

Question 10

An electromagnetic wave is created by

A) an electrical charge moving at a certain velocity.
B) all electric charges.
C) a magnet moving with a certain velocity.
D) an accelerating electric charge.

Question 11

The background radiation in space corresponds to blackbody radiation of what temperature?

A) 0 K
B) 2.7 K
C) 273 K
D) 5800 K
E) 98.6°F

Question 12

The wavelengths associated with thermal radiation at everyday temperatures are in which spectral region?

A) infrared
B) ultraviolet
C) shorter visual
D) longer visual
E) x-ray

Question 13

The electromagnetic spectrum arranged by increasing wavelength is

A) radio waves, gamma rays, x-rays, visible light, infrared, microwave, ultraviolet.
B) x-rays, gamma rays, ultraviolet, visible light, infrared, microwave, radio waves.
C) infrared, gamma rays, x-rays, ultraviolet, visible light, microwave, radio waves.
D) gamma rays, x-rays, ultraviolet, visible light, infrared, microwave, radio waves.

Question 14

Your favorite radio station is located due north of your house and broadcasts using a horizontal electric dipole antenna directed north-south. To receive this signal you need to

A) move to a town that is farther to the east or west.
B) use a magnetic dipole antenna instead of an electric dipole antenna.
C) orient the receiving antenna horizontally north-south.
D) use a vertical receiving antenna.

Question 15

Electromagnetic waves of wavelength 1000 nm are classified as

A) radio waves.
B) microwaves.
C) infrared.
D) x-rays.
E) gamma rays.

Question 16

Light of 10 cm wavelength is classified as what type of radiation?

A) gamma ray
B) x-ray
C) UV
D) microwave
E) visible

Question 17

Radio waves were discovered by

A) Hertz.
B) Maxwell.
C) Penzias.
D) Dumont.
E) Marconi.

Question 18

Electromagnetic wavelengths of a few centimeters are classified as what type of radiation?

A) visible light
B) ultraviolet
C) microwaves
D) radio waves
E) gamma rays

Question 19

A dipole radio transmitter has its rod-shaped antenna oriented vertically. At a point due south of the transmitter, the radio waves have their magnetic field

A) oriented in any horizontal direction.
B) oriented east-west.
C) oriented vertically.
D) oriented north-south.

Question 20

The highest frequency in visible light is associated with the color

A) red.
B) orange.
C) green.
D) blue.
E) violet.

Question 21

What index of refraction halves the wavelength that light has in a vacuum?

A) 1.33
B) 1.50
C) 1.41
D) 2.00
E) 5.00

Question 22

If light has a wavelength of 600 nm in a vacuum, what is its wavelength in a material with an index of refraction of 1.50?

A) 600 nm
B) 900 nm
C) 400 nm
D) 1.35 × 10³ nm
E) 267 nm

Question 23

In vacuum, the components of an EM wave are E_y = (50 V/m) cos [(5.00 m⁻¹)x + ωt], E_x = 0, and E_z = 0. In what direction is this wave moving?

A) positive x-direction
B) negative x-direction
C) positive y-direction
D) negative y-direction
E) positive or negative z-direction

Question 24

What is the wave number for a wavelength of 600 nm?

A) 600 nm
B) 1.67 × 10⁶ m⁻¹
C) 1.05 × 10⁷ m⁻¹
D) 9.55 × 10⁻⁸ m
E) 9.55 × 10⁻⁸ m⁻¹

Question 25

Which of the following is the direction of an electromagnetic wave having electric and magnetic field vectors E and B?

A) E
B) B
C) E + B
D) E × B
E) B × E

Question 26

What is the wavelength from a radio station having frequency 108 MHz?

A) 2.78 m
B) 27.8 m
C) 32.3 m
D) 323 m
E) 108 m

Question 27

The Sun is 1.50 × 10⁸ km from the Earth. How long does it take for light to reach us from the Sun?

A) 2.00 ms
B) 2.00 s
C) 0.50 s
D) 8.33 min
E) almost instantaneous

Question 28

In vacuum, the components of an EM wave are E_y = (50 V/m) cos [(5.0 m⁻¹)x + ωt], E_x = 0, and E_z = 0. What is the amplitude of the magnetic field for this wave?

A) 5.00 × 10⁻⁷ T
B) 1.67 × 10⁻⁷ T
C) 2.00 × 10⁻⁷ T
D) 1.50 × 10⁻⁶ T
E) 1.33 × 10⁻⁶ T

Question 29

When light passes from one medium into another with a different index of refraction, which of the following does or do NOT change?

A) the speed
B) the wavelength
C) the frequency
D) the speed and the wavelength
E) the frequency and the wavelength

Question 30

If the wavelength of light is 400 nm in a substance where the speed of light is 2.35 × 10⁸ m/s, what is the wavelength of this light in air?

A) 313 nm
B) 400 nm
C) 511 nm
D) 635 nm
E) 940 nm

Question 31

What is the speed of light in a material that has an index of refraction of 2.0?

A) 6.0 × 10⁸ m/s
B) 1.5 × 10⁸ m/s
C) 2.0 × 10⁸ m/s
D) 5.0 × 10⁸ m/s
E) No such index can exist.

Question 32

The speed of an electromagnetic wave in vacuum depends on

A) the amplitude of the electric and magnetic fields.
B) the frequency of the wavelength.
C) the angle between the electric and magnetic fields.
D) none of these choices are correct.

Question 33

Approximately how long does it take for light to travel one foot?

A) 1 ms
B) 1 μs
C) 1 ns
D) 1 ps
E) 1 fs

Question 34

Household wiring can emit electromagnetic waves of frequency 60 Hz. What is the wavelength of these waves?

A) 5.0 × 10² m
B) 5.0 × 10³ m
C) 5.0 × 10⁴ m
D) 5.0 × 10⁵ m
E) 5.0 × 10⁶ m

Question 35

If the rms value of the electric field is 9.0 V/m at a distance of 6.0 m from an isotropic source, what is the rms value of the magnetic field at a distance of 12 m from the source?

A) 3.0 × 10⁻⁸ T
B) 1.5 × 10⁻⁸ T
C) 0.75 × 10⁻⁸ T
D) 0.38 × 10⁻⁸ T
E) 0.19 × 10⁻⁸ T

Question 36

In vacuum, the components of an EM wave are E_y = (50 V/m) cos [(5.00 m⁻¹)x + ωt], E_x = 0, and E_z = 0. What is ω?

A) 1.50 × 10⁹ rad/s
B) l.20 × 10⁹ rad/s
C) 1.00 × 10⁹ rad/s
D) 8.00 × 10⁸ rad/s
E) More information is needed.

Question 37

Light passes from medium one (v₁) to medium two (v₂). If v₁ < v₂ then at the boundary

A) f increases, λ decreases.
B) f does not change, λ increases.
C) f decreases, λ increases.
D) f does not change, λ decreases.

Question 38

At a distance of 10 m from a light bulb, the rms value of the electric field is 16 V/m. What is the rms value of the electric field at a distance of 20 m from the light bulb?

A) 32 V/m
B) 16 V/m
C) 8.0 V/m
D) 4.0 V/m
E) 2.0 V/m

Question 39

How far does light travel in a century?

A) 9.5 × 10²⁰ m
B) 9.5 × 10¹⁷ m
C) 9.5 × 10¹⁵ m
D) 9.5 × 10¹⁴ m
E) 9.5 × 10¹² m

Question 40

In vacuum, the components of an EM wave are E_y = (50 V/m) cos [(5.00 m⁻¹)x + ωt], E_x = 0, and E_z = 0. What is the wavelength of the wave?

A) 5.00 m
B) 0.200 m
C) 1.26 m
D) 2.52 m
E) 0.796 m

Question 41

Plane-polarized light of intensity I₀ is sent through two polarizers, the first of which is rotated 45° clockwise relative to the original polarization, while the second is rotated 45° counterclockwise relative to the original polarization. What is the transmitted intensity?

A) 0.71 I₀
B) 0.50 I₀
C) 0.35 I₀
D) 0.25 I₀
E) 0

Question 42

Unpolarized light of intensity I₀ is sent through an arrangement of 4 polarizers. Each polarizer (after the first) is rotated 30° relative to the preceding one, so that the last polarizer is perpendicular to the first. What is the intensity transmitted by this system?

A) 0.75 I₀
B) 0.50 I₀
C) 0.42 I₀
D) 0.21 I₀
E) 0

Question 43

If unpolarized light with E_rms = 20 V/m is passed through a polarizer, what is the transmitted E_rms?

A) 20 V/m
B) 14 V/m
C) 10 V/m
D) 7.1 V/m
E) 5.0 V/m

Question 44

The direction of polarization of an electromagnetic wave is taken by convention to be

A) the electric field direction.
B) the magnetic field direction.
C) the direction of E × B.
D) perpendicular to the electric field direction.
E) None of these choices are correct.

Question 45

Using the approximate formula for the Doppler shift, f_o ≈ f_s (1 + v_rel/c) one finds that a source is moving away at 0.60 c. What result would the exact formula yield for the recession speed?

A) 0.84 c
B) 0.72 c
C) 0.60 c
D) 0.54 c
E) 0.40 c

Question 46

At the summer solstice in June, at what latitude is the Sun directly overhead at solar noon?

A) 0.0° (the equator)
B) 23.5° N
C) 23.5° S
D) 66.5° N
E) 40.0° N

Question 47

For electromagnetic radiation of intensity of 1.00 W/m², what is the rms value of the electric field?

A) 19.4 V/m
B) 9.69 V/m
C) 6.45 × 10^-8 V/m
D) 9.11 × 10^-8 V/m
E) 3.00 × 10⁷ V/m

Question 48

If you are facing north with the Sun directly overhead, the light coming to your eyes from the sky just above the horizon is

A) partially polarized east-west.
B) partially polarized north-south.
C) randomly polarized.
D) partially polarized up-down.

Question 49

Randomly polarized light of intensity I₀ is passed through two polarizers whose transmission axes differ by 45°. What is the intensity of the light that has passed through both polarizers?

A) 0.70 I₀
B) 0.50 I₀
C) 0.35 I₀
D) 0.25 I₀
E) 0.00

Question 50

A 20 W laser beam, with an initial diameter of 2.0 mm, spreads out to a diameter of 2.0 m after traveling 10,000 m. What is the initial E_rms of the beam?

A) 4.9 × 10⁴ V/m
B) 2.5 × 10³ V/m
C) 1.2 × 10³ V/m
D) 1.9 × 10² V/m
E) 1.7 × 10² V/m

Question 51

Light from an astronomical source is found to be shifted to a frequency one percent lower than had the source been at rest. What relative velocity could cause such a shift?

A) 3.0 × 10⁷ m/s moving away from the observer
B) 3.0 × 10⁶ m/s moving away from the observer
C) 3.0 × 10⁶ m/s moving toward the observer
D) 1.5 × 10⁶ m/s moving toward the observer
E) 1.5 × 10⁶ m/s moving away from the observer

Question 52

Plane-polarized light of intensity I₀ is passed through two polarizers, the first one at 45° to the original polarization direction and the second rotated an additional 45° so that it is perpendicular to the original polarization. What intensity is transmitted?

A) 0.50 I₀
B) 0.35 I₀
C) 0.25 I₀
D) 0.13 I₀
E) 0

Question 53

How fast would you have to drive in order to see a red light as green? Assume λ = 630 nm for red and λ = 530 nm for green.

A) 5 × 10⁷ m/s
B) 5 × 10⁶ m/s
C) 5 × 10⁵ m/s
D) 5 × 10⁴ m/s

Question 54

Plane-polarized light of intensity I₀ is passed through a polarizer oriented at 45° to the original plane of polarization. What is the intensity transmitted?

A) 0.70 I₀
B) 0.50 I₀
C) 0.35 I₀
D) 0.25 I₀
E) 0.00

Question 55

Radiation of intensity 1.00 kW/m² is incident on an area perpendicular to the incoming light. The area is now rotated 20° about an axis perpendicular to the incoming light. What is the resulting power per unit area now reaching the surface?

A) 1.00 kW/m²
B) 0.940 kW/m²
C) 0.880 kW/m²
D) 0.800 kW/m²
E) 0.340 kW/m²

Question 56

What is the intensity of the electromagnetic radiation 2.00 m from a 100 W isotropic source?

A) 25.0 W/m²
B) 6.25 W/m²
C) 4.00 W/m²
D) 1.99 W/m²
E) 1.00 W/m²

Question 57

If a 20 W laser beam, which has an initial diameter of 2.0 mm, spreads out to a diameter of 2.0 m after traveling 10,000 m, what is the final intensity of the beam?

A) 20 × 10⁻⁴ W/m²
B) 20 × 10⁻⁶ W/m²
C) 20 × 10⁻⁸ W/m²
D) 6.4 W/m²
E) 6.4 × 10⁻⁶ W/m²

Question 58

A 20 W laser beam, with an initial diameter of 2.0 mm, spreads out to a diameter of 2.0 m after traveling 10,000 m. What is the initial intensity of the beam?

A) 20 × 10⁶ W/m²
B) 6.4 × 10⁶ W/m²
C) 3.2 × 10⁶ W/m²
D) 2.0 × 10⁵ W/m²
E) 1.0 × 10⁵ W/m²

Question 59

Light of wavelength 661.1 nm is received from a star that is emitting light of wavelength 659.6 nm. How fast is the star moving relative to the Earth, and is it moving toward or away?

A) 580 km/s away
B) 580 km/s toward
C) 680 km/s away
D) 680 km/s toward

Question 60

If the intensity of radiation from the Sun is 1.4 kW/m² at a distance of 150 × 10⁶ km, at what distance is the intensity 5.6 kW/m²?

A) 300 × 10⁶ km
B) 100 × 10⁶ km
C) 75 × 10⁶ km
D) 38 × 10⁶ km
E) less than 30 × 10⁶ km

Question 61

A circularly polarized EM wave has an electric field magnitude 3.0 mV/m. If the wave passes through a polarizer, what will be the amplitude of the electric field in the transmitted wave?

A) 3.0 mV/m
B) 2.1 mV/m
C) 1.5 mV/m
D) 0

Question 62

What is the smallest possible value for an index of refraction?

A) 0
B) 0.5
C) 1
D) just less than 1
E) (1/2)^1/2

Question 63

The Sun has a radius of 6.955 × 10⁸ km and a rotational period of 25.38 days. What is the difference in the wavelength of the peak of the observed spectrum between light received from the approaching side of the Sun and from the receding side of the Sun? Observations of the middle of the Sun place the spectral peak at 480 nm.

A) 13 nm
B) 6.4 nm
C) 1.6 nm
D) 3.2 nm

Question 64

It is possible to detect Doppler shifts in the light emitted from each side of a rapidly spinning star. Suppose the light from a nearby star of radius 2.5 × 10⁸ km is blue-shifted on one side of the star and red-shifted on the other side. Assume the center of mass of the star is stationary relative to Earth. If the star emits light of wavelength 525 nm that is shifted, in the received light, to 523 nm on one side of the star and to 527 nm on the other side, what is the rotational period of the star?

A) 34 days
B) 32 days
C) 24 days
D) 16 days

Question 65

What total power is radiated by a light bulb that provides 12 mW on a 15 cm × 25 cm sheet of paper, which is 1.7 m from the bulb? Assume that the paper is oriented with its surface perpendicular to the light rays.

A) 2.9 W
B) 12 W
C) 1.7 W
D) 6.6 W

Question 66

Nexrad radar uses electromagnetic radiation of frequency of 2.7 GHz to measure the speed of cloud systems. Radio waves from a transmitter are sent outward toward a cloud system, and the waves are reflected back toward the source. If the cloud system is moving, the reflected waves will be Doppler shifted. Suppose a Nexrad system observes clouds moving toward the transmitter at 21 m/s. What is the difference between the transmitted and received frequencies of the radar in this case?

A) 380 Hz
B) 760 Hz
C) 570 Hz
D) 190 Hz

Question 67

If the distance from a point source of EM radiation is doubled, the intensity is

A) doubled
B) the same as it was before.
C) reduced by a factor of four.
D) increased by a factor of four.
E) cut in half.

Question 68

At the Earth's surface, the intensity of solar radiation is approximately 1000 W/m². At a latitude of 44.5 degrees north, when the Sun is highest in the sky on the winter solstice, the Sun's rays are incident at an angle of 68.0 degrees from the vertical. What power is incident upon a 1.00 m × 1.50 m window in a vertical, south-facing wall at this time?

A) 1390 W
B) 927 W
C) 562 W
D) 375 W

Question 69

You wish to read by the light of a lamp with a single lightbulb at your desk. In order to get the same brightness on the page from a 25 W bulb as you can from 100 W bulb, how much closer do you need to bring the lamp to your book? Assume that the light bulbs are equally efficient.

A) 1/2 the distance
B) 1/8 the distance
C) 1/4 the distance
D) 1/16 the distance

Question 70

The rms electric field strength at a distance r from a 25 W light source is found to be 4.33 N/C. What is the distance r?

A) 22 m
B) 6.3 m
C) 30 m
D) 40 m

Question 71

A distant star emits light of wavelength 620 nm. As measured by an Earth observer, the light is shifted to 635 nm. Assuming the star is moving either away (positive radial velocity) or toward (negative radial velocity) Earth, what is its radial velocity relative to Earth?

A) −0.11c
B) −0.024c
C) 0.11c
D) 0.024c

Question 72

Light of intensity I₀ passes through a polarizer at normal incidence and the intensity of the transmitted light is observed to be ½ I₀. It is further found that the intensity does not change as the polarizer's transmission axis is rotated. Which of the following could be the polarization state of the incident light?

A) elliptical
B) linear
C) circular
D) unpolarized
E) more than one of the above

Question 73

A distant star emits light with a peak intensity at 620 nm. If the star is moving toward Earth at 0.024c, what is the wavelength of the peak intensity as measured by an Earth observer?

A) 605 nm
B) 591 nm
C) 650 nm
D) 635 nm

Question 74

At the Earth's orbital distance from the Sun, the intensity of solar radiation is approximately 1400 W/m2. What is the rms electric field?

A) 726 V/m
B) 363 kV/m
C) 527 kV/m
D) 513 kV/m

Question 75

At the Earth's orbital distance from the Sun, the intensity of solar radiation is approximately 1400 W/m². What is the intensity at Mercury's orbital distance, which is 38.7% that of Earth?

A) 2280 W/m²
B) 3800 W/m²
C) 9300 W/m²
D) 540 W/m²

Question 76

A circularly polarized EM wave with intensity I₀ is incident on a polarizer. What is the intensity of the transmitted wave?

A) I₀
B) 0.71 I₀
C) 0.5 I₀
D) 0

Question 77

A radio station utilizes a horizontal electric dipole antenna directed north-south as a transmitter. You are located due west of the station and wish to use a magnetic dipole antenna to receive the broadcast. What should the orientation of your antenna be?

A) axis horizontal east-west
B) axis horizontal in any direction
C) axis horizontal north-south
D) Unable to answer given the information.
E) axis vertical

Question 78

In an unpolarized EM wave, the electric field direction is

A) perpendicular to the magnetic field direction
B) parallel to the magnetic field direction.
C) perpendicular to the direction of wave propagation, but randomly oriented with respect to the magnetic field direction.
D) parallel to the direction of wave propagation.