Download Chapter 24

Chapter 24 Electromagnetic Waves
Chapter 24
ELECTROMAGNETIC WAVES
PREVIEW
Electromagnetic waves are produced and propagated through space by vibrating electric
and magnetic fields. Light is the visible portion of the electromagnetic spectrum, which
includes many other electromagnetic waves such as radio waves, ultraviolet light, and x –
rays. All electromagnetic waves travel at 3 x 108 m/s in a vacuum. Visible light can be
dispersed into its component colors, with the longest wavelength being red, and the
shortest wavelength violet. All of the terms we apply to any waves can be applied to light
and all other electromagnetic waves, such as wavelength, polarization, diffraction, and
interference.
The content contained in sections 1, 2, 3, and 6 of chapter 24 of the textbook is included
on the AP Physics B exam.
QUICK REFERENCE
Important Terms
electromagnetic spectrum
the range of electromagnetic waves from low frequency to
high frequency, or colors when white light is passed through a prism
electromagnetic wave
a wave which is produced by vibrating charges and propagates itself through
space by the mutual generation of changing electric and magnetic fields
polarized light
light in which the electric fields are all in the same plane
ultraviolet
electromagnetic waves of frequencies higher than those of violet light
speed of light
in a vacuum, 3 x 108 m/s
Equations and Symbols
c  f
where
c = speed of light = 3 x 108 m/s
f = frequency of light
λ = wavelength of light
276
Chapter 24 Electromagnetic Waves
Ten Homework Problems
Chapter 24 Problems 2, 8, 13, 14, 15, 16, 17, 19, 31, 43
DISCUSSION OF SELECTED SECTIONS
24.1 - 24.2 The Nature of Electromagnetic Waves and The
Electromagnetic Spectrum
As we briefly discussed in the last chapter, an electromagnetic wave is a vibration of
electric and magnetic fields that move through space at an extremely high speed. The
electric and magnetic fields in an electromagnetic wave vibrate perpendicular to each
other. The electromagnetic wave spectrum, listed from lowest frequency to highest
frequency, include radio waves, microwaves, infrared, visible light, ultraviolet, x-rays,
and gamma rays. If the visible colors of light are listed from long wavelength (low
frequency) to short wavelength (high frequency), they would follow the order red,
orange, yellow, green, blue, and violet (ROYGBV).
Low freq
Long λ
High freq
Short λ
microwave
IR
UV
Radio
AM, FM, TV
X - ray
Gamma ray
VISIBLE
ROYGBV
24.3 The Speed of Light
All electromagnetic waves are a result of the same phenomena, and although they have
different wavelengths and frequencies, they all travel through a vacuum at exactly the
same speed: 3 x 108 m/s, or about 670 million miles per hour. This speed is often referred
to as the speed of light, although light is just one example of an electromagnetic wave.
More accurately, this speed is the speed of any electromagnetic wave in a vacuum. In any
case, the speed of an electromagnetic wave is given the symbol c, from the Latin word
celeritas, meaning “swift”. According to Einstein’s theory of special relativity, the speed
of an electromagnetic wave in a vacuum is constant for all observers regardless of any
motion between the source of the electromagnetic wave and the observer.
Electromagnetic waves travel at slower speeds in more dense media, such as water or
glass.
277
Chapter 24 Electromagnetic Waves
The speed, frequency, and wavelength of any electromagnetic wave traveleing through a
vacuum is given by the wave equation:
c  f
24.5 The Doppler Effect and Electromagnetic Waves
Even though the speed of an electromagnetic wave is the same for all observers, all
observers will not measure the same value for the frequency of an electromagnetic wave,
just as all listeners will not measure the same pitch from a moving sound source. The
Doppler effect applies to any wave in which there is relative motion between a source
and an observer. For visible light, for example, if a light source such as a star is moving
toward you, you would measure its frequency a little higher than if the source were at
rest, that is, the electromagnetic spectrum would be shifted toward the high frequency
end (blue). Similarly, the frequency of a light source moving away from you would
appear shifted to the low frequency end of the spectrum (red). Astronomers use the blueshift and red-shift of light to measure the speeds of stars.
24.6 Polarization
Light is a transverse vibration of electric and a magnetic fields. If you could watch light
waves coming toward you, you would see that they actually vibrate in many directions.
We say the light is polarized when it is forced to vibrate in only one plane:
Light can be polarized by passing it through a polarizing lens or filter. Many sunglasses
are actually polarizing filters which will only pass certain vibrations of light.
278
Chapter 24 Electromagnetic Waves
CHAPTER 24 REVIEW QUESTIONS
For each of the multiple choice questions below, choose the best answer.
1. Which of the following best describes
the vibration of an electromagnetic
wave?
(A) Electric and magnetic fields vibrate
perpendicular to one another.
(B) Electric and magnetic fields vibrate
parallel to one another.
(C) The electric field is vibrating and the
magnetic field is stationary.
(D) The magnetic field is vibrating and
the electric field is stationary.
(E) Both the electric field and magnetic
field are stationary.
4. Violet light has a frequency of 7.5 x
1014 Hz. The wavelength of violet light
is most nearly
(A) 2.25 x 1023 m
(B) 2.5 x 106 m
(C) 4 x 10-7 m
(D) 7.5 x 10-7 m
(E) 2.25 x 107 m
5. The frequency of light emitted by a
certain start is known to be 7 x 1014 Hz.
An astronomer measures the frequency
of the light from the star to be 6 x 1014
Hz. The star is
(A) moving toward the astronomer
(B) moving away from the astronomer
(C) stationary
(D) expanding
(E) orbiting Earth
2. Which of the following lists the
electromagnetic waves from longest
wavelength to shortest wavelength?
(A) x-ray, ultraviolet, visible light
(B) visible light, ultraviolet, x-ray
(C) ultraviolet, x-ray, visible light
(D) ultraviolet, visible light, x-ray
(E) x-ray, visible light, ultraviolet
3. In a vacuum, all electromagnetic
waves have the same
(A) amplitude
(B) frequency
(C) wavelength
(D) intensity
(E) speed
6. We say that light is polarized when it
(A) vibrates in all planes
(B) moves away from the observer
(C) does not vibrate
(D) vibrates in only one plane
(E) moves toward an observer
279
Chapter 24 Electromagnetic Waves
ANSWERS AND EXPLANATIONS TO CHAPTER 24 REVIEW QUESTIONS
Multiple Choice
1. A
The electric field in an electromagnetic wave vibrates perpendicular to the vibrating
magnetic field.
2. B
Visible light has the longest wavelength and lowest frequency of the three, and x-rays
have the shortest wavelength and the highest frequency of the three.
3. E
All electromagnetic waves travel at 3 x 108 m/s in a vacuum.
4. C
c
3x108 m / s
 
 4 x10 7 m
14
f 7.5x10 Hz
5. B
Since the frequency the astronomer measures is lower than the known frequency, the
light is red-shifted and the star must be moving away.
6. D
All of the light passed through a polarizing filter is blocked except for that vibrating in
one plane.
280