CHAPTER 7
LESSON 1
Waves
What are waves?
Essential Questions
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before
Statement
After
• What is a wave?
• How do different types of
waves make particles of
matter move?
• Can waves travel through
empty space?
Waves carry matter as they travel from one
place to another.
Sound waves can travel where there is no
matter.
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What are waves?
Imagine a warm summer day. You are floating on a raft in
the middle of a calm pool. Suddenly, a friend does a cannonball
dive into the pool. You probably know what happens next—you
are no longer resting peacefully on your raft. Your friend’s dive
causes you to start bobbing up and down on the water.
However, after you stop moving up and down, you haven’t
moved forward or backward in the pool.
Mark the Text
Underline Main Ideas As
you read, underline the
main ideas under each
heading. After you finish
reading, review the main
ideas that you have
underlined.
Why did your friend’s dive make you move up and down?
Your friend’s jump into the water creates waves. Throwing a
stone into a calm pool of water makes waves on the water’s
surface. When the stone hits the water, the waves spread out
over the surface of the water. But what is a wave? A wave is a
disturbance that transfers energy from one place to another
without transferring matter. You moved up and down because
these waves transferred energy.
A Source of Energy
When raindrops fall into a pond they produce water waves.
The impact of the raindrops on the water is the source of energy
for these water waves. Waves transfer energy away from the
source of the energy. Light waves also spread out in all
directions from a candle flame. The burning wick is the energy
source for the light waves.
Reading Essentials
Waves 133
Energy Transfer
1. Explain What happens
to water when an object
hits it?
Direction of wave
When a diver does a cannonball into the water, the diver’s
energy transfers, or is given, to the water. The energy
transferred to the water produces waves. The waves then
transfer energy in all directions. The waves transfer energy from
the place where the diver hits the water to the place where your
raft is floating. The energy transferred by a wave lifts your raft
when the wave reaches it.
The waves created in the pool caused you to move up and
then down on your raft. However, after the waves passed, you
were in the same place in the pool. The waves didn’t carry you
along. Waves transfer energy, but they leave matter in the same
place after they pass. Because the water under
your raft was not carried along with the
waves, you remained in the same place in the
pool, as shown at left.
The raft is at rest in its initial position.
Direction of wave
A wave begins to lift the raft upward
when it reaches the raft.
The wave transfers energy to the raft
as it lifts it upward.
How do waves transfer energy without
transferring matter? Like all materials, water
is made of tiny particles. When the diver hit
the water, the impact of the diver exerted a
force on water particles. The force of the
impact transferred energy to the water by
pushing and pulling on water particles. These
particles then pushed and pulled on
neighboring water particles, transferring
energy outward from the point of impact. In
this way, the energy of the falling diver is
transferred to the water. This energy then
travels through the water, from particle to
particle, as a wave.
Mechanical Waves
Direction of wave
The wave passes the raft and continues
to move across the pool.
Direction of wave
A water wave is an example of a
mechanical wave. A wave that can travel only
through matter is called a mechanical wave.
Mechanical waves travel through solids,
liquids, and gases. Mechanical waves cannot
travel through a vacuum. A material in which a
wave travels is called a medium. Mechanical
waves can be either transverse waves or
longitudinal waves.
The raft returns to its initial position after
the wave passes.
134 Waves
Reading Essentials
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Direction of wave
How Waves Transfer Energy
Transverse Waves
You can make a wave on a rope by shaking one end of the
rope up and down, as shown in the figure above. A wave
traveling through a rope is a transverse wave. A transverse wave
is a wave in which the disturbance is perpendicular to the
direction the wave travels.
In the figure below, the dotted line shows where the rope
was before it was shaken. This is the rest position. When you
shake the rope, the particles in the rope move up and down,
and the wave moves forward or away from the source of energy.
The rope moves in a direction that is perpendicular, or at right
angles, to the direction the wave moves. All transverse waves
move like this.
Crest
Identify
2. Underline the medium
in each example.
A. When a pebble is
thrown into a pond, water
waves form.
B. Sound waves travel
through the air from a
speaker to your ear.
Direction wave moves
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Rest
position
Direction
rope moves
Trough
The highest points on a transverse wave are crests. The
lowest points on a transverse wave are troughs. As a transverse
wave moves through a rope, it makes crests and troughs in the
rope.
Reading Essentials
Waves 135
Longitudinal Waves
Explain
3. How do particles move
in a longitudinal wave?
Another kind of mechanical wave is a longitudinal (lahn juh
TEWD nul) wave. A longitudinal wave makes the particles in a
medium move parallel to the direction that the wave travels. The
figure below shows a longitudinal wave moving along a spring.
The wave travels along the spring when the hand moves back
and forth. As the wave passes, the coils of the spring move
closer together, then move farther apart, and back again. Before
a wave moves through the spring, the coils of the spring are all
the same distance apart. This is the rest position of the spring.
This changes when a wave moves through the spring. The
wave produces regions in the spring where the coils are closer
together than they are in the rest position and regions where
they are farther apart. The regions of a longitudinal wave where
the particles in the medium are closest together are
compressions. The regions of a longitudinal wave where the
particles of the medium are farthest apart are rarefactions. The
coils move parallel to the direction that the wave moves.
A back-and-forth movement of the
hand on the left produces a longitudinal
wave that travels to the right.
Rarefaction Compression
Wave motion
The wave makes the coil with the yarn
move to the right as the compression
of the wave reaches that coil.
Rarefaction Compression
Wave motion
Motion of coil
The wave makes the coil with the yarn
move to the left as the rarefaction of the
wave reaches that coil.
Rarefaction Compression
Wave motion
The coil with the yarn returns to its
original position after the wave passes.
Rarefaction Compression
Wave motion
136 Waves
Reading Essentials
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Motion of coil
Vibrations and Mechanical Waves
When you hit a drum with a drumstick, the surface of the
drum moves up and down, or vibrates. Vibrating objects are the
sources of energy that produce mechanical waves.
One Wave per Vibration Imagine that you are making a
transverse wave through a rope. Each time you move your hand
up and down, a single vibration occurs. One vibration produces
a transverse wave with one crest and one trough.
Vibrations Stop—Waves Go What happens when you stop
moving your hand? No new waves are produced. The waves that
are already moving through the rope will keep moving along the
rope.
Types of Mechanical Waves
All mechanical waves travel only in matter. Sound waves,
water waves, and waves made by earthquakes are mechanical
waves. Sound waves are longitudinal waves that travel in solids,
liquids, and gases. Water waves are a combination of transverse
waves and longitudinal waves. Earthquake waves are also called
seismic (SIZE mihk) waves. There are both longitudinal and
transverse seismic waves.
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Electromagnetic Waves
Light from the Sun is a wave, but it is not a mechanical wave.
A mechanical wave cannot travel through the space between the
Sun and Earth. Light is an electromagnetic wave. An
electromagnetic wave is a wave that can travel through empty
space and through matter.
Types of Electromagnetic Waves
Make a two-tab book to
organize information about
mechanical and
electromagnetic waves.
Mechanical
Waves
Electromagnetic
Waves
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There are other kinds of electromagnetic waves besides light
waves. Radio waves, microwaves, infrared waves, and ultraviolet
waves are electromagnetic waves. Cell phones use microwaves
that carry sounds from one phone to another. When you stand
by a fire, infrared waves striking your skin cause the warmth
you feel. Ultraviolet waves from the Sun cause sunburns.
Reading Essentials
Waves 137
Sources of Electromagnetic Waves
What do you think of when you hear the term
electromagnetic radiation, or radiant energy? You might imagine
dangerous rays that you should avoid. But you are a source of
electromagnetic waves! All matter contains charged particles
that constantly vibrate. As a result, all matter—including you—
produces electromagnetic waves, and therefore radiant energy.
Electromagnetic Waves and Objects The type of
electromagnetic waves that an object gives off depends mostly
on the temperature of the object. Objects near human body
temperatures give off mostly infrared waves. Hotter objects, such
as a piece of glowing metal, give off visible light waves as well as
infrared waves. Some animals, such as the timber rattlesnake,
have specialized detectors for perceiving the infrared waves
given off by their prey.
Electromagnetic Waves from the Sun Earth’s most
important energy source is the Sun. The Sun emits energy by
giving off electromagnetic waves. Only a tiny amount of these
waves reach Earth. Like all waves, electromagnetic waves carry
energy. Scientists often call this radiant energy. Infrared and
visible light waves carry about 92 percent of the radiant energy
that reaches Earth from the Sun. Ultraviolet waves carry about
7 percent of the Sun’s energy.
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138 Waves
Reading Essentials
Mini Glossary
compression (kum PRE shun): the region of a
medium: a material in which a wave travels
longitudinal wave where the particles in the
medium are closest together
rarefaction (rur uh FAK shun): the region of a
longitudinal wave where the particles in the
medium are farthest apart
crest: the highest point on a transverse wave
electromagnetic (ih lek troh mag NEH tik)
wave: a wave that can travel through empty
transverse (tranz VURS) wave: a wave in which
longitudinal (lahn juh TEWD nul) wave: a wave
trough (TRAWF): the lowest point on a transverse
the disturbance is perpendicular to the direction
the wave travels
space and through matter
that makes the particles of a medium move
parallel to the direction that the wave travels
wave
wave: a disturbance that transfers energy from one
mechanical (mih KA nih kul) wave: a wave that
place to another without transferring matter
can travel only through matter
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes some
of the differences between transverse waves and longitudinal waves.
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2. Use a Venn diagram to compare and contrast sound waves and electromagnetic waves.
Sound Waves
Both
Electromagnetic Waves
What do you think
Reread the statements at the beginning of
the lesson. Fill in the After column with an A
if you agree with the statement or a D if you
disagree. Did you change your mind?
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END OF
LESSON
Waves 139
CHAPTER 7
LESSON 3
Waves
Wave Interactions
Essential Questions
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before
Statement
• How do waves interact
with matter?
• What are reflection,
refraction, and diffraction?
• What is interference?
After
When light waves strike a mirror, they change
direction.
Light waves travel at the same speed in all
materials.
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Interaction of Waves with Matter
Waves interact with matter in many ways. Waves can be
reflected by matter. Waves can change direction when they
travel from one material to another. Also, as waves move
through matter, some of the energy they carry can be
transferred to the matter.
Waves also interact with each other, or work together. Two
different waves can overlap, or cross over each other. When this
occurs, a new wave forms. The new wave has different
properties from those of either original wave.
Absorption
When you shout, you create sound waves. As the waves
travel in air, some of their energy transfers to particles in the air.
The energy the waves carry decreases as they travel through
matter. Absorption is the transfer of energy by a wave to the
medium through which it travels. The amount of energy
absorbed depends on the type of wave and the material in
which it moves.
Absorption also occurs for electromagnetic waves. All
materials absorb electromagnetic waves. Darker materials absorb
more visible light waves than lighter materials.
Use an Outline As you
read, make an outline to
summarize the information
in the lesson. Use the main
headings in the lesson as
the main headings in the
outline. Complete the
outline with the information
under each heading to help
you study.
Relate
1. Give one reason why the
energy carried by sound
waves decreases as the
sound waves travel through
the air.
Reading Essentials
Waves 147
Transmission
Make a tri-fold book about
the interactions of waves
with matter: absorption,
transmission, and reflection.
Transmission Re
flectio
ption
Absor
n
Light waves are absorbed by some materials but can pass
through other materials. For example, you can see through glass,
but you cannot see through metal. Metal absorbs almost all the
energy of the visible light rays. However, light waves pass through
glass because it absorbs only a small amount of the waves’ energy.
Transmission is the passage of light through an object.
Reflection
When waves reach the surface of a material, they can also be
reflected. Reflection is the bouncing of a wave off a surface. An
object that reflects all visible light appears white. An object that
reflects no visible light appears black.
FL7_C09_022A_904406
All types of waves can reflect when they hit a surface.
Reflection causes waves to change direction. When you drop a
basketball at an angle, it bounces up at the same angle but in
the opposite direction. When waves reflect from a surface, they
change direction like a basketball bouncing off a surface.
The Law of Reflection
The direction of an incoming wave that hits a surface and the
reflected wave are related. A line that is perpendicular, at a 90°
angle, to a surface is called the normal. The angle between the
direction of the incoming wave and the normal is the angle of
incidence. The angle between the direction of the reflected wave
and the normal is the angle of reflection.
Normal
Reflected
angle
148 Waves
Incident
angle
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The law of reflection states that when a wave is reflected
from a surface, the angle of reflection is equal to the angle of
incidence. All reflected waves, including light waves, sound
waves, and water waves, obey this law. Objects that bounce off a
surface sometimes behave like waves that are reflected.
Refraction
Waves also change direction when they travel from one
material to another material. Light waves travel at different
speeds in different materials. So, when light waves travel from
one material into another, they change speed. A change in
speed causes a change in direction. Refraction occurs when a
wave changes direction because its speed changes. The greater
the change in speed, the more the wave changes direction.
light wave to change
direction when it moves
from one material into
another material?
Refraction of Seismic Waves
Waves produced by earthquakes refract as they travel
through different parts of Earth. Primary waves (P-waves) and
secondary waves (S-waves) change speed and direction
depending on the material they travel through. By comparing
these measurements to the densities of different Earth materials,
scientists have determined the composition of Earth’s layers.
2. Explain What causes a
Diffraction
Waves can also change direction as they travel by objects.
Have you ever heard people talking in a room before you got to
the open doorway? You heard some of the sound waves because
they changed direction and spread out as they traveled through
the doorway.
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What is diffraction?
Diffraction is the change in direction of a wave when it
travels by the edge of an object or through an opening.
Diffraction causes waves to travel around the edges of an object.
Diffraction also causes waves to spread out after they travel
through an opening. More diffraction occurs as the size of the
object or opening becomes similar in size to the wavelength of
the wave.
Diffraction of Sound Waves
The wavelengths of sound waves are similar in size to many
common objects. Because of this size similarity, you often hear
sound from sources that you can’t see. For example, the
wavelengths of sound waves are about the same size as the
width of a doorway. Therefore, sound waves spread out as they
travel through a doorway.
Reading Essentials
Waves 149
Identify
3. What is the effect on
diffraction as the size of an
object, or opening,
becomes similar in size to
the wavelength of the
wave?
Diffraction of Light Waves
The wavelengths of light waves are more than a million times
smaller than the width of a doorway. Light waves do not spread
out as they travel through a doorway. Because the wavelengths
of light waves are so much smaller than sound waves, you can’t
see into a room until you reach the doorway. You can hear
sounds much sooner.
Interference
Waves not only interact with matter, but they also interact
with each other. As shown in the figure below, when two waves
meet, they overlap for a while as they travel through each other.
Interference occurs when waves that overlap combine and form
a new wave. After the waves travel through each other, they
continue moving in the same direction.
Two waves approach each other from opposite directions.
Wave A
Wave B
The waves interfere with each other and form a large amplitude wave.
Wave A + Wave B
Wave B
150 Waves
Wave A
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The waves keep traveling in opposite directions after they move through each other.
Constructive and Destructive Interference
ACADEMIC VOCABULARY
When two waves meet, their crests might overlap. If this
happens, a new wave forms. This new wave has an amplitude
that is greater than either of the two original waves. This type of
interference is called constructive interference. It occurs when
crests overlap with crests and troughs overlap with troughs.
constructive (adjective)
pertaining to building or
putting parts together to
make a whole
Destructive interference occurs when a crest of one wave
overlaps the trough of another wave. The new wave that forms
has a smaller amplitude than that of the original waves added
together. If the two waves have the same amplitude, they cancel
each other when their crests and troughs overlap. Both
constructive and destructive interference are shown in the
figure below.
Wave 1 + Wave 2
Wave 1 + Wave 2
Wave 1
Wave 1
Wave 2
Wave 2
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Constructive Interference
Destructive Interference
Standing Waves
Imagine shaking one end of a rope that has the other end
attached to a wall. You will create a wave in the rope that travels
away from you. When the wave reaches the wall, it reflects off
the wall. As the wave you create and the reflected wave interact,
interference occurs. If this happens in just the right way, the two
waves interfere with each other. When they do, the wave that
forms from the combined waves seems to stand still. This is
called a standing wave. If you pluck the string of a musical
instrument and look closely, you will see what looks like a single
wave on the whole length of the string.
Reading Essentials
Waves 151
Mini Glossary
absorption (ub SORP shun): the transfer of
energy by a wave to the medium through which it
travels
diffraction (dih FRAK shun): change in direction
of a wave when it travels by the edge of an object
or through an opening
interference (ihn tur FIR unts): occurs when
reflection (rih FLEK shun): the bouncing of a wave
off a surface
refraction (rih FRAK shun): the change in
direction of a wave as it changes speed
transmission (trans MI shun): the passage of light
through an object
waves that overlap combine to form a new wave
law of reflection: states that when a wave is
reflected from a surface, the angle of reflection is
equal to the angle of incidence
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains three
possibilities for what might happen when a wave reaches a surface.
2. Complete the sentences below. Draw a model of the wave interference below each definition.
The new wave has a
­amplitude than either original wave.
Destructive Interference:
The new wave has a
­ mplitude than the sum of the original waves.
a
What do you think
Reread the statements at the beginning of
the lesson. Fill in the After column with an A
if you agree with the statement or a D if you
disagree. Did you change your mind?
152 Waves
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lesson’s resources.
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LESSON
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Constructive Interference: