Wave Energy Types and Motion

Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook 365 The Energy of Waves
SECTION
1 The Nature of Waves
The Energy of Waves
Name Class Date
CHAPTER 20
After you read this section, you should be able to answer
these questions:
• What is a wave, and how does it transmit energy?
• How do waves move?
• What are the different types of waves?
What Is Wave Energy?
A wave is any disturbance that transmits energy
through matter or empty space. Energy can be carried
away from its source by a wave. However, the material
through which the wave moves is not transmitted. For
example, a ripple caused by a rock thrown into a pond
does not move water out of the pond.
A wave travels through a material or substance called
a medium. A medium may be a solid, a liquid, or a gas.
The plural of medium is media.
Wave motion
A wave travels through the medium, but the medium does not travel. In
a pond, lake or ocean, the medium through which a wave travels is the
water. The waves in a pond travel towards the shore. However, the water
and the leaf ﬂoating on the surface only travel up and down.
How Can Waves Do Work?
As a wave travels, it does work on everything in its
path. The waves traveling through a pond do work on
the water. Anything floating on the surface of the water
moves up and down. The fact that any object on the
water moves indicates that the waves are transferring
energy. Waves can transfer energy through a medium or
without a medium.
BEFORE YOU READ National Science
Education Standards
PS 3a
STUDY TIP
As you read the section,
make a table of the types of
waves. Have columns for the
type of wave, what it moves
through, its direction of mo-
tion, and how it transmits
energy.
READING CHECK
1. Identify What does a
wave move through?
READING CHECK
2. Describe What indicates
that a water wave transfers
energy to a ﬂoating object?

SECTION 1
Name Class Date
The Nature of Waves continued
WAVES CAN TRANSFER ENERGY THROUGH A MEDIUM
When a particle vibrates (moves back and forth), it
can pass its energy to the particle next to it. The second
particle will vibrate like the first particle and may pass
the energy on to another particle. In this way, energy is
transmitted through a medium.
A particle can pass energy to the particle next to it. The particle receiving
the energy will vibrate like the first particle. This is shown by the Newton’s
pendulum above. When the moving steel ball collides with another steel
ball, its energy is given to that ball. Notice that the first ball stops, but its
energy is passed on to a third ball.
Waves that require a medium are called mechanical
waves. Mechanical waves include sound waves, ocean
waves, and earthquake waves. For example, consider a
radio inside a jar. If all of the air from inside the jar is
removed to create a vacuum, the radio can not be heard.
WAVES CAN TRANSFER ENERGY WITHOUT A MEDIUM
Waves that transfer energy without a medium are
called electromagnetic waves. Examples of electromag-
netic waves include visible light, microwaves, TV and
radio signals, and X-rays used by dentists and doctors.
Electromagnetic waves may also go through matter, such
as air, water, or glass. Light waves travel from the sun through
space toward Earth. Light waves then travel through the air
in the atmosphere to reach the surface of Earth.
To reach the Earth, light travels from the sun, through the vacuum
of space. The light then travels through the particles of the atmo-
sphere before reaching the surface of the earth.
Sun
Vacuum of space: no medium
Atmosphere:
a medium
Earth
TAKE A LOOK
3. Describe How did the
last ball in the figure on the
right gain energy?
STANDARDS CHECK
PS 3a Energy is a property
of many substances and is
associated with heat, light,
electricity, mechanical motion,
sound, nuclei, and the nature of
a chemical. Energy is transferred
in many ways.
4. Identify What kinds of
waves need a medium to
transfer their energy? What
kinds of waves don’t need
a medium to transfer their
energy?

SECTION 1
Name Class Date
What Are the Different Types of Waves?
Waves transfer energy through vibrations. However,
the way particles in a wave vibrate depends on the type
of wave. Waves are classified based on the direction in
which wave particles vibrate compared with the direc-
tion in which waves move. There are two main types of
waves, transverse waves and longitudinal waves.
TRANSVERSE WAVES
Waves in which the particles vibrate in an up-and-
down motion are called transverse waves. Particles in
a transverse wave move at right angles relative to the
direction of the wave. See the figure below. The highest
point in a transverse wave is a crest. The lowest point in
a transverse wave is a trough.
Crests
Troughs
The wave travels to the right.
The points along
the rope vibrate
up and down.
Motion of a Transverse Wave
A wave traveling down a length of rope is an example of a transverse wave.
The wave travels to the right. The particles in the medium, the rope, travel
up-and-down. The particles in the wave and the medium are moving at right
angles to each other.
All electromagnetic waves are transverse waves.
Remember, electromagnetic waves can travel through
space or through a medium. Electromagnetic waves are
transverse waves because the wave vibrations are at right
angles to the direction the wave is traveling.
READING CHECK
5. Identify What are the two
main types of waves?
READING CHECK
6. Identify What is the
direction of a transverse
wave relative to its direction
of motion?
READING CHECK
7. Describe Why is an
electromagnetic wave
identiﬁed as a transverse wave?

SECTION 1
Name Class Date
LONGITUDINAL WAVES
Waves in which the particles of the medium vibrate
back and forth along the path of the wave are called
longitudinal waves. For example, pushing together two
ends of a spring causes the coils to crowd together. When
you let go, a longitudinal wave is created in the spring
that travels along the length of the spring.
In a longitudinal wave, a compression is the location
of the crowded particles. A rarefaction is where the
particles are spread apart.
Compressions and rarefactions are similar to crests
and troughs in a transverse wave. See the figure below.
Troughs Crests
Transverse wave
Rarefactions Compressions
Longitudinal wave
Comparing Longitudinal and Transverse Waves
A wave traveling along the length of a spring is an example of a longitudinal
wave. The wave travels to the right. The particles in the medium, the spring,
move back-and-forth. The particles in wave and the medium are moving
along the same direction as each other.
The troughs and crests of a transverse wave represent an up-and-down mo-
tion around a central point. Similarly, the rarefactions and compressions of a
longitudinal wave represent a back-and-forth motion around a central point.
A sound wave is an example of a longitudinal wave.
Sound waves travel by compressions and rarefactions of
air particles.
Sound energy is carried away from a drum by a longitudinal wave
through the air.
When the drumhead moves out
after being hit, a compression is
created in the air particles.
When the drumhead moves
back in, a rarefaction is
created.
READING CHECK
8. Describe How do the
particles of a longitudinal
wave vibrate?
CriticalThinking
9. Describe How could you
produce a transverse wave in
a spring?
TAKE A LOOK
10. Identify Circle the
compression part of the
wave in the second ﬁgure.

SECTION 1
Name Class Date
SURFACE WAVE
When waves move at or near the surface between
two media a surface wave may form. For example, this
occurs when an ocean wave comes into shallow water
at the shore. Surface waves travel in both transverse and
longitudinal motion. A particle in a surface wave will
appear to move in a circular motion.
Wave Motion
Ocean waves are surface waves. A ﬂoating bottle shows the circular motion of
particles in a surface wave.
Summary of Wave Types and Their Motion Through Space
wave
mechanical wave electromagnetic wave
transverse wave longitudinal wave surface wave transverse wave
particles
travel at right
angles to
the direction
of the wave
motion
particles
travel back-
and-forth in
the direction
of the wave
motion
particles travel
both at right
angles to, and
back-and-forth in
the direction of
the wave motion
particles travel
at right angles
to the direction
of the wave
motion
needs a
medium
does not need
a medium
CriticalThinking
11. Identify An ocean wave
forms a surface wave as it
comes into shallow water.
What are the two media in-
volved in forming the surface
wave?
TAKE A LOOK
12. Identify Which type of
wave must have a medium in
order to travel?

Name Class Date
SECTION VOCABULARY
medium a physical environment in which
phenomena occur
longitudinal wave a wave in which the particles
of the medium vibrate parallel to the direction
of wave motion
transverse wave a wave in which the particles
of the medium move perpendicularly to the
direction the wave is traveling
wave a periodic disturbance in a solid, liquid, or
gas as energy is transmitted through a medium
1. Describe How does energy travel through a wave in a medium?
2. Identify Determine the method of energy transfer, and the wave type for each
wave source.
Wave source Wave energy transfer
(electromagnetic wave or
mechanical wave)
Wave type (transverse
wave, longitudinal wave,
or surface wave)
Light emitted from a light bulb
Sound coming from a violin
Rock dropped in a pond
3. Apply Concepts A ribbon is tied to the first loop of a spring as a marker. The
spring is pulled and then released to create a longitudinal wave. Where is the
ribbon after three complete vibrations?
4. Recall Label each wave part as a crest, trough, compression, or rarefaction
according to its description.
Wave Part Description
particles are crowded toward
each other
particles are at their highest
point
particles are at their lowest
point
particles are spread away from
each other
Section 1 Review NSES PS 3a

SECTION
2 Properties of Waves
The Energy of Waves
Name Class Date
CHAPTER 20
National Science
Education Standards
PS 3a
STUDY TIP
With a partner, discuss the
properties of a wave and
how they affect the energy of
a wave.
READING CHECK
1. Describe What is the
amplitude of a wave?
READING CHECK
2. Explain Why do waves of
large amplitude carry more
energy?
these questions:
• What are ways to describe a wave?
• What determines the energy of a wave?
What Is the Amplitude of a Wave?
An earthquake in the ocean can make a transverse
wave called a tsunami. The waves can get very tall as
they reach land. The amplitude of a transverse wave
is related to the height of the wave. Amplitude is the
maximum distance the particles of the wave vibrate
away from their rest position. The rest position, shown
in the figure below, is the location of the particles of the
medium before the wave gets there.
The amplitude of a transverse wave is measured from the rest position
to the crest or the trough of the wave.
Rest position
Amplitude
RELATIONSHIP BETWEEN AMPLITUDE AND ENERGY
Taller waves have larger amplitudes, and shorter
waves have smaller amplitudes. It takes more energy to
create a wave with a large amplitude. Therefore, it carries
more energy.
A wave with a larger amplitude
carries more energy.
A wave with a smaller amplitude
carries less energy.
BEFORE YOU READ

SECTION 2
Name Class Date
Properties of Waves continued
What Is the Wavelength of a Wave?
The wavelength is the distance between any point
on a wave and the identical point on the next wave. A
wavelength is the distance between two neighboring
crests or two compressions. The distance between two
troughs or two rarefactions next to each other is also a
wavelength. See the figure below.
Measuring Wavelengths
Wavelength
Wavelength
Longitudinal wave
Transverse wave
Wavelength
Wavelength
A wavelength can be measured between any two identical points on
neighboring waves.
RELATIONSHIP BETWEEN WAVELENGTH AND ENERGY
Suppose you have two waves of the same amplitude.
The wave with a shorter wavelength carries more energy
than the wave with a longer wavelength.
A wave with a short wavelength carries more energy.
A wave with a long wavelength carries less energy.
Math Focus
3. Infer What is the distance
between a crest and a trough
next to a crest? Hint: use the
ﬁgure for help.
STANDARDS CHECK
PS 3a Energy is a property of
many substances and is associated
with heat, light, electricity,
mechanical motion, sound,
nuclei, and the nature of a
chemical. Energy is transferred
in many ways.
4. Identify There are
two groups of waves with
the same amplitude. One
contains waves of short
wavelength and the other
has waves of long wave-
length. Which waves have
the most energy?

SECTION 2
Name Class Date
Properties of Waves continued
What Is the Frequency of a Wave?
The frequency of a wave is the number of waves
produced in a given amount of time. Frequency is often
expressed in hertz (Hz). One hertz equals one wave per
second (1 Hz 1/s).
Frequency is the number of
waves that pass a certain
point in a given amount
of time. Frequency can be
measured by counting the
waves that pass a certain
point in a certain amount of
time. In the image shown
here, two waves passed by
in 10 s. The frequency is
2/10 s 0.2 1/s 0.2 Hz.
RELATIONSHIP BETWEEN FREQUENCY AND ENERGY
When the amplitudes of two waves are equal, the
wave with the higher frequency has more energy.
Lower-frequency waves carry less energy.
What Is Wave Speed?
Wave speed is the speed at which a wave travels
through a medium. Wave speed is symbolized by v. The
speed of a wave is a property of the medium. Changing
the medium of a wave changes its speed. For example,
light travels faster in air than in water.
The equation for calculating speed is v f. is
the Greek letter lambda and means the wavelength. f is
the frequency of the wave.
Let’s do a problem. What is the speed of a wave whose
wavelength is 5 m and frequency is 4 Hz (or 4 1/s)?
Step 1: write the equation v l f
Step 2: replace l and f with their values
v 5 m 4 1/s 20 m/s
Math Focus
5. Determine If a source of
waves produces 30 waves per
second, what is the frequency
in hertz?
CriticalThinking
6. Applying Concepts What
would the time of the lower
clock read if the frequency of
the wave were 0.1 Hz?
READING CHECK
6. Identify When the am-
plitudes of waves are equal,
which frequency waves have
the most energy?
Math Focus
7. Calculate What is the speed
of a wave whose wavelength is
30 m and frequency is 20 Hz?
Show your work.

Name Class Date
SECTION VOCABULARY
amplitude the maximum distance that the
particles of a wave’s medium vibrate from
their rest position
frequency the number of waves produced in a
given amount of time
wavelength the distance from any point on a
wave to an identical point on the next wave
wave speed the speed at which a wave travels
through a medium
1. Apply Concepts The distance between the crest and trough of an ocean wave is
1 meter. What is the amplitude of the wave?
2. Identify Indicate whether the wave description should result in higher-energy
wave, or a lower-energy wave.
Wave description Wave energy
high amplitude
low frequency
low wavelength
3. Apply Concepts Explain how to produce a longitudinal wave on a spring that has large
energy. There are two answers; one involves wavelength and the other frequency.
4. Math Concepts What is the speed of a wave that has a wave length of 100 m and a
frequency of 25 Hz? Show your work.
5. Apply Concepts A sound wave has a frequency of 125 Hz and a speed of 5000 m/s.
What is the wavelength of the wave? Show your work.

SECTION
3 Wave Interactions
The Energy of Waves
Name Class Date
CHAPTER 20
these questions:
• How do waves interact with objects?
• How do waves behave when they move between
two media?
• How do waves interact with other waves?
Why Do Waves Reflect?
A reflection occurs when a wave bounces back after
hitting a barrier. All waves can be reflected. Light waves
reflecting off an object allow you to see that object. For
example, light waves from the sun reflecting off the moon
allow you to see the moon. Sound wave can also reflect.
Sound waves reflecting off a barrier are called an echo.
The waves in this photograph were formed by drops of water that fell
into a container of water. When the waves caused by the drops of water
hit one side of the container, they reflect off. The shape of the reflected
waves is opposite that of the waves that struck the side of the tank.
Waves are not always reflected when they hit a barrier.
Sometimes they pass through a substance. When a wave
passes through a substance, it is transmitted. Light waves
transmitted through a glass window allow light to enter a
room. Light waves transmitted through eyeglasses allow
the wearer to see through them.
National Science
Education Standards
PS 3a
STUDY TIP
In your science notebook,
define each new vocabulary
word. Include sketches
illustrating reflection,
refraction, diffraction, and
both kinds of interference.
CriticalThinking
1. Infer How does your
reflection in a bathroom
mirror look when you raise
you right arm?
BEFORE YOU READ

SECTION 3
Name Class Date
Wave Interactions continued
Why Do Waves Diffract?
Diffraction is the bending of waves around a barrier
or through an opening. Waves usually travel in a straight
line. When a wave reaches the edge of an object or an
opening in a barrier, it may curve or bend.
The amount of diffraction of a wave depends on its
wavelength and the size of the barrier opening. Sound
waves are relatively long. You can hear voices from one
classroom diffract through the opening of a door into
another classroom. Light waves are relatively short. You
cannot see who is speaking in the other classroom.
If the barrier or opening is
larger than the wavelength of
the wave, there is only a small
amount of diffraction.
If the barrier or opening is the
same size or smaller than the
wavelength of an approaching
wave, the amount of diffraction
is large.
Why Do Waves Refract?
Refraction is the bending of a wave as the wave
passes from one medium to another. The wave changes
speed as it passes from one material to the other. The
change in speed causes the wavelength to change. The
resulting wave bends and travels in a new direction.
This light wave is refracted as
it passes into a new medium.
The light wave is passing
from air into water. The wave
is refracted because the
speed of the wave changes.
READING CHECK
2. Describe What is
diffraction?
TAKE A LOOK
3. Describe Suppose the
opening in the lower ﬁgure
were made larger. What
would happen to the shape
of the diffracted wave?
READING CHECK
4. Describe What happens
to a wave because of
refraction?

SECTION 3
Name Class Date
REFRACTION OF DIFFERENT COLORS
When light waves from the sun pass through a droplet
of water in the air, the light is refracted. The different
colors of light travel at different speeds through the drop.
Therefore, the different colors are refracted by different
amounts. The light is dispersed, or spread out, into its
separate colors. The result is a rainbow.
White light
(from the sun)
Glass prism
Red
Yellow
Green
Violet
White light is separated into its component colors when it passes
through a prism. The red light is refracted the least. The violet light is
refracted the most.
What Is Wave Interference?
All matter has volume. Therefore, objects cannot be
in the same space at the same time. However, waves are
made up of energy, not matter. So, more than one wave
may be in the same space at the same time. Two waves can
meet, share the same space, and pass through each other.
When two or more waves meet and share the same
space, they overlap. Interference is the combination of
two or more waves to form a single wave.
CONSTRUCTIVE INTERFERENCE
Constructive interference occurs when the crests of
one wave overlap with the crests of another wave or
waves. The troughs of both waves will also overlap.
The energy of the waves adds together to make a
higher-energy wave. The new wave has higher crests,
deeper troughs, and, therefore, higher amplitude.
Waves continuingWaves approaching Waves overlapping
Constructive Interference When waves combine by constructive interference,
the combined wave has a larger amplitude.
READING CHECK
5. Describe What does light
do when it disperses?
TAKE A LOOK
6. Identify The order of
the dispersed colors can be
remembered by the mne-
monic ROY G. BIV. Draw a
line coming from the prism
that shows the direction that
orange would move in.
READING CHECK
7. Identify What is the
combination of two or more
waves to form a single wave
called?
TAKE A LOOK
8. Describe What does the
medium do after the waves
have overlapped and are
continuing their movement?

SECTION 3
Name Class Date
DESTRUCTIVE INTERFERENCE
Destructive interference occurs when the crests of
one wave overlap with the troughs of another wave. The
energy of the new wave is less than the energy of both
waves. The new wave has lower amplitude than the original
waves. If a crest and trough of the same amplitude meet and
cancel, the result is no wave at all.
Destructive Interference When two waves with the same amplitude combine by
destructive interference, they cancel each other out.
Waves approaching Waves overlapping Waves continuing
How Is a Standing Wave Created?
In a standing wave, the pattern of vibrations makes
it appear as if the wave is standing still. A standing wave
is caused by interference between a wave and a reflected
wave. For example, pluck a guitar string. The string
makes a standing wave like the top wave shown in the
figure below.
Destructive interference
Constructive interference
A rope vibrating at certain frequencies
can create a standing wave. The initial
wave travels down the rope and will be
reﬂected back when it reaches the end.
In a standing wave, certain parts of the
wave are always at the rest position.
This point is caused by destructive inter-
ference between the waves. Constructive
interference can be seen at points in the
wave where there is large amplitude.
Remember, a standing wave only looks as if it is stand-
ing still. Waves are actually moving in two directions.
Standing waves can be formed with transverse waves or
with longitudinal waves.
READING CHECK
9. Describe What parts of
a wave overlap during
destructive interference?
READING CHECK
10. Describe What causes a
standing wave?
TAKE A LOOK
11. Identify Draw two
arrows in the bottom ﬁgure
that show the locations of
destructive interference.

SECTION 3
Name Class Date
RESONANCE
Resonant frequencies are the frequencies at which
standing waves are created. Resonance occurs when two
objects naturally vibrate at the same frequency. The reso-
nating object absorbs energy from the vibrating object
and vibrates also. For example, when a guitar string is
plucked, the wood body vibrates at the same frequency
as the string.
When you pluck the guitar string, you hear a musical
note. The vibrating wood body makes sound waves in the
air. The sound waves that reach your ear make parts of
your ear vibrate, so you hear the sound of the note.
Refraction
Constructive
Interference
Destructive
Interference
Standing Wave
Wave Interactions
Reﬂection
Diffraction
Interference
crests of two
waves overlap
crest and trough of
two waves overlap
crests and troughs
overlap such that
the wave looks to
be standing still
Interaction with
another wave
Interaction with a barrier
Interaction
between two
mediums
bounces
back
bends
around
Summary of Wave Interactions
READING CHECK
12. Describe When does
resonance occur?
TAKE A LOOK
13. Identify What two
interactions can occur when
a wave strikes a barrier?

Name Class Date
SECTION VOCABULARY
diffraction a change in the direction of a wave
when the wave ﬁnds an obstacle or an edge,
such as an opening
interference the combination of two or more
waves that results in a single wave
reﬂection the bouncing back of a ray of light,
sound, or heat when the ray hits a surface that
it does not go through
refraction the bending of a wavefront as the
wavefront passes between two substances in
which the speed of the wave differs
resonance a phenomenon that occurs when two
objects naturally vibrate at the same frequency;
the sound produced by one object causes the
other object to vibrate.
standing wave a pattern of vibration that
simulates a wave that is standing still
1. Describe Suppose two students grab an end of a rope. Both shake the rope once
in an upward direction to create crests traveling at each other. What would you
see when the crests meet? What type of interference does this show?
2. Describe Suppose two students grab an end of a rope. Both shake the rope once,
but one makes a crest and the other a trough traveling at each other. What would
you see when the crest and trough meet? What type of interference does this show?
3. Identify Using the vocabulary words above, fill in the table below by writing
which wave interaction most likely caused the situation described.
Wave situation Wave interaction
The image of an object in a mirror
A straight pencil appears to bend
when the bottom half is placed in a
glass of water
Two rocks dropped in the water a
meter apart produce a large wave
centered between them
A radio turned on in one classroom
can be heard down the hall in a
second classroom

Wave Energy Types and Motion

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