Name
CHAPTER 20
Class
Date
The Energy of Waves
SECTION
3 Wave Interactions
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
PS 3a
• 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.
STUDY TIP
In your science notebook,
define each new vocabulary
word. Include sketches
illustrating reflection,
refraction, diffraction, and
both kinds of interference.
Critical Thinking
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.
1. Infer How does your
reflection in a bathroom
mirror look when you raise
you right arm?
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.
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Wave Interactions continued
Why Do Waves Diffract?
READING CHECK
2. Describe What is
diffraction?
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.
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?
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?
READING CHECK
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.
4. Describe What happens
to a wave because of
refraction?
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.
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The Energy of Waves
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Wave Interactions continued
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)
Red
READING CHECK
5. Describe What does light
do when it disperses?
Yellow
Green
Violet
Glass prism
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.
TAKE A LOOK
6. Identify The order of
the dispersed colors can be
remembered by the mnemonic 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?
Waves approaching
Waves overlapping
Waves continuing
Constructive Interference When waves combine by constructive interference,
the combined wave has a larger amplitude.
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Wave Interactions continued
DESTRUCTIVE INTERFERENCE
READING CHECK
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.
9. Describe What parts of
a wave overlap during
destructive interference?
Waves approaching
Waves overlapping
Waves continuing
Destructive Interference When two waves with the same amplitude combine by
destructive interference, they cancel each other out.
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.
READING CHECK
10. Describe What causes a
standing wave?
Destructive interference
TAKE A LOOK
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 interference between the waves. Constructive
interference can be seen at points in the
wave where there is large amplitude.
11. Identify Draw two
arrows in the bottom figure
that show the locations of
destructive interference.
Remember, a standing wave only looks as if it is standing still. Waves are actually moving in two directions.
Standing waves can be formed with transverse waves or
with longitudinal waves.
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The Energy of Waves
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SECTION 3
Date
Wave Interactions continued
RESONANCE
Resonant frequencies are the frequencies at which
standing waves are created. Resonance occurs when two
objects naturally vibrate at the same frequency. The resonating 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.
READING CHECK
12. Describe When does
resonance occur?
Summary of Wave Interactions
Constructive
Interference
Destructive
Interference
Standing Wave
crests and troughs
overlap such that
the wave looks to
be standing still
crest and trough of
two waves overlap
crests of two
waves overlap
Interference
Refraction
Interaction with
another wave
Interaction
between two
mediums
Wave Interactions
Interaction with a barrier
bounces
back
Reflection
TAKE A LOOK
bends
around
13. Identify What two
interactions can occur when
a wave strikes a barrier?
Diffraction
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Section 3 Review
NSES
PS 3a
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
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The Energy of Waves
Physical Science Answer Key continued
4. Mechanical waves need a medium to
5.
6.
7.
8.
9.
10.
11.
12.
7. those with the highest frequency
8. v f 30 m 20/s 600 m/s
transfer their energy, while electromagnetic
waves don’t.
transverse and longitudinal waves
at right angles to the direction of motion
The wave vibrations are at right angles to
the direction the wave is traveling.
back and forth along the path that the wave
moves
Shake the spring up and down.
Densely packed group of particles should be
circled.
water and sand in the shallow water
mechanical wave
Review
1. The amplitude is 0.5 meters.
2. Wave description
Wave energy
Wave energy
transfer
(electromagnetic
wave or
mechanical
wave)
Wave type
(transverse
wave,
longitudinal
wave, or
surface wave)
Light emitted
from a light
bulb
electromagnetic
wave
transverse wave
Sound coming
from a violin
mechanical wave
longitudinal
wave
Rock dropped
in a pond
mechanical wave
transverse wave
Low wavelength
high
SECTION 3 WAVE INTERACTIONS
1. It looks like the opposite. When you raise
2.
3.
4.
5.
6.
3. The ribbon is in the same position on the
7.
first loop of the spring.
4. Compression: Particles are crowded toward
each other.
Crest: Particles are at their highest point.
Trough: Particles are at their lowest point.
Rarefaction: Particles are spread away from
each other.
8.
9.
10.
11.
12.
SECTION 2 PROPERTIES OF WAVES
1. the maximum distance the particles of a
2.
3.
4.
5.
6.
low
(make short-wavelength waves) before
releasing the spring. The other is to make
the frequency high.
4. v f 100 m 25/s 2,500 m/s
5,000 m/s
5. _v_ _________ 40 m
f
125/s
away from the wave source. Energy is transferred from one particle to the particle next
to it.
Wave source
high
Low frequency
3. One way is to compress coils closer together
Review
1. Energy moves in the direction of the wave
2.
High amplitude
13.
wave vibrate away from their rest position
It took more energy to make the wave.
one-half wavelength
the group of waves with short wavelengths
30 hertz, or 30 Hz
0:20
your right arm, it looks like you raised your
left arm.
the bending of waves around a barrier or
through an opening
It would become straighter (have less
diffraction).
It slows down and bends to travel in a new
direction.
spreads out
A line should be drawn between red
and yellow.
interference
The medium returns to its rest, or undisturbed, position.
crests and troughs
interference between a wave and a reflected
wave
Arrows should be drawn at the two cancellation points, not at the end points.
when two objects naturally vibrate at the
same frequency
reflection and diffraction
Review
1. When the crests meet, a wave with larger
amplitude is formed. This is constructive
interference.
2. When the crest and trough meet, a wave
with smaller amplitude is formed. This is
destructive interference.
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Physical Science
Physical Science Answer Key continued
3.
Wave situation
Wave interaction
The image of an object is seen
in a mirror.
reflection
A straight pencil appears to bend
when the bottom half is placed
in a glass of water.
refraction
Two rocks dropped in the
water a meter apart produce a
large wave centered between
them.
interference
A radio turned on in one
classroom can be heard down
the hall in a second classroom.
diffraction
3. They can damage the hair cells in the
cochlea. Damaged hair cells do not grow
back, so damage can result in permanent
hearing loss.
4. The compressions are regions where
particles are packed more closely. The
rarefactions are regions where particles
are spread farther apart.
5. a medium
6. Small vibrations may not be increased
enough to be heard.
SECTION 2 PROPERTIES OF SOUND
Chapter 21 The Nature of
Sound
1. solids and liquids
2. equal to 10 compressions or rarefactions per
second
3. ultrasonic sounds
4. The source of the sound is moving relative
to the listener.
5. The waves at the rear of the car are farther
apart; they have lower frequency than sound
waves at the front of the car.
6. When energy... Amplitude...
Loudness...
SECTION 1 WHAT IS SOUND?
1. the complete back-and-forth motion of an
2.
3.
4.
5.
6.
7.
8.
9.
10.
object
Two areas where particles are clustered
close together should be circled.
The paths are the same.
any substance that a wave can travel
through; yes
tiny hair cells
The eardrum makes the hammer vibrate. This
makes the anvil vibrate and the stirrup vibrate.
Sound is made by making an object vibrate;
detecting a sound means hearing the sound.
ringing or buzzing in the ear
They can damage the inner ear and cause
permanent hearing loss.
Stay away from loud sounds; use earplugs
when needed; lower the volume when using
headphones.
Work done by the organ
Stirrup
It vibrates the oval window in
the entrance to the inner ear.
Cochlea
Vibrations of the liquid inside it
cause hair cells to bend.
Hair cells
When they bend, nerves are
stimulated to send electrical
signals to the brain.
Ear canal
It funnels sound waves into the
middle ear.
Eardrum
Membrane stretched over the
opening to the middle ear
vibrates with the sound waves.
...increases
...decreases
...decreases
...decreases
represent rarefactions.
9. The graph on the left has lower amplitude
than the graph on the right, but the wavelengths are the same.
10. The graph on the left has higher frequency
and shorter wavelength than the graph on
the right.
Review
1. The compressions and rarefactions of the
sound wave are pushed closer together. This
makes the frequency higher. Therefore, the
pitch is higher.
forth. They do not move through space but
vibrate around a resting position.
Organ in ear
...increases
7. decibel
8. Crests represent compressions, and troughs
Review
1. The particles in the air vibrate back and
2.
...increases
2.
Medium
Relative speed
A glass window
faster
A glass of lemonade
faster
Air on a cold winter day
slower
3. The drum struck with a lot of energy will
be louder. The more energy added to the
vibration, the farther the particles in the
medium move from their rest position. The
amplitude of the wave is larger, and the
sound is louder.
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Physical Science