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WAVES
23.3 Wave Motion
Sometimes your car radio fades out. Why? It’s because the radio waves are affected by objects. For
example, if you drive into a tunnel, some or all of the radio waves get blocked. In this section, you
will learn how waves move and discover what happens when they encounter objects or collide with
other waves.
When a wave encounters objects
Wave fronts A wave front is the leading edge of a moving wave and is often considered
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VOCABULARY
wave front - the leading edge of a
moving wave.
plane wave - moving waves that
have crests in parallel straight lines.
circular wave - moving waves that
have crests that form circles around a
single point where the wave began.
to be a wave crest rather than a trough. You can make waves in all shapes but
plane waves and circular waves are easiest to create and study (Figure 23.15).
The crests of a plane wave look like parallel lines. The crests of a circular
wave are circles. A plane wave can be started by disturbing water in a line. A
circular wave can be started by disturbing water at a single point.
The direction a wave The shape of the wave front determines the direction the wave moves.
moves Circular waves have circular wave fronts that move outward from the center.
Plane waves have straight wave fronts that move in a line perpendicular to the
wave fronts.
The four wave Both circular and plane waves eventually hit surfaces. Four interactions are
interactions possible when a wave encounters a surface—reflection, refraction,
diffraction, or absorption.
Figure 23.15: Plane waves move
perpendicular to the wave fronts. Circular
waves radiate outward from a single point.
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Wave interactions
Boundaries A boundary is an edge or surface where one material meets a different
material. The surface of a glass window is a boundary. A wave traveling in
the air experiences a sudden change when it encounters the boundary
between the air and the glass of a window. Reflection, refraction, and
diffraction usually occur at boundaries. Absorption also occurs at a
boundary, but happens to a greater extent within the body of a material.
Reflection When a wave bounces off an object we call it reflection. A reflected wave is
like the original wave but moving in a new direction. The wavelength and
frequency are usually unchanged. An echo is an example of a sound wave
reflecting from a distant object or wall. People who design concert halls pay
careful attention to the reflection of sound from the walls and ceiling.
VOCABULARY
reflection - the process of a wave
bouncing off an object.
refraction - the process of a wave
bending as it crosses a boundary
between two materials.
diffraction - the process of a wave
bending around a corner or passing
through an opening.
absorption - what happens when
the amplitude of a wave gets smaller
and smaller as it passes through a
material.
Refraction Refraction occurs when a wave bends as it crosses a boundary. We say the
wave is refracted as it passes through the boundary. The process of refraction
of light through eyeglasses helps people see better. The lenses in a pair of
glasses bend incoming light waves so that an image is correctly focused
within the eye.
Diffraction The process of a wave bending around a corner or passing through an
opening is called diffraction. We say a wave is diffracted when it is changed
by passing through a hole or around an edge. Diffraction usually changes the
direction and shape of the wave. When a plane wave passes through a small
hole, diffraction turns it into a circular wave (Figure 23.16). Diffraction
explains why you can hear sound through a partially closed door. Diffraction
causes the sound wave to spread out from any small opening.
Absorption Absorption is what happens when the amplitude of a wave gets smaller and
smaller as it passes through a material. The wave energy is transferred to the
absorbing material. A sponge can absorb a water wave while letting the
water pass. Theaters often use heavy curtains to absorb sound waves so the
audience cannot hear backstage noise. The tinted glass or plastic in the lenses
of your sunglasses absorbs some of the energy in light waves. Cutting down
the energy of light makes your vision more comfortable on a bright, sunny
day so you don’t have to squint!
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Figure 23.16: An illustration of
diffraction.
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Transverse and longitudinal waves
Wave pulses A wave pulse is a short “burst” of a traveling wave. A pulse can be produced
with a single up-down movement. The illustrations below show wave pulses
in springs. You can see the difference between the two basic kinds of waves—
transverse and longitudinal—by observing the motion of a wave pulse.
Transverse waves The oscillations of a transverse wave are not in the direction the wave
moves. For example, the wave pulse in the illustration below moves from left
to right. The oscillation (caused by the boy’s hand) is up and down. Water
waves are an example of a transverse wave (Figure 23.17 top).
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VOCABULARY
transverse wave - a wave is
transverse if its oscillations are not in
the direction it moves.
longitudinal wave - a wave is
longitudinal if its oscillations are in
the direction it moves.
Longitudinal waves The oscillations of a longitudinal wave are in the same direction that the
wave moves (Figure 23.17 bottom). A sharp push-pull on the end of the
spring makes a traveling wave pulse as portions of the spring compress then
relax. The direction of the compressions are in the same direction that the
wave moves. Sound waves are longitudinal waves.
Figure 23.17: Transverse and
longitudinal waves.
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Constructive and destructive interference
Wave pulses If you have a long elastic string attached to a wall, you can make a wave
pulse. First you place the free end of the string over the back of a chair. The
string should be straight so that each part of it is in a neutral position. To
make the pulse, you pull down a short length of the string behind the chair
and let go. The pulse then races away from the chair all the way to the wall.
You can see the wave pulse move on the string. Each section of string
experiences the pulse and returns to the neutral position after the wave pulse
has moved past it.
Constructive Suppose you make two wave pulses on a stretched string. One comes from
interference the left and the other comes from the right. When the waves meet, they
combine to make a single large pulse. Constructive interference happens
when waves combine to make a larger amplitude (Figure 23.18).
VOCABULARY
constructive interference when waves add up to make a larger
amplitude.
destructive interference - when
waves add up to make a smaller, or
zero, amplitude.
Figure 23.18: This is an example of
constructive interference.
Destructive There is another way to add two pulses. Sometimes one pulse is on top of the
interference string and the other is on the bottom. When these pulses meet in the middle,
they cancel each other out (Figure 23.19). One pulse pulls the string up and
the other pulls it down. The result is that the string flattens and both pulses
vanish for a moment. In destructive interference, waves add up to make a
wave with smaller or zero amplitude. After interfering, both wave pulses
separate again and travel on their own. This is surprising if you think about
it. For a moment, the middle of the cord is flat, but a moment later, two wave
pulses come out of the flat part and race away from each other. Waves still
store energy, even during destructive interference. Noise cancelling
headphones are based on technology that uses destructive interference.
Figure 23.19: This is an example of
destructive interference.
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23.3 Section Review
1. How does the motion of a plane wave differ from the motion of a circular
wave?
2. For each of the examples below, identify whether reflection, refraction,
diffraction, or absorption is happening.
a. During a total solar eclipse, the Moon is in front of the Sun but you
can still see some sunlight around the edges of the Moon.
b. The black surface of a parking lot gets hot in the summer when
exposed to sunlight.
c. The image at the right of a straw in a glass looks
funny.
d. When you look in a mirror, you can see yourself.
e. Sound seems muffled when it is occurring on the
other side of a wall.
f. Light waves bend when they move from water to
air.
g. A ball bounces back when you throw it at a wall.
3. When a wave is being absorbed, what happens to the
amplitude of the wave? Use the term energy in your explanation.
4. Compare and contrast transverse waves and longitudinal waves.
5. Two waves combine to make a wave that is larger than either wave by
itself. Is this constructive or destructive interference?
6. When constructive interference happens between two sound waves, the
sound will get louder. What does this tell you about the relationship
between amplitude and volume of sound?
7. One wave on a string is moving toward the right and another is moving
toward the left. When, they meet in the middle, half of the cycle of the
wave from the right overlaps with half of the cycle of the wave from the
left. The result is that the string gets flat when the two waves meet. What
happened? What will happen after the waves meet?
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TECHNOLOGY
Noise-Cancelling Headphones
The graphic below illustrates how
noise-cancelling headphones work.
Study the graphic and write a
description that explains why noisecancelling technology is a good way
to reduce noise. Verify your
description by doing some research
about these special headphones.
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