Waves
Waves
• The Nature of
Waves
– What’s in a Wave
– Waves and Energy
– Mechanical Waves
What is a Wave?
• A wave is a repeating disturbance or
movement that transfers energy through
matter or space.
• Important: Waves transfer energy. The
evidence is that they can do mechanical work
(or destruction) or cause things to become
warmer (transferring energy as heat).
• Example: During Earthquakes, energy is
transferred in powerful waves that travel
through Earth. Light is a type of wave that
can travel through empty space to transfer
energy from one place to another, such as
from the Sun to Earth.
What’s in a Wave?
•Have you ever "done the wave" as part of a large crowd at a
football or baseball game?
•A group of people jumps up and sits back down, some nearby
people see them and they jump up, some people further away
follow suit and pretty soon you have a wave travelling around
the stadium.
•The wave is the disturbance (people jumping up and sitting
back down), and it travels around the stadium. However,
none of the individual people in the stadium are carried
around with the wave as it travels - they all remain at their
seats.
Waves and Energy
• Suppose a pebble falls
into a pool of water and
ripples form.
• Because it is moving, the
falling pebble has
energy.
• As it splashes into the
pool, the pebble
transfers some of its
energy to nearby water
molecules, causing them
to move.
• What you see is energy
traveling in the form of
a wave on the surface of
the water.
Waves and Matter
• Imagine you’re in a boat
on a lake.
• Approaching waves bump
against your boat, but
they don’t carry it along
with them as they pass.
• The waves don’t even
carry the water along
with them. Only the
energy carried by the
waves moves forward.
• All waves have this
property - they carry
energy without
transporting matter
from place to place.
Making Waves
• A wave will travel only
as long as it has
energy to carry.
• Suppose you are
holding a rope at one
end, and you give it a
shake.
• You would create a
pulse that would
travel along the rope
to the other end, and
then the rope would
be still again.
Making Waves (2)
• It is the up-and-down
motion of your hand
that creates the wave.
• Anything that moves up
and down or back and
forth in a rhythmic way
is vibrating.
• The vibrating movement
of your hand at the end
of the rope created the
wave. In fact, all waves
are produced by
something that vibrates.
Mechanical Waves
• Most waves travel through a medium.
• A medium is the matter through which a wave
travels.
• Sound waves travel through air to reach your
ears. Ocean waves travel through water to
reach the shore. In both cases, the matter
the wave traveled through is the medium.
• The medium can be a solid, a liquid, a gas, or a
combination of these.
• For sound waves, the medium is air, and for
ocean waves, the medium is water.
Mechanical Waves (2)
• Waves that require a medium are called
mechanical waves.
• Mechanical waves can only travel through
matter.
• There are two types of mechanical waves:
transverse waves and compressional waves.
• Light does not require a medium. Light waves
are also called electromagnetic waves.
• An electromagnetic wave is a wave caused by
a disturbance in electric and magnetic fields
and does not require a medium.
Transverse Waves
• In a transverse
wave, matter in the
medium moves back
and forth at right
angles to the
direction that the
wave travels.
• For example, a water
wave travels
horizontally as the
water moves
vertically up and
down.
Transverse Waves (2)
The animation below shows a one-dimensional transverse plane wave
passing from left to right. The particles do not move along with the wave;
they simply vibrate up and down about their individual positions as the
wave passes by.
Pick a single particle and watch its motion.
Compressional Waves
• Compressional waves
are also called
longitudinal waves.
• In a compressional
wave, matter in the
medium moves back
and forth along the
same direction that
the wave travels.
• You can model
compressional waves
with a coiled spring
toy.
Compressional Waves (2)
The animation below shows a one-dimensional compressional wave moving
down a tube. The particles do not move down the tube with the wave; they
simply vibrate back and forth about their individual positions.
Pick a single particle and watch its motion. The wave is seen as the motion of
the compressed region which moves from left to right.
Sound Waves
• Sound waves are
compressional waves.
• When a noise is made,
such as when a locker door
slams shut and vibrates,
nearby air molecules are
pushed together by the
vibrations.
• The air molecules are
squeezed together like
the coils in a coiled spring
toy are when you make a
compressional wave with
it.
• The compressions travel
through the air to make a
wave.
Sound in Other Materials
• Sound waves also can travel through
other mediums, such as water and wood.
• When a sound wave reaches your ear, it
causes your eardrum to vibrate.
• Your inner ear then sends signals to
your brain, and your brain interprets
the signals as sound.
Water Waves
• Water waves are not purely transverse
waves.
• A water wave causes water to move
back and forth, as well as up and down.
• Water is pushed back and forth to form
the crests and troughs.
Water Waves (2)
• The low point of a water
wave is formed when
water is pushed aside
and up to the high point
of the wave.
• The water that is
pushed aside returns
to its initial position.
Water Waves (3)
The wave below is simulation of a water wave.
As a wave travels through the water, the particles travel in
clockwise circles.
The simulation below shows a water wave travelling from left to
right.
The two particles in blue show that each particle indeed travels in
a clockwise circle as the wave passes.
Water Waves (4)
• Ocean waves are formed
most often by wind
blowing across the
ocean surface.
• The size of the waves
that are formed depend
on the wind speed, the
distance over which the
wind blows, and how long
the wind blows.
Seismic Waves
• Forces in Earth’s
crust can cause
regions of the crust
to shift, bend, or
even break.
• The breaking crust
vibrates, creating
seismic (SIZE mihk)
waves that carry
energy outward.
Seismic Waves (2)
• Seismic waves are a
combination of
compressional and
transverse waves. They
can travel through
Earth and along Earth’s
surface.
• The more the crust
moves during an
earthquake, the more
energy is released.
The Parts of a Wave
• A transverse wave
has alternating high
points, called crests,
and low points, called
troughs.
Parts of a Wave (2)
• A compressional wave has no
crests and troughs.
• When you make
compressional waves in a
coiled spring, a compression
is a region where the coils
are close together.
• The coils in the region next
to a compression are spread
apart, or less dense. This
less-dense region of a
compressional wave is called
a rarefaction.
Wavelength
• A wavelength is the
distance between
one point on a wave
and the nearest
point just like it.
• For transverse
waves the
wavelength is the
distance from crest
to crest or trough to
trough.
Wavelength (2)
• A wavelength in a
compressional wave
is the distance
between two
neighboring
compressions or two
neighboring
rarefactions.
Frequency and Period
• The frequency of a
wave is the number of
wavelengths that pass
a fixed point each
second.
• You can find the
frequency of a
transverse wave by
counting the number
of crests or troughs
that pass by a point
each second.
• Frequency is
expressed in hertz
(Hz).
• The period of a wave
is the amount of time
it takes one
wavelength to pass a
point.
• As the frequency of a
wave increases, the
period decreases.
• Period has units of
seconds.
Wavelength is Related to
Frequency
• As frequency increases,
wavelength decreases.
• The frequency of a wave
is always equal to the
rate of vibration of the
source that creates it.
• If you move the rope up,
down, and back up in 1 s,
the frequency of the
wave you generate is 1
Hz.
Wavelength is Related to
Frequency (2)
As frequency increases, wavelength decreases.
Wave Speed
• Wave speed is simply how fast a wave moves.
• You can calculate the speed of a wave
represented by v by multiplying its frequency
times its wavelength.
Wave Speed Depends on Medium
• The speed of a wave depends on the medium it
is traveling through.
• Sound waves usually travel faster in liquids and
solids than they do in gases. However, light
waves travel more slowly in liquid and solids
than they do in gases or in empty space.
• Sound waves usually travel faster in a material
if the temperature of the material is increased.
• The arrangement of particles in a medium
determines how well waves travel through it.
(Remember how particles are arranged in solids,
liquids, and gases).
Amplitude and Energy
• The greatest distance that particles are
displaced from their normal resting positions
because of a wave is called the amplitude.
• Amplitude is related to the energy carried by
a wave.
• The greater the wave’s amplitude is, the more
energy the wave carries.
• Amplitude is measured differently for
compressional and transverse waves.
Amplitude of Compressional
Waves
• The amplitude of a
compressional wave is
related to how tightly
the medium is pushed
together at the
compressions.
• The denser the
medium is at the
compressions, the
larger its amplitude is
and the more energy
the wave carries.
Amplitude of Compressional
Waves (2)
• The closer the
coils are in a
compression, the
farther apart
they are in a
rarefaction.
• So the less dense
the medium is at
the rarefactions,
the more energy
the wave carries.
Amplitude of Transverse Waves
• The amplitude of any transverse wave is
the distance from the crest or trough
of the wave to the rest position of the
medium.
The Behavior of Waves
•
•
•
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•
Reflection
Refraction
Diffraction
Interference
Standing Waves
Reflection
• Reflection is the
bouncing back of a
wave when it meets a
surface or boundary.
• The wave that
reflects is like the
original wave but
moving in a new
direction.
• All types of waves including sound,
water, and light waves
- can be reflected.
Reflection (2)
• How does the reflection
of light allow you to see
yourself in the mirror?
It happens in two steps.
First, light strikes your
face and bounces off.
Then, the light
reflected off your face
strikes the mirror and is
reflected into your
eyes.
Echoes
• A similar thing happens to sound waves when
your footsteps echo.
• Sound waves form when your foot hits the
floor and the waves travel through the air to
both your ears and other objects.
• Sometimes when the sound waves hit another
object, they reflect off it and come back to
you.
• Your ears hear the sound again, a few seconds
after you first heard your footstep.
The Law of Reflection
• The beam striking the
mirror is called the
incident beam.
• The beam that
bounces off the
mirror is called the
reflected beam.
• According to the law
of reflection, the
angle of incidence is
equal to the angle of
refection. All
reflected waves obey
this law.
Refraction
• When a wave passes from
one medium to
anothersuch as when a
light wave passes from air
to waterit changes
speed.
• If the wave is traveling at
an angle when it passes
from one medium to
another, it changes
direction, or bends, as it
changes speed.
• Refraction is the bending
of a wave caused by a
change in its speed as it
moves from one medium to
another.
Refraction of Light in Water
• Light waves travel
slower in water than
in air. This causes
light waves to
change direction
when they move
from water to air or
air to water.
• When light waves
travel from air to
water, they slow
down and bend
toward the normal.
Refraction of Light in Water (2)
• When light waves travel from water to air,
they speed up and bend away from the normal.
Refraction of Light in Water (3)
• You may have noticed that
objects that are underwater
seem closer to the surface
than they really are.
• In the figure, the light
waves reflected from the
swimmer’s foot are
refracted away from the
normal and enter your eyes.
• However, your brain
assumes that all light waves
have traveled in a straight
line.
• The light waves that enter
your eyes seem to have
come from a foot that was
higher in the water.
Diffraction
• Diffraction occurs
when an object causes
a wave to change
direction and bend
around it.
• Diffraction and
refraction both cause
waves to bend. The
difference is that
refraction occurs
when waves pass
through an object,
while diffraction
occurs when waves
pass around an object.
Diffraction (2)
• Waves also can be
diffracted when
they pass through a
narrow opening.
• After they pass
through the opening,
the waves spread
out.
Diffraction and Wavelength
• The amount of diffraction
that occurs depends on how
big the obstacle or opening is
compared to the wavelength.
• When an obstacle is smaller
than the wavelength, the
waves bend around it.
• If the obstacle is larger than
the wavelength, the waves do
not diffract as much. In
fact, if the obstacle is much
larger than the wavelength,
almost no diffraction occurs.
Interference
• When two or more
waves overlap and
combine to form a
new wave, the
process is called
interference.
• Interference occurs
while two waves are
overlapping.
Constructive Interference
• In constructive
interference, the
waves add together.
• This happens when the
crests of two or more
transverse waves
arrive at the same
place at the same time
and overlap.
• The amplitude of the
new wave that forms
is equal to the sum of
the amplitudes of the
original waves.
Destructive Interference
• In destructive interference,
the waves subtract from
each other as they overlap.
• This happens when the
crests of one transverse
wave meet the troughs of
another transverse wave.
• The amplitude of the new
wave is the difference
between the amplitudes of
the waves that overlapped.
• Waves undergoing
destructive interference are
said to be out of phase.
Constructive and Destructive
Interference
The sum wave is the blue wave traveling from left to
right.
When the two gray waves are in phase the result is
larger amplitude (constructive interference).
When the two gray waves become out of phase the
sum wave is zero (destructive interference).
Standing Waves
• A standing wave is a
special type of wave
pattern that forms
when waves equal in
wavelength and
amplitude, but
traveling in opposite
directions,
continuously interfere
with each other.
• The places where the
two waves always
cancel are called
nodes.
Sound
Sound
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What causes sound?
Sound Waves
Speed of Sound
Frequency and Pitch
The Doppler Effect
What Causes Sound?
• Every sound is produced by an object
that vibrates.
• For example, your friends’ voices are
produced by the vibrations of their
vocal cords, and music from a carousel
and voices from a loudspeaker are
produced by vibrating speakers.
Sound Waves
Sound waves are
compressional waves.
• A compressional wave is
made up of two types of
regions called
compressions and
rarefactions.

• You’ll see that when a
radio speaker vibrates
outward, the nearby
molecules in the air are
pushed together to
form compressions.
Sound Waves (2)
• As the figure shows,
when the speaker moves
inward, the nearby
molecules in the air have
room to spread out, and
a rarefaction forms.
• As long as the speaker
continues to vibrate
back and forth,
compressions and
rarefactions are
formed.
Sound Waves (3)
• Compressions and rarefactions move
away from the speaker as molecules in
the air collide with their neighbors.
• A series of compressions and
rarefactions forms that travels from
the speaker to your ear.
• This sound wave is what you hear.
Moving through Materials
• Most sounds you hear travel through air to reach your
ears.
• If you’ve ever been swimming underwater and heard
garbled voices, you know that sound also travels
through water.
• Sound waves can travel through any type of
mattersolid, liquid, or gas.
• Remember the matter that a wave travels through is
called a medium.
• Sound waves cannot travel through empty space.
The Speed of Sound in Different
Materials
• The speed of a sound
wave through a medium
depends on the
substance the medium is
made of and whether it
is solid, liquid, or gas.
• In general, sound
travels the slowest
through gases, faster
through liquids, and
even faster through
solids.
The Speed of Sound in Different
Materials (2)
• Sound travels faster in
liquids and solids than in
gases because the individual
molecules in a liquid or solid
are closer together than the
molecules in a gas.
• However, the speed of sound
doesn’t depend on the
loudness of the sound.
• Loud sounds travel through a
medium at the same speed as
soft sounds.
Pitch
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If you were to sing a
scale, your voice would
start low and become
higher with each note.
Pitch is how high or low
a sound seems to be.
The pitch of a sound is
related to the
frequency of the sound
waves.
Frequency and Pitch
Frequency is a measure of how many
wavelengths pass a particular point each
second.
 For a compressional wave, such as sound,
the frequency is the number of
compressions or the number of
rarefactions that pass by each second.
 Frequency is measured in hertz (Hz).

Frequency and Pitch (2)
When a sound wave with high frequency
hits your ear, many compressions hit
your eardrum each second.
 Your brain interprets these fast
vibrations caused by high-frequency
waves as a sound with a high pitch.
 As the frequency of a sound wave
decreases, the pitch becomes lower.

Frequency and Pitch (3)
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This figure shows
different notes and
their frequencies.
A healthy human ear can
hear sound waves with
frequencies from about
20 Hz to 20,000 Hz.
The human ear is most
sensitive to sounds in
the range of 440 Hz to
about 7,000 Hz.
The Doppler Effect
The change in pitch or wave frequency
due to a moving wave source is called
the Doppler effect.
 The Doppler effect occurs when the
source of a sound wave is moving
relative to a listener.

The Doppler Effect (2)
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

As a race car moves, it
sends out sound waves in
the form of compressions
and rarefactions.
The race car creates a
compression, labeled A.
Compression A moves
through the air toward
the flagger standing at
the finish line.
The Doppler Effect (3)


By the time compression
B leaves the race car,
the car has moved
forward.
Because the car has
moved since the time it
created compression A,
compressions A and B
are closer together than
they would be if the car
had stayed still.
The Doppler Effect (4)
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
As a result, the flagger hears a higher pitch.
The Doppler effect happens any time the source of a
sound is changing position compared with the observer.
It occurs no matter whether it is the sound source or
the observer that is moving.
The faster the change in position, the greater the
change in frequency and pitch.
Using the Doppler Effect
The Doppler effect also occurs for
other waves besides sound waves.
 For example, the frequency of
electromagnetic waves, such as radar
waves, changes if an observer and wave
source are moving relative to each
other.

Using the Doppler Effect (2)


Radar guns use the
Doppler effect to
measure the speed
of cars.
Weather radar also
uses the Doppler
shift to show the
movement of winds
in storms, such as a
tornado.
Light and
Matter
 Prisms
 Colors
 Lenses
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What you see depends on the amount of light in
the room and the color of the objects.
For you to see an object, it must reflect some
light back to your eyes.
Remember reflection occurs when a light wave
strikes an object and bounces off.
Objects can absorb light, reflect light, and
transmit light (allow light to pass through them).
The type of matter in an object determines the
amount of light it absorbs, reflects, and
transmits.
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Opaque material only
absorbs and reflects
light (no light passes
through it).
As a result, you
cannot see the
candle inside.
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Materials that allow
some light to pass
through them are
described as
translucent.
You cannot see
clearly through
translucent
materials.

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Transparent
materials transmit
almost all the light
striking them, so you
can see objects
clearly through
them.
Only a small amount
of light is absorbed
and reflected.
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Refraction causes a
prism to separate a beam
of white light into
different colors.
Remember refraction is
caused by a change in
the speed of a wave as it
passes from one material
to another.
How does the bending of
light create these
colors?
It occurs because the
amount of bending
usually depends on the
wavelength of the light.
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Wavelengths of visible
light range from the
longer red waves to
the shorter violet
waves.
White light, such as
sunlight, is made up of
this whole range of
wavelengths.
The animation shows
what happens when
white light passes
through a prism.
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The triangular prism refracts
the light twice – once when it
enters the prism and again when
it leaves the prism and reenters
the air.
Because the longer wavelengths
of light are refracted less than
the shorter wavelengths are,
red light is bent the least.
As a result of these different
amounts of bending, the
different colors are separated
when they emerge from the
prism.
Which color of light would you
expect to bend the most?
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Does the light leaving
the prism remind you
of a rainbow?
Like prisms, rain
droplets also refract
light.
The refraction of the
different wavelengths
can cause white light
from the Sun to
separate into the
individual colors of
visible light.
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In a rainbow, the
human eye usually
can distinguish only
about seven colors
clearly.
In order of
decreasing
wavelength, these
colors are red,
orange, yellow, green,
blue, indigo, and
violet.
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Why do some apples
appear red, while
others look green or
yellow?
An object’s color
depends on the
wavelengths of light
it reflects.
Remember white
light is a blend of all
colors of visible
light.
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When a red apple is
struck by white light,
it reflects red light
back to your eyes
and absorbs all of
the other colors.
The figure shows
white light striking a
green leaf. Only the
green light is
reflected back to
your eyes.
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Although some objects
appear to be black,
black isn’t a color that
is present in visible
light.
Objects that appear
black absorb all colors
of light and reflect
little or no light back
to your eye.
White objects appear
to be white because
they reflect all colors
of visible light.
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What do your eyes have in common with
cameras, eyeglasses, and microscopes?
Each of these things contains at least one
lens.
A lens is a transparent material with at least
one curved surface that causes light rays to
bend, or refract, as they pass through.
The image that a lens forms depends on the
shape of the lens.
Lenses can be convex or concave.
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Convex lenses are
thicker in the middle
than at the edges.
A convex lens focuses
light at a focal point.
The focal length of
the lens depends on
the shape of the lens.
If the sides are less
curved, light rays are
bent less.
As a result, lenses with
flatter sides have
longer focal lengths.
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The type of image a
convex lens forms
depends on where the
object is relative to the
focal point of the lens.
When the candle is more
than two focal lengths
away from the lens, its
image is real, reduced,
and upside down.
A real image is an image
formed by light rays
that converge to pass
through the place where
the image is located.

When the candle is between one and two
focal lengths from the lens, its image is
real, enlarged, and upside down.
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
When the candle is less than one focal length from the lens,
its image is virtual, enlarged, and upright.
A virtual image is an image formed by diverging light rays that
is perceived by the brain, even though no actual light rays pass
through the place where the image seems to be located.
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A concave lens is thinner
in the middle and thicker
on the edges.
Light rays that pass
through a concave lens
bend outward away from
the optical axis.
The image formed is
always virtual, upright,
and smaller than the
actual object is.
Concave lenses are used
in some types of
eyeglasses and some
telescopes.
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Light enters your eye through a
transparent covering on your eyeball
called the cornea.
The cornea causes light rays to bend so
that they converge.
The light then passes through an opening
called the pupil.
Behind the pupil is a flexible convex lens.
The lens helps focus light rays so that a
sharp image is formed on your retina.
The retina is the inner lining of your eye.
It has cells that convert the light image
into electrical signals, which are then
carried along the optic nerve to your
brain to be interpreted.