Waves
Unit 8
Chapter 11
Topics to be covered in this unit
1. Types of Waves
2. Characteristics of Waves
3. Wave Interactions
What is a wave?
• Waves are disturbances that transmit
energy through matter or space.
– Example – when you throw a stone into a
pond… it creates a ripple on the surface of
the water.
How waves travel
• Ripples in the pond
• Sound traveling through air
• Earthquakes in the ground
• These are all examples of mediums in
which waves can travel.
• A medium is the matter through which a
wave travels.
How waves travel
• Waves that require a medium are called
mechanical waves.
• Almost all waves require a medium
– Exceptions are electromagnetic waves
Electromagnetic wave
• Electromagnetic wave is a wave
caused by a disturbance in
electric and magnetic fields and
does not require a medium.
Light
• Light can travel from the sun to the Earth
through the empty space between the two.
• Light does not need a medium
• Light waves consist of changing electric
and magnetic fields in space.
• They are also called electromagnetic
waves
– Examples: visible light and radio waves
Waves transfer energy
• Remember – energy is the ability to do
work.
• Waves carry energy because they can do
work.
– Water waves can move a boat or leaf
– Sound waves impact your eardrum
– Light waves work on your eye
• The bigger the wave, the
more energy it carries.
Tsunami
Waves transfer energy
• Tsumani waves carry enough energy to
cause a lot of damage to coastal towns
and shorelines.
• Normal-size waves do work on the shore
to break up rocks into tiny pieces forming
sandy beaches.
Energy may spread out as a wave travels
• The closer you are to speakers, the louder
the music.
• Waves spread out in circles that get bigger
as the waves move farther from the
center.
• Each circle (called a wave front) has the
same amount of total energy. But as the
circle get larger, the energy spreads out
over a larger area.
Vibrations and Waves
• Waves are related to vibrations
– Example: a singer sings (vocal cords move back and
forth) creating sound. The sound waves reach your
ears causing your eardrum to vibrate. This vibration
is interpreted by your brain as sounds.
• Most waves are caused by a vibrating object
• Electromagnetic waves may be caused by
vibrating charged particles
• In a mechanical wave, the particles in the
medium also vibrate as the wave passes
through the medium.
Vibrations involve transformations
of energy
• Vibration: in a general sense, anything
that switches back and forth, to and from,
side to side, in and out, off and on, loud
and soft, or up and down is vibrating. A
vibration is a wiggle in time.
• Wave: a wiggle in both space and time is
a wave. A wave extends from one place to
another.
Vibrations involve transformations
of energy
• Vibrations and waves: the source of all
waves is something that is vibrating.
Waves are propagations of vibrations
throughout space.
• Example is a pendulum in
a clock.
simple harmonic motion is a type of
motion where the restoring force is
directly proportional to the
displacement
Simple Harmonic Motion
• A wave whose source vibrates with
simple harmonic motion is called a
sine wave
• As the mass moves down, the spring
is exerting force which pushes it back
to the original position. This will allow
the mass to keep bouncing up and
down forever.
Damped Harmonic Motion
• Unlike the simple harmonic motion which
will continue forever. The damped
harmonic motion will transfer its energy to
the second mass, it slows down and then
returns to its resting position.
• http://www.animations.physics.unsw.edu.a
u/jw/oscillations.htm#Damped
Wave Types
Transverse waves vibrate across from
direction of travel
Longitudinal waves vibrate along the
direction of travel (as in a spring)
Longitudinal Wave
• The wave we see here is a longitudinal wave.
• In a longitudinal wave, the medium
particles vibrate parallel to the motion of
the pulse.
• This is the same type of wave that we use
to transfer sound.
20
Transverse waves
• A second type of wave is a transverse
wave.
• We said in a longitudinal wave the pulse
travels in a direction parallel to the
disturbance.
• In a transverse wave the pulse travels
perpendicular to the disturbance.
21
What’s the difference?
• The differences between the two can be
seen
22
Surface Wave
• Surface waves occur at the boundary
between two different mediums such as
between water and air.
• Example: The waves on the ocean are not
simply transverse or longitudinal waves.
Characteristics of Waves
• Waves can be observed with many
different properties:
– Large or small
– Close together or very far apart
• However, they can all be described with
their properties using a sine curve.
Wave Properties
Wavelength
Wavelength, l, is the distance between any
two successive identifical parts of a wave.
Wave Properties
Amplitude
Amplitude is the greatest distance that
particles in a medium move from their normal
position when a wave passes.
Parts of a Wave
• Crest: The highest point of a transverse
wave
Parts of a Wave
• Trough: The lowest point of a transverse
wave.
• These are the points where the wave exhibits its
maximum negative or downward displacement.
Longitudinal wave parts:
• Compressions- a region in a longitudinal
wave where the particles are closest
together. It is an area of high density
• Rarefactions- A decrease in density and
pressure in a medium.
Wave Properties
Period
The wave period, P, the time required for one full
wavelength to pass a certain point.
Wave Properties
Frequency
Frequency, f, is the number of vibrations that
occur in a 1 second time interval.
Frequency-Period Equation
• The frequency and period of a wave are
related.
– If more vibrations are made in a second, each
one takes a shorter amount of time.
– The frequency is inverse of the period.
• SI unit is Hertz (Hz)
Frequency = 1
period
f = 1/T
Visible Light
• Our eyes can detect light with frequencies
from 4.3 x 1014 Hz to 7.5 x 1014 Hz.
• This is called the visible light
Electromagnetic Spectrum
• Light occurs at other ranges we cannot
see with our eyes.
• The full range is called the
electromagnetic spectrum.
Type of
wave
Mechanical
Electromagnetic
Form
Longitudinal
Transverse
Modeled as
transverse
Medium
Solids, liquids,
gases
Solids and
liquids
None required
Travel as
Compression
and
rarefactions in
matter
Back & forth
movement of
matter
Examples
Sound waves
and some
earthquake
waves
Water waves,
rope waves,
some
earthquake
waves
Oscillating
electric and
magnetic fields
Visible light
waves, radio
waves, X-rays
Wave speed
• Wave speed is the speed at which a wave
passes through a medium.
• Formula:
Wave speed = frequency x wavelength
v=fxl
SI unit for wave speed is meters per second.
m/s
Practice problem
• The average wavelength in a series of
ocean waves is 15.0 m. A wave arrives on
average every 10.0 s, so the frequency is
0.100 Hz. What is the average speed of
the waves?
• v=fxl
Practice problem
• The average wavelength in a series of
ocean waves is 15.0 m. A wave arrives on
average every 10.0 s, so the frequency is
0.100 Hz. What is the average speed of
the waves?
v=fxl
= 0.100 Hz x 15.0 m
= 1.5 m/s
Practice problems
v=fxl
P=1
t
Sound Waves
Molecules in the air vibrate about some
average position creating the compressions
and rarefactions. We call the frequency of
sound the pitch.
Doppler Effect
• Doppler effect is an observed change in
the frequency of a wave when the source
or observer is moving.
• Pitch of a sound, how high or low it is, is
determined by the frequency at which
sound waves strike the eardrum in your
ear.
Of course…..
• There's this take on it
Doppler Effect
• Refers to the change in frequency when
there is relative motion between an
observer of waves and the source of the
waves
• Doppler with Sound
Red shift?
Expand that…..
• Wrap your mind around this
Wave Interactions
• Two different material objects can never
occupy the same space at the same time.
• When two waves come together they do
not bounce back as bumper boats.
• Because mechanical waves are not matter
but rather the displacement of matter, two
waves can occupy the same space at the
same time
Wave Interactions
• Reflection is the bouncing back of a wave
as it meets a surface or boundary
angle of incidence=angle of reflection
Reflection of a wave
from a free end
Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com
Reflection of a wave
from a fixed end
Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com
Wave Interactions
• Diffraction is the bending of a wave as it
passes an edge or an opening
Wave Interactions
• Refraction is the bending of waves as the
pass from one medium to another.
What is the difference?
• http://www.brainpop.com/science/energy/r
efractionanddiffraction/
Wave Interaction
• All we have left to discover is how waves
interact with each other.
• When two waves meet while traveling
along the same medium it is called
INTERFERENCE.
• Interference is the combination of two or
more waves that exist in the same place at
the same time.
59
Wave Interference
When two wave pass each other their
superposition causes reinforcement or
cancellation.
Constructive interference
Reinforcement when the crest of one wave
overlaps the crest of another
Their individual effects adds together, resulting
in a wave increased in amplitude
In phase- two waves crests and troughs arrive
at a place at the same time, effects reinforce
each other
Destructive Interference
Cancellation when crest of one wave overlaps trough
of another reducing their individual effects
Water waves show these best
Out of phase- the crest of one wave arrives at a
point at the same time as a trough of the second
wave arrives, effects cancel each other
Sound Wave Interference
•Interference occurs when two sounds of difference frequency
are heard superposed.
•Constructive interference (compressions align) causes louder sound an
destructive inference cause fainter sound
•This alternating pattern produces a beat.
Shock Waves
Water Wave Interference
• Left side is theoretical drawing of an
interference pattern.
• Right side is the actual interference pattern.
Standing Waves
Standing wave is a wave form caused by
interference that appears not to move along
the medium and that shows some regions of
no vibrations (nodes)….
Standing Waves
And other regions of maximum vibrations called
antinodes.
Antinodes form where the crests of the original
waves line up with the crests of the reflected
waves so that complete constructive
interference occurs.
Standing Waves
Change the frequency in a standing wave and
more nodes/antinodes appear in the event
Review Questions
What dictates the frequency of a
sound wave?
(a) wavelength
(b) medium
(c) source of the sound
(d) speed
(e) amplitude
What dictates the frequency of a
sound wave?
(a) wavelength
(b) medium
(c) source of the sound
(d) speed
(e) amplitude
What determines the speed of a
wave?
(a) the frequency
(b) the wavelength
(c) the amplitude
(d) the period
(e) the medium of transmission
What determines the speed of a
wave?
(a) the frequency
(b) the wavelength
(c) the amplitude
(d) the period
(e) the medium of transmission
A skipper on a boat notices wave
crests passing his anchor chain
every 5 seconds. If the wave crests
are 15 m apart, what is the speed
of the water waves in m/s?
(a) 5
(b) 15
(c) 75
(d) 10
(e) 3
A skipper on a boat notices wave
crests passing his anchor chain
every 5 seconds. If the wave crests
are 15 m apart, what is the speed
of the water waves in m/s?
(a) 5
(b) 15
(c) 75
(d) 10
(e) 3
For a medium transmitting a
longitudinal wave, the areas of the
medium where the density of the
medium is temporarily increased
are called...
(a) rarefactions
(b) compressions
(c) density holes
For a medium transmitting a
longitudinal wave, the areas of the
medium where the density of the
medium is temporarily increased
are called...
(a) rarefactions
(b) compressions
(c) density holes
When you move away from a fixed
source of sound, the frequency of
the sound you hear...
(a) is greater than what the source emits
(b) is less than what the source emits
(c) is the same as what the source emits
When you move away from a fixed
source of sound, the frequency of
the sound you hear...
(a) is greater than what the source emits
(b) is less than what the source emits
(c) is the same as what the source emits
Sonic booms from a plane are
produced...
(a) because the plane breaks through the sound
barrier.
(b) when the plane reaches the speed of sound.
(c) by the plane traveling faster than the speed of
sound.
(d) by the plane traveling slower than the speed
of sound.
Sonic booms from a plane are
produced...
(a) because the plane breaks through the sound
barrier.
(b) when the plane reaches the speed of sound.
(c) by the plane traveling faster than the speed of
sound.
(d) by the plane traveling slower than the speed
of sound.
Type of
wave
Mechanical
Electromagnetic
Form
Longitudinal
Transverse
Modeled as
transverse
Medium
Solids, liquids,
gases
Solids and
liquids
None required
Travel as
Compression
and
rarefactions in
matter
Back & forth
movement of
matter
Examples
Sound waves
and some
earthquake
waves
Water waves,
rope waves,
some
earthquake
waves
Oscillating
electric and
magnetic fields
Visible light
waves, radio
waves, X-rays