A wave is a disturbance that carries energy through
matter or space.
The matter through which a wave travels is called a
medium.
• For waves in a lake, water is the medium.
• For sound from a television, air is the medium.
• For earthquakes, the earth is the medium.
Waves that require a medium are called mechanical
waves.
– Mechanical waves CANNOT travel through a vacuum.
Electromagnetic waves consist of oscillating electric
and magnetic fields, which radiate outward at the
speed of light.
– Do not require a medium
Types of electromagnetic waves:
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


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Radio waves
Microwaves
Infrared
Visible light
Ultraviolet (UV)
X-rays
Gamma rays
Recall that energy is the ability to do work, or to exert
a force over a distance. Waves transfer energy.
The larger a wave is, the more energy it carries.
Tsunami waves are
created by
underwater
disturbances such as
earthquakes, volcanic
eruptions, and
underwater
landslides. These
waves harness a great
amount of energy and
are very dangerous.
Energy may spread out as a wave travels.
Each of the circles in the wave above is called a wave front.
They each carry the same amount of energy, but in the larger
circles, this energy is spread out over a greater distance.
Mechanical waves spread out spherically from the source, if
the source is open to the medium in all directions.
Most waves are caused by vibrations.
Vibration: in a general sense, anything that switches back
and forth, to and fro, side to side, in and out, off and
on, or up and down is vibrating.
Only the energy travels in the wave, not the medium. The
medium only vibrates around its equilibrium position.
Holt Science Spectrum: Physical Science
Holt Science Spectrum: Physical Science
The image above displays the energy changes as a mass on
a spring vibrates around its equilibrium position.
This photo and the animation on the previous slide are
examples of simple harmonic motion.
Transverse waves are waves in which the motion of the
particles is perpendicular to the direction the wave is
traveling.
Longitudinal waves are waves in which the particles of the
medium vibrate parallel to the direction of the wave
motion.
Crest
Trough
Rarefaction
Compression
Mechanical waves
– Can be transverse or longitudinal
Electromagnetic waves
– transverse
Particles in a surface wave move both
perpendicular and parallel to the direction in
which the wave travels. These waves occur at the
boundary of two different media, such as water
and air.
• The amplitude of a wave measures the amount of
particle vibration.
amplitude: the maximum distance that the particles of a wave’s
medium vibrate from their rest position
– for a transverse wave, measured from the rest position to the
crest or the trough
– expressed in the SI unit meters (m)
– Ideal transverse waves are shaped as sine curves.
wavelength: the distance from any point on a
wave to an identical point on the next wave
– for a transverse wave, measured from crest to crest or trough to
trough
– for a longitudinal wave, the distance between two compressions
– represented by the Greek letter lambda, l
– expressed in the SI unit meters (m)
Amplitude and wavelength tell you about energy.
larger amplitude = more energy
shorter wavelength = more energy
Frequency is a measurement of the vibration
rate.
frequency: the number of cycles or vibrations per second;
also the number of waves produced in a second.
– represented by the symbol f
– expressed in the SI unit hertz (Hz), which equals 1/s
Example:
Suppose I wiggle a slinky back and forth, and count that 6
waves pass a point in 2 seconds. What would the
frequency be?
– 6 cycles / 2 seconds = 3 cycles / 1 second
– 3 Hz
Period: in physics, the time that it takes for one full
wavelength to pass a given point.
– represented by the symbol T
– expressed in the SI unit seconds (s)
The period of a wave is related to the frequency by the
following equation:
T=1
f
or
f=1
T
Period of wave (a)
T = 1/4Hz = 0.25s
Period of wave (b)
T = 1/8Hz = 0.125s
• Wave speed equals wavelength divided by
period.
speed = distance/time
v=l
T
• Wave speed equals frequency times
wavelength.
v=fl
Wave Speed
The string of a piano that produces the note middle C
vibrates with a frequency of 262 Hz. If the sound waves
produced by this string have a wavelength in air of 1.30
m, what is the speed of the sound waves?
Given: frequency, f = 262 Hz
wavelength, l = 1.30 m
Unknown: wave speed, v = ? m/s
v=fl
v = 262 Hz  1.30 m
v = 341 m/s
• The speed of a wave depends on the medium.
– In general (for mechanical waves), wave speed is
greatest in solids and least in gases.
– In a given medium, the speed of waves is constant.
• Shaking your hand faster while holding a rope will not
increase the wave speed along the rope. It simply increases
the frequency and decreases the wavelength.
Kinetic theory explains differences in wave
speed.
• In gases, molecules are farther apart. They
need to travel through more empty space
before hitting another molecule than they
would in liquids and solids.
• Light has a finite speed.
– the speed of light in empty space (c) = 3.00  108 m/s
(186,000 miles per second)
– for electromagnetic waves, c = f  l
Electromagnetic waves travel fastest in a vacuum and
slower through mediums such as air or water.
Why does the pitch of an ambulance siren change as
the ambulance rushes past you?
〉Motion between the source of waves and the
observer creates a change in observed frequency.
• Pitch is determined by the frequency of sound
waves.
• The 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.
• A higher-pitched sound is caused by sound waves of
higher frequency.
• Frequency changes when the source of waves
is moving.
– Doppler effect: an observed change in the
frequency of a wave when the source or observer
is moving
– The Doppler effect occurs for many types of waves,
including sound waves and light waves.
Stationary wave source. The
sound waves move
symmetrically out from the
source.
Moving wave source. Since
the source of the sound is
moving to the right, the
wavefronts seem to bunch up
to the right and spread out
toward the left. This causes
the observed pitch to be
different depending on where
the source is in relation to the
observer.
This is an illustration of an object
moving with a speed that is
equal to the speed of sound.
(Mach 1)
An observer in front of the object
would not hear anything until
the object arrived.
http://www.acs.psu.edu/drussell/Demos
/doppler/mach1.html
An F/A-18 Hornet creating a vapor cone
at transonic speed just before reaching the
speed of sound
This animation is an example of
an object moving faster than
the speed of sound.
The image above shows a sonic boom created by the THRUST
SSC team car as it broke the land speed record and the sound
barrier.
An observer will hear the sonic boom from an object moving
faster than the speed of sound when the shock wave, on the
edges of the cone, crosses his or her location.
Wave Interactions
How do waves behave when they hit a boundary, when
they pass around an edge or opening, and when they
pass from one medium to another?
〉When a wave meets a surface or a boundary, the
wave bounces back.
〉When a wave passes the edge of an object or passes
through an opening, the wave bends.
〉 A wave also bends when it passes from one medium
to another at an angle.
Reflection occurs when a wave meets a
boundary.
– reflection: the bouncing back of a wave when it
hits a surface that it does not go through
Examples:
– The reflection of light waves in a lake can create a
mirror image of a landscape.
– Water waves are reflected when they hit the side
of a boat.
The animation below shows a wave pulse on a string being
reflected off of a fixed boundary. The wave exerts an upward
force on the end of the string. But, since the end is clamped, it
cannot move. According to Newton's third law, the wall must
be exerting an equal downward force on the end of the string.
This new force creates a wave pulse that propagates from
right to left, with the same speed and amplitude as the
incident wave, but with opposite polarity (upside down).
The wave in the animation below is traveling from a lower
density to higher density medium.
This wave is traveling from a higher to lower density medium.
Diffraction is the bending of waves around an
edge.
– diffraction: a change in the direction of a wave
when the wave finds an obstacle or an edge,
such as an opening
Examples:
– Water waves
diffract around a
block in a tank of
water.
– Sound waves
passing through a
door diffract.
Waves can also bend by refraction.
refraction: the bending of a wavefront as the wavefront
passes between two substances in which the speed of the
wave differs
All waves are refracted when they pass from one medium to
another at an angle.
interference: the combination of two or more waves
that results in a single wave
• Constructive interference increases amplitude.
– constructive interference: a superposition of two or
more waves that produces an intensity equal to the
sum of the intensities of the individual waves
• Destructive interference decreases amplitude.
– destructive interference: a superposition of two or
more waves that produce an intensity equal to the
difference of the intensities of the individual waves
http://serc.carleton.edu/NAGTWorkshops/dee
pearth/activities/40826.html
Standing waves
Occurs when a wave reflects upon itself and interference causes
the pattern
Each loop of a standing wave is separated from the next loop
by points that have no vibration, called nodes.
Midway between the nodes lie points of maximum vibration,
called antinodes.