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AP PHYSICS 2
MECHANICAL WAVES
UNIT 7
Quantum Physics,
atomic, and nuclear
physics
CHAPTER 24
Electromagnetic
Waves
MECHANICAL WAVES
MECHANICAL WAVES
Polarization of waves
 Polarization is a property of waves that describes
the orientation of the oscillations of the wave.
Transverse waves can
be polarized
Longitudinal waves
cannot be polarized
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Testing the polarization of light waves
Polarization shows that light phenomena can be better explained
by a transverse wave model
ELECTROMAGNETIC WAVES
A wave that DOES NOT
require a medium through
which to travel.
Models that tried to explain how light
propagate through air
(outdated models)
1. Light is a mechanical vibration that travels
through an elastic medium. This medium is
completely transparent and has exactly zero
mass. This medium is called ether.
2. A light wave is some new type of vibration that
does not involve physical particles vibrating
around equilibrium positions due to restoring
forces being exerted on them.
Discovery of electromagnetic waves
 In 1865, Maxwell suggested a new field
relationship: a changing electric field can
produce a magnetic field.
 This magnetic field was first detected in 1929,
but was not measured precisely until 1985 due
to its extremely tiny magnitude.
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Maxwell's equations
Maxwell's equations
1. Stationary electric charges produce a
constant electric field.
D = v
2. There are no magnetic charges (no
magnetic monopoles).
B = 0
3. A magnetic field is produced either
by electric currents or by a changing
electric field.
xE =
4. A changing magnetic field produces  x H =
an electric field.
+J
Maxwell's equations
CONSEQUENCES OF MAXWELLS EQUATIONS
Producing an electromagnetic wave
 A changing electric field can produce a changing
magnetic field, which in turn can produce a
changing electric field, and so on.
 This feedback loop does not require the
presence of any electric charges or currents.
CONSEQUENCES OF MAXWELLS EQUATIONS
Vacuum permittivity and the speed of light
Testing Maxwell’s Equations
Henry Hertz (1857 – 1894)
 The constant εo is the vacuum permittivity:
 The constant μo is the vacuum permeability.
k = 9x109 Nm2/C2
μo = 4 x10-7 N/A2
WHITEBOARD
 Show magnitude of v
 Show unit analysis
 Switch connects a charged capacitor to a the
primary coil of a transformer (transmitter).
 Capacitor discharges, potential difference across the
primary coil induces a huge potential difference
across the secondary coil.
 Metal spheres charged and generated a spark
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Testing Maxwell’s Equations
Henry Hertz (1857 – 1894)
Henry Hertz: Testing the hypothesis that light
can be modeled as an electromagnetic wave
 Hertz
characterized
the
wave
nature
of
electromagnetic
disturbances
by
performing
experiments similar to those used to determine the
wave nature of visible light. For example, he
observed reflection, refraction, and diffraction.
 The spark would indicate a large electric field
between the spheres of the transmitter.
 The
changing
electric
field
produces
an
electromagnetic wave.
 When the wake reaches the receiver, it induces and
electric current causing a weak spark.
 He also performed a double-slit experiment and
observed interference.
Antennas are used to start electromagnetic
waves
Antennas are used to start electromagnetic
waves
 He observed polarization and measured their speed
to be the same as the speed of light (3×108 m/s).
 An antenna is commonly
used to produce
electromagnetic waves.
 A simple type of antenna
can be made from a pair
of electrical conductors,
one connected to each
terminal of a power supply
that is producing an
alternating emf.
 The alternating emf leads
to the continuous charging
and discharging of the two
ends of the antenna
Antennas are used to start electromagnetic
waves
Antennas are used to start electromagnetic
waves
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Antennas are used to start electromagnetic
waves
Radar
 Radar is a way of determining the distance to a
faraway object by reflecting radio wave pulses
off the object.
Crossed, oscillating electric and magnetic fields
will propagate indefinitely and without loss of
energy at speed c through a vacuum.
Global Positioning System
 The GPS receiver detects signals from at least three
satellites to determine your position on the ground.
 Using the known positions of the satellites, the GPS
unit is able to calculate your position by a process
called trilateration.
Hearing FM radio waves
 The high-frequency EM waves used by FM radio
stations are known as carrier waves.
 FM stands for “Frequency Modulation“
 AM stands for “Amplitude Modulation”
 The information converted into the sounds we
hear is encoded as tiny variations in the
frequency/amplitude of the carrier wave.
Microwave cooking
 Water is a polar molecule and is a permanent
electric dipole.
 Water, fat, and other substances in food
absorb energy from microwaves in a process
called dielectric heating.
The electromagnetic spectrum
 The range of frequencies and wavelengths of
electromagnetic
waves
is
called
the
electromagnetic spectrum.
 A receiver decodes the variations and
converts them into an electric signal that a
speaker can then convert into sound.
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• Radio Waves
–
–
–
–
–
AM Radio
Shortwave radio
FM Radio
Television
Radar
The Electromagnetic Spectrum!
• Microwaves
• Infrared
• Visible light
• Ultraviolet
• X-rays
• Gamma rays
A mnemonic to help you remember the spectrum!
(in order of increasing frequency)
Radio Microwave Infrared Visible Ultraviolet X-ray Gamma
Rattlesnakes May Inject Venom Upon eXtreme aGitation
All of these frequencies of light travel at
speed c in a vacuum (3 x 108 m/s).
Human eyes are only able to detect light of
wavelength 480-720 nm.
That is why this is called the visible range of the spectrum.
The wavelength that we perceive as red is about 480 nm.
The wavelength that we perceive as violet is about 720 nm.
Different animals are able to detect
different ranges of EM waves!
The image on the right shows the
ultraviolet light given off by a dandelion.
Bees and other insects have eyes that are
capable of detecting UV light!
The electromagnetic spectrum
Why are we able to detect 480 – 720 nm
electromagnetic waves with our eyes?
That is the peak range of wavelengths
emitted by our Sun!!!
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Mathematical description of EM waves and
EM wave energy
Mathematical description of EM waves
 The wave equation tells us:
 Maxwell's equations
predict that the
amplitudes of the
changing electric
and magnetic field
vectors are related:
Producing unpolarized light
 Light emitted by a lightbulb
consists of many waves that
originate at random times with
random polarizations.
 If we could observe the
many separate EM waves
as a beam of unpolarized
light moving directly toward
our eyes, the oscillations of
both the electric and
magnetic fields would look
something like a porcupine
How polarized glasses work
 The lenses of polarized glasses are coated with
a polarizing film that only transmits light whose
electric field oscillates in the vertical direction.
Light polarizers
 A polarizer absorbs one component of the E field
of the EM wave passing through it, allowing the
perpendicular component to pass.
Brewster's law
 Light is traveling from medium 1
when reflects off medium 2.
 The reflected light is totally
polarized an axis in the plane
parallel to the surface when the
tangent of the incident polarizing
angle P equals the ratio of the
indexes of refraction of the two
media
𝑡𝑎𝑛𝜃𝑃 =
𝑛2
𝑛1
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Example 24.4
Polarization by scattering
 You are facing the Sun and looking at the light
reflected off the ocean. At which angle above the
horizon should the Sun be so that you get the
most benefit from your polarizing sunglasses?
 If you look through polarized sunglasses at the
clear sky in an arbitrary direction and rotate the
glasses, the intensity of the light passing through
the glasses changes.
  = 36.94
Polarization by scattering
Polarization by scattering
© 2014 Pearson Education, Inc.
Polarization by reflection
 Consider light produced by the LCD screen of a
calculator, cell phone, or laptop computer, or
reflected off a body of water.
 If you look at this light through a polarizer, the
intensity of reflected light varies depending on
the orientation of the polarizer relative to the
surfaces.
 This indicates that the light is partially
polarized.
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Polarized LCDs
 Nearly all computer, TV, calculator, and cell phone
screens are LCDs—liquid crystal displays.
3D movies
 The 3D projector produces two distinct images
on the screen.
 Each image consists of polarized light.
 The two images have their polarizing axes
rotated by 90° relative to each other.
Polarized LCDs
1. Unpolarized light shines on the back.
2. A horizontal polarizing filter in front of the light blocks out all
light waves except those vibrating horizontally.
3. Only light waves vibrating horizontally can get through.
4. A transistor switches on/off this pixel by switching on/off the
electric current flowing through its liquid crystal. That makes
the crystal twist. The twisted crystal rotates (or not) light
waves by 90° as they travel through it.
5. Light waves that entered the liquid crystal vibrating
horizontally emerge from it vibrating horizontally/vertically
6. The vertical polarizing filter in front of the liquid crystal
blocks out all light waves except those vibrating vertically.
The vertically vibrating light that emerged from the liquid
crystal can now get through the vertical filter.
7. The pixel is lit up. A red, blue, or green filter gives the pixel
its color
3D movies
The film is recorded using two camera lenses sat side by side.
But in the cinema, the two reels of film are projected through
different polarized filters. So images destined for viewers' left
eyes are polarized on a horizontal plane, whereas images
destined for their right eyes are polarized on a vertical plane.
Cinema goers’ glasses use the same polarizing filters to
separate out the two images again, giving each eye sees a
slightly different perspective and fooling the brain into 'seeing'
Avatar's planet Pandora as though they were actually there.
“Light is a wave” or “light behaves like a wave”?
 Saying that “light is a wave”, claims to know the
absolute nature of light, which is not possible.
 Saying that “light behaves like a wave”, claims
to know how to describe light, which is
possible.
 The second statement is more accurate because
it reflects the capacity of science to continually
fine tune itself.
Light behaves like a wave:
The Wave Model of Light
 Light is a transverse electromagnetic wave!
 It is composed of perpendicular electric and
magnetic fields that propagate one another.
 Light waves can constructively and destructively
interfere with one another.
 Light waves obey c
= 𝝀𝒇
 Light waves propagate according to Huygens’
Principle.
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Frequency and wavelength of
electromagnetic waves
Frequency of a wave does not change
upon entering a new medium!
 All electromagnetic waves travel at the speed of
light c in a vacuum.
 In media other than a vacuum, the speed of
electromagnetic waves is 𝒗
=
Frequency is a property of the wave, and is
set once the wave is produced.
𝒄
.
𝒏
Wave speed and wavelength will change
inversely upon entering a new medium!
 The speed, frequency, and wavelength are
related by 𝝀
=
The frequency of an EM wave governs how
much energy it carries.
𝒗
:
𝒇
Wave properties
Models of Light
Particle Model
 Reflection involves a change in direction of
waves when they bounce off a barrier.
 Refraction of waves involves a change in the
direction of waves as they pass from one
medium to another.
 Diffraction involves a change in direction of
waves as they pass through an opening or
around a barrier in their path
• Reflection
Wave Model
• Reflection
• Refraction
• Shadows and semishadows
• Shadows and semi-shadows
• Light travels in a
straight line
• Interference (Double slit -
• Light travels in a straight line
small openings)
• Diffraction (Single slit - small
openings)
• Polarization
• Doppler Effect
Conceptual Whiteboard
What happens to the speed and the wavelength of light as it
crosses the boundary in going from air into water?
Speed
(A)
(B)
(C)
(D)
(E)
Increases
Remains the same
Remains the same
Decreases
Decreases
Wavelength
Remains the same
Decreases
Remains the same
Increases
Decreases
(E)
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