Download A wave that DOES NOT require a medium through which to travel.

3/21/2016
Chapter 24 Lecture
Electromagnetic
Waves
ELECTROMAGNETIC
WAVES
A wave that DOES NOT
require a medium through
which to travel.
Prepared by
Dedra Demaree,
Georgetown University
© 2014 Pearson Education, Inc.
ELECTROMAGNETIC
WAVES
Electromagnetic waves are created by the
vibration of an electric charge.
This vibration creates a wave which has
both an electric and a magnetic
component.
An electromagnetic wave transports its
energy through a vacuum at a speed of
light (c).
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 v = λf
 Light waves propagate according to Huygens’
Principle.
c = 3x108 m/s
• Radio Waves
– AM Radio
– Shortwave radio
– FM Radio
– Television
– Radar
• Microwaves
• Infrared
• Visible light
Crossed, oscillating electric and magnetic fields
will propagate indefinitely and without loss of
energy at speed c through a vacuum.
• Ultraviolet
• X-rays
• Gamma rays
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The Electromagnetic Spectrum!
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 10 8 m/s).
Human eyes are only able to detect light of
wavelength 480-720 nm.
Why are we able to detect 480 – 720 nm
electromagnetic waves with our eyes?
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!
That is the peak range of wavelengths
emitted by our Sun!!!
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!
Quick Conceptual Whiteboard Review!
Frequency of a wave does not change upon entering a
new medium!
What happens to the speed and the wavelength of light as it
crosses the boundary in going from air into water?
(A)
(B)
(C)
(D)
(E)
Speed
Increases
Remains the same
Remains the same
Decreases
Decreases
Wavelength
Remains the same
Decreases
Remains the same
Increases
Decreases
The frequency of an EM wave governs how
much energy it carries.
Frequency is a property of the wave, and is
set once the wave is produced.
Wave speed and wavelength will change
inversely upon entering a new medium!
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Youtube Links
DOPPLER
EFFECT
• http://www.youtube.com/watch?v=Y5KaeCZ_AaY
THE DOPPLER EFFECT ALSO APPLIES TO
LIGHT!
If a source of light is moving toward an observer, the
light that the observer receives will have a higher
frequency and shorter wavelength than would normally
be received!
DOPPLER EFFECT
It is the apparent change in the frequency
of a wave caused by relative motion
between the source of the wave and the
observer
This is called blueshift. (Light is shifted toward the
blue end of
the spectrum – higher frequency)
If a source of light is moving away from an observer, the
received light will have a lower frequency and longer
wavelength than normal!
This is called redshift. (Light is shifted toward the
red end of
the spectrum – lower frequency)
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What's new in this chapter
 We found that a wave model explained reflection,
refraction, and interference.
 Every other type of wave we have encountered so
far involves the vibration of the medium through
which the wave travels.
 What is vibrating in a light wave?
 We continue to investigate that question in this
chapter. We'll resolve the question when we
learn about special relativity (in Chapter 25).
© 2014 Pearson Education, Inc.
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Polarization of waves
 Polarization is a property of waves that describes
the orientation of the oscillations of the wave.
Testing the polarization of waves
Polarization shows that light phenomena can be better explained
by a transverse wave model
Two hypotheses concerning light and
polarization
(two 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
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.
Frequency and wavelength of
electromagnetic waves
 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 = c/n.
 The speed, frequency, and wavelength are
related by:
Radar
 Radar is a way of determining the distance to a
faraway object by reflecting radio wave pulses
off the object.
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.
Hearing FM radio waves
 The high-frequency EM waves used by FM radio
stations are known as carrier waves.
 FM stands for "frequency modulation"; the
information converted into the sounds we
hear is encoded as tiny variations in the
frequency of the carrier wave.
 A receiver decodes the variations and
converts them into an electric signal that a
speaker can then convert into sound.
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The electromagnetic spectrum
The electromagnetic spectrum
 The range of frequencies and wavelengths of
electromagnetic
waves
is
called
the
electromagnetic spectrum.
Mathematical description of EM waves and
EM wave energy
 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
Mathematical description of EM waves
 The wave equation tells us:
Light polarizers
 A polarizer absorbs one component of the E field
of the EM wave passing through it, allowing the
perpendicular component to pass.
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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.
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
𝑛2
𝑡𝑎𝑛𝜃𝑃 =
𝑛1
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.
  = 36.9
Polarization by scattering
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.
© 2014 Pearson Education, Inc.
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Polarized LCDs
 Nearly all computer, TV, calculator, and cell phone
screens are LCDs—liquid crystal displays.
 They can flow like a liquid, but their molecules are
aligned or oriented in an orderly crystal-like manner.
 Polarization plays an important role in the operation
of these screens.
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.
© 2014 Pearson Education, Inc.
Wave properties
 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
Models of Light
Particle Model
• Reflection
Wave Model
• Reflection
• Refraction
• Shadows and semishadows
• Shadows and semi-
• Light travels in a
straight line
• Light travels in a
shadows
straight line
• Diffraction and
interference effects
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