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Electromagnetic Radiation
Lecture 10 - Light and Color
What exactly are they?
(Giancoli Chapter 22)
Electromagnetic Induction
Electromagnetic Waves
Speed of Light
The EM spectrum
Light and Color
Radio
Electromagnetic Induction demonstration
James Clerk Maxwell
Electromagnetic
Waves
•  What does Maxwell have in his hand?
•  Why is he one of the 3 greatest
scientists of all time?
•  Who are the others?
•  What on earth do magnets have to do
with light and color?
James Clerk Maxwell
From his studies on electromagnetic
induction, Maxwell realized:
“Accelerating Charges give rise to
Electromagnetic Waves”
Maxwell's equations demonstrate that
electricity, magnetism and light are all
manifestations of the same phenomenon,
namely the electromagnetic field."
22.2 Production of Electromagnetic Waves
Maxwell invented the field called “Dimensional
Analysis”
Realized that the ratio of Electric to Magnetic Field had
the dimensions of a velocity (m/s), and this term
appeared in an expression for the velocity at which EM
waves should travel
E=v=c
B
This is the speed of light in a vacuum.
ConcepTest 22.2
The electric field in an EM
wave traveling northeast
oscillates up and down. In
what plane does the
magnetic field oscillate?
Oscillations
1) In the north-south plane.
2) In the up-down plane.
3) In the NE-SW plane.
4) In the NW-SE plane.
5) In the east-west plane.
ConcepTest 22.2
The electric field in an EM
wave traveling northeast
oscillates up and down. In
what plane does the
magnetic field oscillate?
Oscillations
1) In the north-south plane.
2) In the up-down plane.
3) In the NE-SW plane.
4) In the NW-SE plane.
5) In the east-west plane.
The magnetic field oscillates perpendicular to BOTH the
electric field and the direction of the wave. Therefore the
magnetic field must oscillate in the NW-SE plane.
Light as an Electromagnetic Wave and the
Electromagnetic Spectrum
Light was known to be a wave, and experiments demonstrated that EM
waves share all of the same behaviors.
The frequency of an electromagnetic wave is related to its wavelength:
c = 3x108 m/s
This wave-speed equation is the same one that applies to sound, and
any other waves.
A few years after Maxwell’s breakthrough, Tesla invented (and
patented) radio technology (1897)
Marconi commercialized it and Wireless communication revolutionized
the world within a few short years. (First TransAt: 1901)
Measuring the Speed of Light
•  The speed of light was known to be very large, (Galilio concluded
that either light traveled instantaneously, or it was too fast to be
measured given human reaction times)
•  Studies of the orbits of Jupiter’s moons (Ole Romer: 1676)
showed that their positions fall successively “behind” where they
are supposed to be, as the distance between Earth and Jupiter
increases.
•  The moons are ~40min late when Earth and Jupiter are on
opposite sides of the Sun .
•  Using the familiar laws of gravity and circular motion to compute
the distance to Jupiter, the speed of light was found.
•  Romer’s result was within a few percent of the modern value
22.4 Measuring the Speed of Light
•  One important measurement, by the Polish-American physicist
Michelson in 1925 used a rotating mirror and a light beam
bounced between two mountain tops in California.
•  Over the years, measurements have become more and more
precise; now the speed of light is defined to be:
If the sun (150x103 km away) were to
somehow lose power and go dark, how long
would it take us to notice?
A. 8 min
B. 1 min
C instantaneously
D. about a year
E. 8 seconds
Light as an Electromagnetic Wave and the
Electromagnetic Spectrum
Electromagnetic waves can have any wavelength;
we have given different names to different parts
of the wavelength spectrum.
What type of EM wave could
have a wavelenght of 1
meter ?
A.  Light
B.  Radio
C.  X-ray
D.  Infrared
Electromagnetic Spectrum
Which Electromagnetic waves travel the fastest?
1. 
2. 
3. 
4. 
5. 
X-rays
Red Light
Radio waves
Microwaves
All the same
Light and Vision
Although Aristotle and other ancient scientists had already written on the nature of light and color vision,
it was not until Newton that light was identified as the source of the color sensation. In 1810, Goethe
published his comprehensive Theory of Colors. In 1801 Thomas Young proposed his trichromatic
theory, based on the observation that any color could be matched with a combination of three lights.
This theory was later refined by James Clerk Maxwell and Hermann von Helmholtz. As Helmholtz puts
it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's
theory of color sensations, like so much else that this marvellous investigator achieved in advance of his
time, remained unnoticed until Maxwell directed attention to it."[4]"
http://lite.bu.edu/vision-flash10/applets/Color/Color/Color.html
Wavelength response curves for
Human Cone Cells
• 
B
G R
• 
We typically think of these
cells as having R, G, B
wavelength coverage, the
actual responses are
shown at left, and overlap
significantly, probably to
give maximum sensitivity
and color discrimination in
the green.
Color perception arises
from the ratio of R/G/B
counts resulting from the
convolution of the incident
spectrum with these
response curves.
Temperature and the EM Spectrum
•  All everyday objects emit electromagnetic radiation
•  The part of the spectrum depends on their temperature
If you heat something up so that it begins to glow,
what color tends to appear first?
1. 
2. 
3. 
4. 
5. 
Blue
Green
Red
Yellow
Orange
What colors are possible for glowing hot objects?
• 
• 
• 
The shape and width of the
spectrum result in a certain
range of possible colors or hues
for incandescent objects.
Natural width of the black-body
distribution is much greater than
the band-pass of the eyes’
“Green” cones, so green stars
for example are not possible.
This color-space diagram shows
the color of a glowing hot object
as a function of its temperature
22.5 Energy in EM Waves
The energy transported through a unit area
per unit time is called the intensity:
(22-7)
Its average value is given by:
Units are Watts per square meter W/m2
For example the “Solar Constant” is ~ 1300 W/m2
Radio, TV, Wireless Communication
This figure illustrates the process by which a radio station transmits
information. The audio signal is combined with a carrier wave:
• Remember those Capacitor “Time Constants” ?
• Radio Transmitters contain a special circuit containing a coil and a
capacitor, designed to “oscillate” at kHz - MHz frequencies.
• This circuit creates the carrier wave.
• The audio signal is added by simply connecting the microphone
output into the circuit.
Amplitude Modulation
The mixing of signal and carrier can be done two ways. First, by using the
signal to modify the amplitude of the carrier (AM):
Note the audio signal is only approximated by the AM
envelope. The closer together the carrier “peaks” the
more faithfully the signal can be reproduced.
ConcepTest 22.4
If a radio transmitter has a vertical
antenna, should a receiver’s
antenna be vertical or horizontal
to obtain the best reception?
Radio Antennas
1) vertical
2) horizontal
3) doesn’t matter
ConcepTest 22.4
If a radio transmitter has a vertical
antenna, should a receiver’s
antenna be vertical or horizontal
to obtain the best reception?
Radio Antennas
1) vertical
2) horizontal
3) doesn’t matter
If a wave is sent out from a vertical
antenna, the electric field oscillates
up and down. Thus, the receiver’s
E field
antenna should also be vertical so
of wave
that the arriving electric field can set
the charges in motion.
E field
of wave
ConcepTest 22.3
Before the days of cable,
televisions often had two
antennae on them, one straight,
and one circular. Which antenna
picked up the magnetic
oscillations?
TV Antennas
1) the circular one
2) the straight one
3) both equally, they were
straight and circular for different
reasons.
ConcepTest 22.3
Before the days of cable,
televisions often had two
antennae on them, one straight,
and one circular. Which antenna
picked up the magnetic
oscillations?
The varying B field in the loop
means the flux is changing and
therefore an EMF is induced.
TV Antennas
1) the circular one
2) the straight one
3) both equally, they were
straight and circular for different
reasons.
FM - Frequency Modulation
Second, by using the signal to modify the
frequency of the carrier (FM):
FM results in much better signal reproduction, partly because the signal
strength is constant, and partly because it is achieved at higher frequencies
(more bits of information per second)
Why use a carrier wave at all? -Why not just broadcast the raw audio signal
as an electromagnetic wave? • To reduce the wavelength for efficient transmission and reception (the
optimum antenna size is ½ or ¼ of a wavelength). • A typical audio frequency of 3000 Hz will have a wavelength of 100 km and So would need an effective antenna length of 25 km! (this goes for both
reception and transmission)
•  By comparison, a typical carrier for FM is 100 MHz, with a wavelength of
3m, and could use an antenna only 80 cm long.
• The energy Density in very long wavelength wave is so small that
interference would be overwhelming.
• Long wavelengths do not behave in a very directional way (which for certain
very specialized applications is extremely useful)
• Digital Radio is even more efficient than FM, it uses a very high frequency,
and basically switches the signal on and off rapidly to transmit 1’s and zero’s.
Many more stations can occupy the the radio spectrum
22.7 Radio and Television;
Wireless Communication
At the receiving end, the wave is received,
demodulated, amplified, and sent to a
loudspeaker:
22.7 Radio and Television;
Wireless Communication
The receiving antenna is bathed in waves of
many frequencies; a tuner is used to select the
desired one:
Summary of Chapter 22
•  Maxwell’s equations are the basic equations
of electromagnetism
•  Electromagnetic waves are produced by
accelerating charges; the propagation speed
is given by:
•  The fields are perpendicular to each other
and to the direction of propagation.
The simplest radio consists of something to rectify the
signal (a diode in this case), and a tiny speaker
A Diode is a one-way valve for electric current. It
isolates the positive side of the radio waveform.
Earpiece
Diode
Diodes today use
germanium or silicon
(semiconductors)
In the past Galena crystals
were used, Even rusty razor
blades and pencil leads can
do the job in a pinch
In a Crystal Radio set, where does the power
come from to make an audible signal in the
earpiece?
A.  Batteries
B.  The radio waves themselves
C.  Magnets in the earpiece
Momentum Transfer and Radiation
Pressure
In addition to carrying energy, electromagnetic
waves also carry momentum. This means that a
force will be exerted by the wave.
The radiation pressure is related to the average
intensity. It is a minimum if the wave is fully
absorbed:
And a maximum if it is fully reflected:
ConcepTest 22.5
Heat Insulation
Imagine you are an alien from another planet with infrared eyes.
What do you see when you look around the room?
1) Bright spots where the bodies are and dark elsewhere.
2) Dark spots where the bodies are and bright elsewhere.
3) The same as what we see, only everything looks red.
4) The same as what we see, except that red is invisible.
ConcepTest 22.5
Heat Insulation
Imagine you are an alien from another planet with infrared eyes.
What do you see when you look around the room?
1) Bright spots where the bodies are and dark elsewhere.
2) Dark spots where the bodies are and bright elsewhere.
3) The same as what we see, only everything looks red.
4) The same as what we see, except that red is invisible.
Bodies are sources of heat and
therefore emit infrared radiation.
An alien with an instrument to
detect infrared would see these
sources as bright spots.
Infrared photo of a building to check
the heat insulation – where are the
problem spots in this case?