Physics 1230: Light and Color
Instructor: Joseph Maclennan
CLASS 1 - Waves and the
Electromagnetic Spectrum
http://www.colorado.edu/physics/phys1230
Sources of Light
in Nature
Sources of Light
in Nature
Today’sTopics
•  Describing waves
•  Light waves - the Electromagnetic Spectrum
•  Speed of light
Waves
We are familiar with waves in everyday life waves on a rope, waves in the ocean, sound waves,
shock waves
Propagating waves
- air is medium
these are called
”mechanical” waves
rope
x
Propagating waves
- rope is medium
More about Waves
•  A wave is a propagating disturbance of an equilibrium
state - for example ripples on water, waves on a rope,
light in vacuum etc.
•  The medium does not have to move far - but the
disturbance moves, often at some characteristic speed
(e.g. cork on water)
Wave on a rope
Light waves
Waves on a Rope
http://phet.colorado.edu/simulations/
Concept Question - Waves on a Rope
The direction of motion of the rope is A.  Parallel to the propagation of the wave
B.  Perpendicular to the wave propagation
C.  Other
Waves
Both the waves in the flag and the ocean waves are waves that you
can see - where we see the medium move. There are other kinds
of waves that we cannot ”see”, but which we experience every day.
These waves are called electromagnetic waves.
Sound Waves
Sound is also a type of wave that we cannot see. Like ocean waves,
sound waves need a medium to travel through. Sound can travel
through air because air is made of molecules. These molecules
carry the sound waves by bumping into each other, like Dominoes
knocking each other over. Sound can travel through anything made
of molecules - even water! There is no sound in space because
there are no molecules there to transmit the sound waves!
Alien Poster
Waves can be either Periodic or Aperiodic
Periodic wave
λ
space
λ
Periodic wave
Aperiodic
wave
Wavelength
For periodic waves, we can identify a wave length, λ
”lambda” is the repeat distance of the wave
Periodic wave
λ
space
λ
Periodic wave
Period and Frequency
For periodic waves, we can identify a period, T, by measuring
the time taken for a complete disturbance to pass a given point -
The frequency, υ, (”nu”) is the inverse of the
period i.e.
υ = 1/T
and is the number of times per second that an
oscillation occurs at any fixed point in space
Wavelength of a Wave
What is the wavelength of the red wave?
A)  1m
B)  2m
C)  3m
meters
Period of a Wave
What is the period of the green wave?
A)  1 second
B)  1.5 seconds
C)  2 seconds
0
5
10
(sec)
Wavelength, Frequency, and Velocity
For periodic waves, we can identify a speed, v, by
Speed = distance/time
Speed = Wavelength/Period
Speed = Wavelength x frequency
v = λυ
So => c = λυ or υ=c/λ or λ=c/υ
So knowing the frequency, we can calculate the wavelength
Or knowing the wavelength, we can calculate the frequency
For light waves, the speed in air or vacuum is
3 x 108 meters/sec
Wavelength
What is the wavelength of this wave?
A)  4 cm
B)  8 cm/second
C)  8 cm
D)  8 m
E)  None of above
Frequency
What is the frequency of this wave?
A)  2 cm/second
B)  8 cm/second
C)  4 seconds
D)  2 cycles/second
E)  0.25 cycles/second
Light Waves part of the EM
Spectrum
Electromagnetic waves are unlike sound or rope waves because they
do not need molecules to travel. This means that electromagnetic
waves can travel through air and solid materials - but they can also
travel through empty space. This is why astronauts on spacewalks
use radios to communicate. Radio waves are a type of
electromagnetic wave.
Electromagnetic Waves
EM waves are periodic, with speed given by
Speed = Wavelength x frequency
c = λυ
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/electromagnetic/index.html
If watch train go by and 3 cars pass by sign post in one second and
each car is 10 meters long, how to calculate speed of train?
(which eqn makes sense?)
a.  Speed of train = (length of car)/(# of cars per second)
b.  Speed of train = (length of car) x (# of cars per second)
c.  Speed of train = (# of cars per second) / (length of car)
b. Speed of train = (length of car) x (# of cars per second)
Speed of light = Wavelength of wave x Frequency OR c = λ ν
Speed of light = 3 x 108 m/s
Wavelength of wave ~ length of car
Frequency of wave ~ # of cars (oscillations) per second
Visible light is only a small part of the electromagnetic spectrum.
Most of the electromagnetic spectrum is not accessible to us,
unless we're aided by special detectors tuned to the desired
energies, much like our eyes are "tuned" to the energy of visible
light.
Scientific Notation
mega, written M, means 106: a 100 MHz FM station is generating waves at 108 Hz
milli, written m, means 10-3: 1/1000 of a meter is a millimeter, or 1 mm
micro, written µ, means 10-6: the wavelength of light is 0.5 micrometers = 0.5 µm
nano, written n, means 10-9: the wavelength of green light is 500 nanometers = 500 nm
400 nm
500 nm
650 nm
0.4 µm
0.5 µm
0.65 µm Waves - the Electromagnetic Spectrum
http://imagers.gsfc.nasa.gov/ems/visible.html
Waves - the Electromagnetic Spectrum
Electromagnetic Waves critical to life as we know it!
• Communications – radio, TV, cell phones, portable phones
• Food prep - microwaves
• Vision (and Life!) – visible light
• AM radio 530 to 1600 kHz.
• FM is 88 to 108 MHz.
• TV is 54-206 MHz (each station gets 6 MHz band (Station 1, 54-60 MHz))
• Microwaves - same thing but few x 109 Hz (oscillations/s),
• Light - same thing but several x 1014 Hz
All are electromagnetic waves, but different frequencies:
AM radio 530 to 1600 kHz.
FM is 88 to 108 MHz.
TV broadcasting is 54-206 MHz - each station gets 6 MHz
wide band (Station 1 54-60 MHz)
Microwaves - same thing but few x 109 Hz (oscillaitons/s),
Light - same thing but several x 1014 Hz
What is a resonance?
•  Many objects oscillate or
vibrate at special frequencies
called resonant frequencies
or resonances
•  When these objects are hit
or "shaken" by an external
agent at a frequency = to
their resonant frequency
they will oscillate at their
resonant frequency.
–  Hand moving back and forth at
same frequency as pendulum’s
resonant frequency (or hit)
–  Tacoma narrows bridge in the
wind
–  Car on a dirt road with regular
bumps (washboard effect)
•  The oscillations of the object
are largest when the
"shaking" occurs at the
object’s resonant frequency.
–  We then say that a resonance
has occurred
e.g. girl on swing being pushed by
her mother (mother’s push
frequency = swing frequency)
•  Energy is transferred from
an external agent to the
object during resonance.
–  Wineglass broken by an opera
singer’s voice
–  due to resonance between voice
sound frequency and natural
frequency of wineglass
What do resonances have to do
with light?
•  When light is absorbed
by atoms we can think
of this as a resonance
–  The light frequency may
match a certain frequency
of resonant vibration in the
atom.
–  When this happens, the
energy of the light is
transferred to the atom
and the light disappears.
–  For example, we see light
rays of 470 nm coming into
our eyes because this light
excites a resonance in
certain atoms inside our
eyes
•  When light is emitted
by atoms we can think
of this as a resonance
–  For example when an
electron hits an atom the
atom can gain energy in
the form of resonances.
–  This energy in the atom
can then be released by
another resonant
interaction in which light
is emitted and the atom
loses energy.
–  Each color of light emitted
corresponds to a
particular atomic
resonance.
Resonance
and the
Creation of Light
•  Absorption of light
•  Emission of light
How does the light from a light bulb
depend on temperature?
•  Light from ideal sources is
generally a mixture of
different wavelengths
–  Think of the light from
the sun, which is broken
up by a prism
–  (Such light is called
black- body radiation)
•  The mixture of
wavelengths can be
understood by asking how
bright is the mixture at
each wavelength?
•  The result is a curve which
peaks at a certain wavelength
and falls off at higher or
lower wavelengths
–  The hotter the source, the
lower the wavelength at
which the peak brightness
occurs
–  Demo using incandescent
bulb with controlled
current (and hence
temperature)
–  This is important in moviemaking
Black Body Radiation
The hotter the source the bluer the white light.
The cooler the source the redder the white light
What can we see?
Concept Question
What is the wavelength of red light?
A)  650 nm
B)  0.650 µm
C)  6.5 m
D)  65 mm
E) none of the above
Concept Question
X-ray wavelengths are
A)  longer
B)  shorter
C)  same
than/as visible light?
How can we "see" using the Infrared?
Since the primary source of infrared radiation is heat or thermal radiation, any
object which has a temperature radiates in the infrared. Even objects that we
think of as being very cold, such as an ice cube, emit infrared. When an object
is not quite hot enough to radiate visible light, it will emit most of its energy in
the infrared. For example, hot charcoal may not give off light but it does emit
infrared radiation which we feel as heat. The warmer the object, the more
infrared radiation it emits.
Humans, at normal body temperature, radiate most strongly in the infrared at a
wavelength of about 10 microns.
http://imagers.gsfc.nasa.gov/ems/infrared.html
Ultraviolet Light
Ultraviolet (UV) light
has shorter wavelengths
than visible light.
Though these waves are
invisible to the human
eye, some insects, like
bumblebees, can see
them!
Our Sun emits light at all the
different wavelengths in
electromagnetic spectrum, but it
is ultraviolet waves that are
responsible for causing sunburns.
At right is a satellite image of
the Sun taken at an Extreme
Ultraviolet wavelength – (171
Angstroms).
http://imagers.gsfc.nasa.gov/ems/
X-Rays
X-rays were first observed and
documented in 1895 by Wilhelm
Conrad Röntgen, a German
scientist who found them quite
by accident when experimenting
with vacuum tubes.
A week later, he took an X-ray
photograph of his wife's hand
which clearly revealed her
wedding ring and her bones. The
photograph electrified the
general public and aroused great
scientific interest in the new
form of radiation. Röntgen called
it "X" to indicate it was an
unknown type of radiation. They
are still often referred to as
Röntgen rays in German-speaking
countries.
http://imagers.gsfc.nasa.gov/ems/
Let’s check the relationship between
wavelength and frequency
Your 101.5 FM radio station broadcasts at a frequency around
101.5 MHz = 101.5 x 106 cycles/second
=> wavelength = velocity /frequency= (3 x 108)/(101.5 x 106) ≈ 3 meters
Let’s check the relationship between
wavelength and frequency
Your 1490 AM radio station broadcasts at a frequency of
1490 kHz = 1490 x 103 cycles/second
⇒ What is the wavelength? A) 200 meters
B) 2 meters
C) 2 mm
Concept Questions
1. If increase amplitude of an electromagnetic wave, wave
will get from a transmitter to a receiver a. sooner than small amplitude wave, b. same time, c. faster
ans. b. ALL electromagnetic waves travel at c = speed of light.
2. If increase wavelength of a wave, it will a.  move up and down with higher frequency
b.  move up and down with lower frequency
c.  frequency stays the same
e. Speed stays the same
ans. b. Since the speed of light is the same at c, if the wavelength
increases, the frequency MUST decrease
Speed of Light - Jupiter's Moon Io
(see http://www.phys.virginia.edu/classes/109N/lectures/spedlite.html)
1676, Ole Rømer, a Danish astronomer,
working at the Paris Observatory. He had
made a systematic study of Io, one of the
moons of Jupiter, which was eclipsed by
Jupiter at regular intervals, as Io went
around Jupiter in a circular orbit at a
steady rate.
Michelson Measures the Speed of Light (1875)
(see http://www.phys.virginia.edu/classes/109N/lectures/spedlite.html)
Michelson in 1875 is commissioned and becomes an instructor in physics and
chemistry at the Naval Academy.
Lecture demonstrations had just been introduced at Annapolis and it was suggested
that it would be a good demonstration to measure the speed of light by Foucault's
method. Michelson soon realized, on putting together the apparatus, that he could
redesign it for much greater accuracy. Instead of Foucault's 60 feet to the far
mirror, Michelson had about 2,000 feet along the bank of the Severn, a distance he
measured to one tenth of an inch. He invested in very high quality lenses and mirrors
to focus and reflect the beam.
His final result was 186,355 miles per
second, with possible error of 30
miles per second or so. This was
twenty times more accurate than
Foucault, made the New York Times,
and Michelson was famous while still
in his twenties. In fact, this was
accepted as the most accurate
measurement of the speed of light
for the next forty years, at which
point Michelson measured it again.
Measuring the speed of light
(Michelson, 1926)
22 miles
Rotating mirror
Reflecting mirror
Telescope
Measuring the speed of light
Rotating
mirror
22 miles
Telescope
Reflecting
mirror
At proper speed, mirror moved 1/8 turn during the 44 mile path of light,
and therefore the return light was visible through the telescope. This
happened at 530 revolutions per second.
=> Speed = distance/time = 44 miles/(1/8 x 1/530) seconds
= 186,000 miles/second
Speed of Light as a Constant – since 1983
•  The speed of light in the
vacuum of free space is an
important physical constant
usually denoted by the symbol
c. The metre is defined such
that the speed of light in free
space is exactly 299,792,458
metres per second (m/s).
The Globe