Physics 1230: Light and Color
Instructor: Joseph Maclennan
TOPIC 3 - Resonance and the
Generation of Light
http://www.colorado.edu/physics/phys1230
•  How do we generate light?
•  How do we detect light?
•  Concept of resonance
In vacuum
distance λ
speed = c =
= = λf
time
T
Static magnetic fields
•  Magnets generate magnetic fields that we can draw as field lines
•  Magnetic field lines form continuous loops
Bar magnet
The Earth
Static electric fields
•  Positive and negative charges generate electric fields that we can draw
as field lines
•  Electric field lines from charges behave like stretched strings
•  Electric fields generate static sparks: first radio transmissions
Positive charge
Negative charge
http://library.thinkquest.org/10796/ch12/ch12.htm
How do we generate a wave on a rope?
A)  Drop one end
B)  Raise and lower one end quickly
C)  Drop both ends
D)  Twang it in the center
http://phet.colorado.edu/new/simulations/sims.php?sim=Wave_on_a_String
How do we generate EM waves?
•  How do we make waves in the electromagnetic field?
•  Let’s wiggle the charges...
•  Light waves are disturbances in the electromagnetic field, a
non-material physical entity whose equilibrium state is vacuum
Heinrich Hertz
(1888)
http://phet.colorado.edu/new/simulations/sims.php?sim=Radio_Waves_and_Electromagnetic_Fields
Electromagnetic Wave Propagation
Electromagnetic waves can be generated by a variety of methods, such as a
discharging spark or by an oscillating electrons in an atom or molecular. As the
current oscillates up and down in the spark gap, at a characteristic circuit
frequency, a magnetic field is created that oscillates in a horizontal plane. The
changing magnetic field, in turn, induces an electric field so that a series of
electrical and magnetic oscillations combine to produce a formation that
propagates as an electromagnetic wave.
http://micro.magnet.fsu.edu/primer/java/polarizedlight/emwave/index.html
Sources of electromagnetic waves
•  So electrons moving up and down will emit electromagnetic waves
because the electrons make an electric field, and the lines are
wiggled when the electrons move
•  What makes electrons move?
1. Heat
2. An oscillating electric field due to current changes
(e.g. radio, TV, microwave oven)
3. Electrons in an excited atom
(e.g. neon sign, or a fluorescent light)
4. Chemical excitation
(e.g. firefly, phosphorescence)
Concept Quiz: How does your cell phone transmit
your voice?
A)  Generating sound waves
B)  Generating heat
C)  Generating electromagnetic waves
(high frequency radio)
D)  Generating neural activity
How does your cell phone generate electromagnetic waves?
A)  Heat
B)  Voltage in a circuit
C)  Excited atoms
D)  Neural activity/heat
Generating electromagnetic waves
SOURCE
MECHANISM
Radio stations
Voltage in a circuit
Microwaves
Voltage in a circuit
Human body
Neural activity/heat
Cell phones
Voltage in a circuit
Neon light
Excited atoms
Laser
Excited atoms
Warm stove
Heat
Sources of electromagnetic waves
What are the frequencies of these sources?
Radio stations
Microwaves
Human body
Cell phones
Neon light
Laser
Warm stove
Creating Light from Atoms
Electrons can absorb energy from external sources, such as lasers, arcdischarge lamps, and tungsten-halogen bulbs, and be promoted to higher energy
levels. Light energy is absorbed by an electron to elevate it into a higher
energy level and the energy can subsequently be released, in the form of a
lower energy photon, when the electron falls back to the original ground state.
The precise difference between the energy levels determines the resonance
frequency or color of light that is emitted or absorbed.
http://micro.magnet.fsu.edu/primer/java/fluorescence/exciteemit/index.html
Detecting Electromagnetic Waves – Resonance
The emission and reception of electromagnetic waves is not
equally efficient for all waves, but depends on the nature
of the emitter and receptor. One phenomenon, called
resonance, is responsible for this selectivity.
Resonance effects cause molecules to vibrate - for
example in your eye. Light of certain frequency
(wavelength) drives the molecules to vibrate, so that
eventually a signal is transmitted to your brain via the optic
nerve.
Detection of electromagnetic waves Resonance and Selectivity
Why do we see only certain colors?
How does tuning of a radio signal work?
Tacoma Narrows Bridge
(State Route 16 spanning the Tacoma Narrows)
In 1940, the Tacoma Narrows Bridge failed from windinduced torsional oscillations. Research of design flaws in
the bridge led to the use of aerodynamic testing as a
standard procedure in suspension span structural analysis.
Can you think of other examples of resonance?
Resonance
Every material (such as glass, steel, concrete) has a natural frequency at which it vibrates,
called a resonant frequency. If you put energy into the substance at its resonant frequency,
you will force it to vibrate or resonate (resonance is a forced vibration). In the case of the
wine glass, your finger slides and sticks along the surface of the glass as you rub the rim (a
wet fingertip has no oil and makes a better contact with the glass). The rubbing imparts
energy to the glass molecules and causes them to resonate. The motion of your hand sets up a
wave of vibration traveling through the glass. The vibrating glass causes air molecules to
vibrate at the same frequency. The vibrating air molecules are the sound wave that you hear
(the frequency or pitch of the sound wave is the same as the resonant frequency of the
glass).
So, how does the water change the pitch of the singing wine glass? As the resonant wave
moves around the glass, it drags the water molecules with it, creating a wave of water that
you can see near the edge of the glass. The dragging water molecules effectively increase the
mass (both the water and the glass molecules) and reduce the energy of the wave traveling
through the glass. When the energy is reduced, so is the frequency of the wave in the glass,
which is reflected in the pitch of the sound wave that you hear.
If you impart enough energy to the glass at its resonant frequency, you can cause the glass to
shatter. However, this takes more energy than you can provide by rubbing the rim. Some
singers can sing a note equal to the resonant frequency of a wine glass and cause it to shatter
http://static.howstuffworks.com/mpeg/wine.mpg
RESONANCE IN SOUND
If you look at a guitar string under a strobe
light (or even a fluorescent light) you can see
it that makes a standing wave. Another
experiment you can do is to stand in the
shower (they reflect sound well) and start
singing while changing the pitch slowly. At
certain pitches the sound will suddenly
amplify, because the sound waves fit an even
number of times between the walls.
http://www.colorado.edu/physics/2000/microwaves/standing_wave2.html
Examples of Resonance
Guitar string - can be tuned by changing the tension
Electrical circuits in radios
Nerve cells in eye - sensitive to red, green
and blue
Chemical dyes
Using Resonance to Shatter a Wineglass
If a singer can match the natural frequency of
the wine glass she can put more energy into the
wine glass than it can handle. As the energy
builds, the glass begins to deform beyond what
its bonds can sustain. The trick is to sing with
the right frequency and being able to sustain
that note. It is not about singing loudly or
horribly. It is also not possible for one note to
shatter all glasses as each glass would have its
own natural frequency.
http://www.blazelabs.com/pics/glass.mov
http://video.google.com/videoplay?docid=-7765557442856739526
Another Example of Resonance
Using Resonance to shatter a Kidney stone.
By tuning ultrasound waves to the natural frequency of a
kidney stone, we can rely on resonance to pulverize the stone
Concept Test on Resonance
You are swinging back and forth on a swing at the
natural frequency. If a friend (or your cat) joins
you on the swing, the new natural frequency will be:
A.
B.
C.
D.
greater
the same
smaller
zero - you won’t be able to swing any more
Concept Test on Resonance
If you rub the rim of a wineglass you can make it
”sing”. This is because:
A.  vibrations from your finger excite a resonant
response in the glass
B.  you need to sing the same note to get the resonance
C.  the glass is a mechanical system in resonance
D.  the table top transmits a musical tone to the glass
E.  otherwise it would shatter
RESONANT ABSORPTION OF LIGHT -
Photosynthesis
Green plants absorb water and carbon dioxide
from the environment, and utilizing energy from
the sun, turn these simple substances into
glucose and oxygen. With glucose as a basic
building block, plants synthesize a number of
complex carbon-based biochemicals used to
grow and sustain life. This process is termed
photosynthesis, and is the cornerstone of life on
Earth. In the applet, water molecules are converted to
molecular hydrogen and oxygen as a result of
photon absorption in the granum. Subsequently,
the hydrogen molecules react with carbon
dioxide in the stroma to produce oxygen and
carbohydrates. http://micro.magnet.fsu.edu/primer/java/photosynthesis/index.html
Color of Plants- Photosynthesis
Chlorophylls absorb blue and red light
and carotenoids absorb blue-green
light, but green and yellow light are
not
effectively
absorbed
by
photosynthetic pigments in plants;
therefore, light of these colors is
either reflected by leaves or passes
through the leaves. This is why plants
are green.
Examples of photosynthetic
organisms: leaves from higher
plants flanked by colonies of
photosynthetic purple bacteria
(left) and cyanobacteria (right).
http://photoscience.la.asu.edu/photosyn/education/photointro.html
COLOR OF PLANTS - Photosynthesis
Chlorophylls absorb blue and red
light and carotenoids absorb bluegreen light, but green and yellow
light are not effectively absorbed
by photosynthetic pigments in
plants; therefore, light of these
colors is either reflected by leaves
or passes through the leaves. This is
why plants are green.
Absorption spectrum of isolated chlorophyll and
carotenoid species. The color associated with the
various wavelengths is indicated above the graph.
http://photoscience.la.asu.edu/photosyn/education/photointro.html
What we see
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
Effect of resonance produced by military
helicopter blade going around at frequency
resonant with the helicopter body
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