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Activities
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Gas lamps and spectrum glasses –explain atomic spectra (several websites help)
BB radiation – spectrum glasses and a filament bulb
ray tracing
o chart of rules
o mirror with pin (image and object)
o basic rays = reflection off mirror and plastic
o basic rays refraction = plastic blocks
o find all five rays
o total internal reflection circular block
o fibers small, large
o many websites which clarify above ideas using rays
o many websites which clarify above ideas using waves
o laser beam shooting across the class unseen followed by chalk dust display
 laser in water stream demo
 optics
o human eye and glasses
 construct a model of nearsighted and far sighted
 use corrective lenses
o single convex lens (ray tracing)
o convex + concave (ray tracing)
o two mirrors that provide image display
 diffraction and refraction
o lasers, gratings , slits
o water tank to show this as a wave phenomena
 bent pencil in water
 hologram
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Light sources:
The key is to recognize that there are two distinct ways to generate light that we sould
like to demonstrate and explain. The two are demonstrated using the tower of light.
The filament is glowing due to BB radiation. The gas tubes generate light based on
the atomic spectra.
BB radiation
 Systems in thermal contact share their energy so that all available systems
eventually systems get their share. Consider the classroom it is maintained at
70oF by heaters and coolers. If a glass of ice tea or hot coffee is brought into
the room they will both eventually reach 70oC. One by interchanging energy
and lowering temperature and the other gains energy.
 Not all systems are in equilibrium.
o A burner on a stove can be at one temperature and the air in the room
at another. Of course eventually there would reach equilibrium if the
two systems were isolated and in thermal contact but for all practical
purposes the two coexist at different temperatures (light in a room is
not in equilibrium with the air)
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o If two systems do not interact they cannot exchange energy and
therefore will never reach equilibrium. A typical approach is to isolate
systems with insulation. A thermos bottle is a device that keeps its
contents from interacting with the air in the room.
Light can be thought of as a sort of billiard ball carrying energy. The energy
that a “LIGHT” billiard ball carries determines its color or frequency, f ,
[ e  hf (h  6.626  10 24 Js ) .
An interesting difference however is that “LIGHT” billiard balls can be
created and destroyed.
Molecules and atoms are made of charged particles. There are a host of ways
that the motion of these charges can generate or absorb “LIGHT” billiard
balls.
There are interactions between E&M fields and matter so that thermal
equilibrium between the two systems will eventually be reached.
One aspect of a Light-matter system can be modeled is by imagining the
billiard balls that represent one system are colliding with the surface of water.
o Collisions will generate waves. The surface will eventually have
bumps of water rising and falling and moving around. Equilibrium is
reached when a ball close to the water’s surface is just as likely to
absorb as it is to give energy to the water. After some time the water
would be wiggling on average with the same average energy and this
would be the average energy carried by the moving air molecules.
o Notice that the water could have no wiggles or many wiggles. With the
energy stored in surface waves one cannot associate a particle with this
energy. The amount of energy in waves can change. This mimics part
of the way that the E&M field absorbs and emits energy. There is no
fixed number of particles associated with a wave or with the E&M
field. This is a profound difference. The billiard ball model keeps the
number of balls fixed and varies their energy by increasing or
decreasing speed. The water surface model has disturbances that can
grow, shrink, appear or disappear so the number of disturbances does
not remain the same.
o Using what we learned with sound we can take any disturbance on the
water surface and describe it as a sum of “tuning fork” or sine waves
each with a different frequency. Each collision can create or add
energy to a particular tuning wave or destroy or remove energy.
{destroy and create imply that a given vibrational mode “specific
tuning fork” may not be vibrating at all.}
o To complete the picture we require a certain base line or quanta of
energy must be added or subtracted for each tuning fork. This
completely defies the classical wave picture which says that a string or
tuning fork can vibrate at any amplitude and therefore at any arbitrary
energy. We are negating this idea and replacing it with the notion that
all increments must be a multiple of some basic chunk or quanta or
energy.
o Assume that the disturbance on the surface of the water at some instant
in time looks like the plot below.
100
80
60
40
20
0
10
60
110
160
210
260
310
-20
This disturbance is built from
20
15
10
5
0
-5
-10
-15
-20
10
60
110
160
210
260
310
+
10
0
1
0
60
1
1
0
1
60
1
0
60
1
1
0
1
60
21
0
260
31
0
-10
+
20
15
10
5
0
21
0
260
31
0
-5
-10
-15
-20
+ a few more tuning fork waves.
We require that any give wave consist of some number of steps of the basic amount
(eg blue wave). This in some sense recovers the particle picture because now I can
talk about having three steps worth of blue and 5 steps worth of pink. But there is no
required fixed value for the number and each step is worth a defienite fixed amount fo
enery. Therefore to add energy to the “water surface” I need to add for example 5
steps in the blue and 4 in the pink or 9 overall steps (PHOTONS). To remove energy I
take some of the steps away.
Atomic spectra
 Our model is a ball rolling into a valley or a pendulum on a string. Here the
energy of the pendulum can be increased or decreased. If the pendulum were
able to release or absorb energy by creating or removing a photon then light
could be generated by the pendulum.
 Note that here all of the pendulum energy is available and directly given to the
photon created. The ideas of thermodynamics are not relevant in this model.
This is similar to throwing a baseball rather than trying to get it to move faster
by heating it up. The atom has energy available that can be directly converted
to light energy. The gas lamps simply break atoms (ionize) apart and then
when they recombine they create photons. (There is no average or sharing.)
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Ray Model
A typical way to analyze and understand optical systems is to analyze using light
rays. The laser is a fairly good replica of the ideal ray. Light rays move in straight
paths until they encounter a change in medium. At these interfaces they follow the
law of refraction and reflection. The way light rays behave can then be
experimentally determined by following rays through various materials and at various
angles.
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