Announcements 10/24/12





Prayer
Slinkies: Please turn-in & cross out your name
Lab 6 due tonight
Exam 2 starts tomorrow, goes through Tuesday
evening
My office hours today: only to 2:30 pm
Foxtrot
From warmup

Extra time on?
a. (Nothing in particular)

Other comments?
a. A Benjamin Franklin and a 12 pack of Diet Coke
says that there will be ranges and no proofs on
the test.
b. I hope we do a cool demo with polarization.
c. Since we had slinkys for waves, does that mean
we get plastic prisms for optics?
From Warmup

In section 35.1, your book says: "The question "Is light a wave or
a particle" is inappropriate, however. Sometimes light acts like a
wave, and other times it acts like a particle." Do your best to
explain to a friend in junior high school what that means and how
it can be possible.
a. Light is what it is, which is neither a wave or a particle. It
BEHAVES sometimes like waves and particles. When we say
that it behaves that way, we mean that modeling light as either
a particle or a wave (depending on the situation) helps us to
understand the behavior of light itself.
b. Depending on the scale with which you view light, it can be
better to describe it as a particle or a wave. When studying
humans, it makes more sense to describe a group of people as
the collective qualities of the group, rather than as thousands of
individual people. When viewing only a few people, it is better
to view them as individuals.
From Warmup
http://www.empiricalzeal.com/wp-content/uploads/2011/06/wave-particle-duality.jpg
Wave-particle duality
xkcd
Advertisement for Phys 222, 451
Wave/particle duality



Textbook: “Sometimes light acts like a wave, and other times it
acts like a particle.”
Colton: Light is made up of quantum-mechanical particles,
called “photons”. Electrons, protons, etc., are also quantum
mechanical particles. Quantum-mechanical particles are neither
waves nor particles in the macroscopic sense, but rather we
should think of the converse: “waves” and “particles” as we
typically use the words are based on our observations of largescale effects of these quantum-mechanical particles. That is,
quantum mechanics says that there are no such things are
"particles". All real particles exhibit both "particle" as well as
"wave" behavior. Macroscopic waves and particles are
manifestations of the underlying quantum effects, not the other
way around.
Colloquium speaker in 2011: “Photons don’t exist. They are only
quantized oscillations of electro-magnetic fields.”
Advertisement for grad school. 
Clicker question:

Which of the following scientists did not attempt
to make a measurement of the speed of light?
a. Einstein
b. Fizeau
c. Galileo
d. Roemer
e. Michelson
How did each attempt to measure it?
Advertisement for Phys 471
From warmup

A beam of light passes through a hole of
diameter d in a metal plate. Under what
condition are we allowed to ignore the diffraction
or “spreading” of the light?
a. when lambda<<d
“Ray approximation”
Diffraction: spreading of light (and interfering with itself)
lambda = d
50
100
150
200
250
300
350
400
450
500
50
100
150
200
250
300
350
400
450
Next three slides: image credit to Dr. Durfee
500
lambda = d/4
50
100
150
200
250
300
350
400
450
500
50
100
150
200
250
300
350
400
450
500
lambda = d/10
50
100
150
200
250
300
350
400
450
500
50
100
150
200
250
300
350
400
450
500
Index of Refraction
v = c/n

Book table
Index of Refraction
Different
wavelengths have
different speeds!
v = lf
linside = lvacuum/n
Different wavelengths (k values)
have different
speeds!
Dispersion!
blue green
Song: Roy G. Biv (start at 0:30)
http://www.youtube.com/watch?v=Gf33ueRXMzQ
red
(l going into material)
Image:
http://en.wikipedia.org/wiki
/Dispersion_(optics)
Clicker question:

Which color of light travels fastest in glass?
a. Red
b. Green
c. Blue
d. Same
Absorption

“Lorentz oscillator model”
“anomalous” index of refraction
index of
refraction
absorption
increasing
frequency
From Peatross & Ware,
textbook for Phys 471
(decreasing l)
Why is blue light slower through glass than red light?
 It’s closer to an absorption region
Index of Refraction

Light ray at boundary
fast light
(smaller n)
slow light
(larger n)
q1
q2
Snell’s Law

n1sinq1 = n2sinq2
fast light
(smaller n)
q1
slow light
(larger n)
Advertisement for Phys 442/471
q2
From warmup

In the movies, you sometimes see an actor
looking in a mirror and you can see a front
view of his/her face in the mirror. During the
filming of such a scene, what does the actor
see in the mirror?
a. The camera!
“Time-reversal symmetry”
Law of Reflection

qrefl. = q1
Reflections occur
off of any boundary,
not just “mirrors”
fast light
(smaller n)
q1 qrefl.
slow light
(larger n)
q2
When will you have no reflection?
Fresnel Coefficients / Fresnel Equations

If near perpendicular (1-D problem)
v2  v1 n1  n2
r

v1  v2 n1  n2
R r

2v2
2n1
t

v1  v2 n1  n2
2
T  1 r
2
Look familiar??
For arbitrary angle (these eqns not needed for HW/exam)
n1 cosq1  n2 cosq2
rs polar . 
n1 cosq1  n2 cosq2
2n1 cosq1
ts polar . 
n1 cosq1  n2 cosq2
n1 cosq2  n2 cosq1
rp polar. 
n1 cosq2  n2 cosq1
2n1 cosq1
t p polar. 
n1 cosq2  n2 cosq1
Advertisement for Phys 442, Phys 471
Clicker question:
top
bottom

I send white light into a prism as shown below
(n>1). Will the red part of the “rainbow” be on
the top or the bottom of the outgoing fan of
light?
a. top
b. bottom
Demos


Reflection/refraction using water-soluble oil
“Blackboard optics”