Announcements 10/29/12
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

Prayer
Clement will be out of town on Friday, no office
hours
Exam 2 ends tomorrow
Pearls
Before
Swine
From warmup
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Extra time on?
a. polarization by reflection (3 students)

Other comments?
a. Do we really have homework due on a national
holiday?
From warmup

What does it mean to say that a given light
beam is polarized? Or unpolarized?
a. If it is polarized, the electric field vibrates in
only one plane. If light isn't polarized, the
electric field is free to vibrate any which way
it likes.
 mostly correct
Horizontally Polarized Light
Credit: the next few slides are from Dr. Durfee
Vertically Polarized Light
Diagonally Polarized Light
Circularly Polarized Light
Elliptically Polarized Light
Unpolarized Light
Clicker question:

What type of polarization is displayed in the
animation?
http://stokes.byu.edu/teaching_resources/emwave_flash.html
a.
b.
c.
d.
e.
Horizontally polarized
Vertically polarized
Diagonally polarized
Circularly polarized
Non-polarized
Circularly Polarized, pictures
Pictures from Wikipedia
Polarizers


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Crystals
Lines
Polaroid
of wires
film
Polaroid Film
Crystals
www.thorlabs.com
Clicker question:

If you send horizontal linearly polarized light
through a (perfect) vertical polarizer, how much
of the light intensity will get through?
a. 0-20%
b. 20-40%
c. 40-60%
d. 60-80%
e. 80-100%
Clicker question:

If you send horizontal linearly polarized light at
45 through a perfect vertical polarizer, how
much of the light intensity will get through?
a. 0-20%
b. 20-40%
c. 40-60%
d. 60-80%
e. 80-100%
Clicker question:

If you send circularly polarized light through a
perfect vertical polarizer, how much of the light
intensity will get through?
a. 0-20%
b. 20-40%
c. 40-60%
d. 60-80%
e. 80-100%
Clicker question:

(Like HW 27-2) If you send horizontal linearly polarized
light through a vertical polarizer, no light gets through
because there is no component of the electric field in
the light wave that is oscillating vertically. If you insert a
diagonal polarizer at 45 between the two, how much of
the light intensity will now get through the final
polarizer?
a. 0-20%
b. 20-40%
c. 40-60%
d. 60-80%
e. 80-100%
Demos

Polarization configurations
From warmup:

In the discussion accompanying the figure shown above,
the text explains how a certain reflection angle can lead to
polarization. In your own words, explain how this works.
a. When waves are reflected off of surfaces, the parallel
[to the surface] component reflects more strongly than
the perpendicular [to the first component] component.
b. At a certain angle, the only light that reflects must be
oriented parallel to the surface. Kind of like how a flat,
horizontal rock is the best to skip across water.
Remember these? (Fresnel Coefficients)

If near perpendicular (1-D problem)
v2  v1 n1  n2
r

v1  v2 n1  n2
When/what is
phase shift?

R r
2v2
2n1
t

v1  v2 n1  n2
2
T  1 r
2
For arbitrary angle (you don’t need to know for this class)
n1 cos1  n2 cos2
rs polar . 
n1 cos1  n2 cos2
ts polar . 
n1 cos2  n2 cos1
rp polar. 
n1 cos2  n2 cos1
2n1 cos1
t p polar. 
n1 cos2  n2 cos1
2n1 cos1
n1 cos1  n2 cos2
What is s-polar? What is p-polar?
Plots for air (n=1) to glass (n=1.5)
s-polarization
p-polarization
field amplitudes vs 
field amplitudes vs 
1.0
t
0.5
t
0.8
0.6
20
40
60
80
r
0.5
0.4
r
0.2
20
1.0
40
60
80
0.2
intensities vs 
Do you always intensities
get a
180 phase shift1.0upon
reflection?0.8
1.0
T
0.8
0.6
0.6
0.4
0.4
R
0.2
20
40
vs 
T
R
0.2
60
80
Brewster’s angle!
20
40
60
80
Fresnel Coefficients, cont.

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
For arbitrary angle (you don’t need to know for this class)
n1 cos1  n2 cos2
rs polar . 
n1 cos1  n2 cos2
ts polar . 
2n1 cos1
n1 cos1  n2 cos2
n1 cos2  n2 cos1
2n1 cos1
rp polar. 
t p polar. 
= 0,
apply Snell’s Law…
n1 cos2  n2 cos1 Set numerator n
1 cos 2  n2 cos1
a bit of algebra/trig…
tan1 = n2/n1
Clicker question:

If you send an unpolarized beam at a piece of glass
at Brewster’s angle, what happens?
a. The reflected beam is partially polarized
b. The reflected beam is completely polarized
c. The transmitted beam is partially polarized
d. The transmitted beam is completely polarized
e. More than one of the above
“Brewster windows”
p-polarization

If you send p-polarized
light towards a piece
of glass at Brewster’s
angle, how much gets
transmitted?
1.0
t
0.8
0.6
0.4
r
0.2
20
40
60
80
0.2
1.0
T
0.8
0.6
0.4
R
0.2
20
40
60
80
Clicker question:

In polarized sunglasses, some of the glare
from the sun is eliminated when the sun is
directly in front (and above) you. Should the
polarizing material be aligned to allow
vertical or horizontal electric fields through?
a. vertical
b. horizontal
Demo

Flashlight reflecting off of glass