Physics 212 Lecture 23, Slide 1

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Physics 212 Lecture 23 Polarization
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Some of your comments
Practical examples of what polarizing and
circular polarizing light does.
A few more examples please and thank you!
More about the wave plate and right/left
polarization.
How exactly does circularly polarized light look?
I'm trying to envision it at the moment, and I'm
failing miserably.
If a wave has only a verticalcomponent and is
passed through a polarizer that is at some angle
between 0 and 90 degrees, how exactly does a
horizontal component result?
Circular polarization and birefringment
materials.
This stuff's pretty easy now, but what's the
point of it all? We now understand how to
polarize EM waves. Big whoop.
How did we change the speed of light? I thought
it was a constant.
Hey I'm sitting in the second row back in the
middle wearing a brown hat and I'm dying of
spontaneous dental hydroplosion and the only cure
is rich dark chocolate. Help me soon my teeth
don't have too long before they turn to liquid
and drip down the back of my throat.
I am unsure of my answers to seven and nine and
would like to go over those problems.
I kinda don't understand this stuff, but I don't
want to go to class either, so maybe I will sleep
with the prelecture running through my headphones
and some sort of miracle will happen.
Scientists have been able to slow down light
(even stop it) by passing it through
bose-einstein condensate. If the speed of light
can be changed by passing through really really
cold things, are our measurements of distances to
other stars/galaxies completely wrong due to
observable EM waves passing through really really
cold space?
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Thanksgiving is just two weeks away... and the
final is just two weeks away from Thanksgiving.
Yay
I think i'm good actually. You'll also notice
(well maybe not, i'm not sure how it works) that
i did in fact hold up to my promise and i
rewatched the past 3 lectures that i skipped due
to insane amounts of midterms. 3 cheers for me!
Why don't we get today off of school? it is a
federal holiday and a state holiday but it isn't
important enough to remember our veterans.
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So far we have considered plane waves that look
like this
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Linear Polarization
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The intensity of a wave does not depend on
itspolarization
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Polarizers
The molecular structure of a polarizer causes the
component of the E field perpendicular to the
Transmission Axis to be absorbed.
Animation
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Two Polarizers
since the second polarizer is orthogonal to the
first, no light will come through. cos(90) 0
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Two Polarizers
you can use a polarizer at an angle so that it
takes a portion of the polarized light and
basically lets the light through AT THAT ANGLE.
Then, when the light reaches the second
polarizer, it is no longer orthogonal to it.
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What if we put many polarizers between?
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There is no reason that f has to be the same for
Ex and Ey
Making fx different from fy causes circular or
elliptical polarization
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Q How do we change the relative phase between
Ex and Ey? A Birefringence
By picking the right thickness we can changethe
relative phase byexactly 90o.
This changes linear to circular polarization
and is called a quarter wave plate
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Right Handed Circular Polarization(at t 0)
This modification does notchange the intensity
either
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Circular Light on Linear Polarizer
Q What happens when circularlypolarized light
is put through apolarizer along the y (or x)
axis ?

• I 0
• I ½ I0
• I I0

X
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In case A you lose your horizontal component of
the wave, however in B you retain both, just at a
shift of pi/2.
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Both Ex and Eyare still there, sointensity is
the same
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Right circularly polarized
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Executive Summary
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Demos
What else can we put in there to change the
polarization?
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Calculation
Light is incident on two linear polarizers and a
quarter wave plate (QWP) as shown. What is the
intensity I3 in terms of I1?
fast
45o
y
x
slow
60o
I1
I2
z
I3

• Conceptual Analysis
• Linear Polarizers absorbs E field component
  perpendicular to TA
• Quarter Wave Plates Shifts phase of E field
  components in fast-slow directions
• Strategic Analysis
• Determine state of polarization and intensity
  reduction after each object
• Multiply individual intensity reductions to get
  final reduction.

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Calculation
Light is incident on two linear polarizers and a
quarter wave plate (QWP) as shown.
fast
45o
y
x
slow
60o
I1
I2
z
I3
Curl fingers of RH back to front Thumb points in
dir of propagationif right hand polarized.
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Calculation
Light is incident on two linear polarizers and a
quarter wave plate (QWP) as shown.
fast
45o
y
RCP
E1
x
slow
Ex
Ey
60o
I1
l/4
I2
z
I3

• What is the intensity I2 of the light after the
  QWP?

(A) I2 I1
(B) I2 ½ I1 (C) I2 ¼ I1
AFTER
No absorption Just a phase change !
Same before after !
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Calculation
Light is incident on two linear polarizers and a
quarter wave plate (QWP) as shown.
fast
45o
y
RCP
E1
x
slow
Ex
Ey
60o
I1
l/4
I2 I1
z
I3
Absorption only passes components of E parallel
to TA (q 60o)
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Calculation
Light is incident on two linear polarizers and a
quarter wave plate (QWP) as shown.
fast
45o
y
RCP
E1
x
slow
Ex
Ey
60o
E3
I1
l/4
I2 I1
I3 ½ I1
z

• What is the intensity I3 of the light after the
  60o polarizer?

(A) I3 I1
(B) I3 ½ I1 (C) I3 ¼ I1
Ey
E3
3
NOTE This does not depend on q !!
60o
Ex