Physics 212 Lecture 23, Slide 1

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Physics 212 Lecture 23
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Music

• Who is the Artist?
• Sarah Vaughn
• Billie Holiday
• Dinah Washington
• Nina Simone
• Ella Fitzgerald

Why? Some time back you picked her when I
played Chet Baker. I promised Id play her some
day and here it is. A true
classic.. dramatic, defiant, and
uncategorizable Important figure in the 60s
If you havent heard her before, you really
should
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Things you identified as difficult
I'm having a lot of trouble with circular
polarization. What does any of this achieve?
Polarizing radiation, what does that accomplish?
I thought finding the intensity and knowing if
it is right or left polarized was confusing.
What are the answers to the next test?
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More Physics Haiku
Exams, Homework, Labs Quizzes, Lectures, and
Preflights My brain is on fire
Begin the machine Double E's love of
circuits Life integrated
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So far we have considered plane waves that look
like this
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This rotation does not change the intensity
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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.
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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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Right Handed Circular Polarization
This modification does notchange the intensity
either
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Q How do we change the relative phase between
Ex and Ey? A Birefringence
This changes linear to circular polarization
and is called a quarter wave plate
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Q What happens when circularlypolarized light
is put through apolarizer along the x (or y)
axis ?
A You kill either Ey (or Ex), so you loose half
the intensity!
X
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Q What happens when circularly polarized light
is put through apolarizer along some arbitrary
axis in the x-y plane?
A Same thing - you loose half the intensity!
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Executive Summary
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1) An EM wave is passed through a linear
polarizer. The component of the E-field
which is absorbed is A) perpendicular to the
transmission axis B) parallel to the
transmission axis C) both components are
absorbed
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3)  An EM wave polarized along the y-axis, is
incident on two orthogonal polarizers. .
What percentage of the intensity gets
through both polarizers? A) 50 B) 25 C) 0
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A quarter wave plate does not change the intensity
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A quarter wave plate changes linearlypolarized
light into circularly polarized light
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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. What is the
intensity I3 in terms of I1?
fast
45o
y
x
slow
60o
I1
I2
z
I3
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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
RCP
E
x
slow
Ex
Ey
60o
I1
l/4
I2
z
I3

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

(A) I1 (B) ½
I1 (C) ¼ I1
AFTER
No absorption Just a phase change !
Intensity lt E2 gt
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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
RCP
E
x
slow
Ex
Ey
60o
I1
l/4
I2 I1
z
I3
BEFORE
AFTER
Intensity lt E2 gt
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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
RCP
E
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. What is the
intensity I3 in terms of I1?
fast
45o
y
RCP
E
x
slow
Ex
Ey
60o
E3
I1
l/4
I2 I1
z

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

(A) I1 (B) ½
I1 (C) ¼ I1
Ey
E3
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NOTE Does not depend on q !!
60o
Ex
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Follow Up 1
Replace the 60o polarizer with another QWP as
shown.
fast
45o
y
slow
RCP
E
x
slow
Ex
Ey
fast
I1
l/4
I2 I1
I3
z
Easiest way Efast is l/4 ahead of Eslow
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Follow Up 2
Replace the 60o polarizer with another QWP as
shown.
fast
45o
y
slow
RCP
E
x
slow
Ex
Ey
E3
fast
I1
Ex
Ey
l/4
I2 I1
z

• What is the intensity I3 of the light after the
  last QWP?

(A) I1 (B) ½
I1 (C) ¼ I1
AFTER
No absorption Just a phase change !
Intensity lt E2 gt