Lab 5 Part 2

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Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Topics
 Use birefringence to generate and quantify elliptically polarized light.
 Understand, measure and characterize the optical activity of syrup.
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Birefringence
270 
0
Fast axis: n2
180 
90 
c
v1 
n1
Slow axis : n1
c
v2 
n2
The birefringent retarder in your box causes a phase delay
of 140nm (approximately 0.22*llaser). Phase delay depends
on the thickness of the birefringent material.
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Retardation in a ”l/4-waveplate” for light polarized linearly at 45 with respect to main axes
(=superposition of two orthogonal in-phase linearly polarized waves)
Fast axis
Slow axis
y
x
F
E
A y
D
C y
B y
x
D
C B A
y
E
x
x
x
y
F
x
Has fallen behind by l/4
y
x
Polarization before the l/4-waveplate:
When looking against the direction of light
propagation: A, B, C, … pass you by
successively.
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Polarization after the ”l/4-waveplate”
Fast axis
Slow axis
C
D
y
x
F
B
E
y
y
y
A
C
B
x
D
x
x
A
y
E
x
y
F
x
y
x
This light is “left-circularly” polarized (What does electric field do at one point in space when
looking against propagation direction – Physics definition; careful: EE is opposite!).
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
V.D Measure transmitted intensity I as a function of Retarder orientation Q
Polarizer ( to laser polarization)
Retarder
To photometer
Laser
 Vary the orientation Q of retarder.
 Measure I(Q) and plot I versus Q .
 Discuss the result in terms of the retardation properties of the retarder.
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
V.D Measure intensity I as a function of polarizer orientation Q
Retarder at 45 angle to laser polarization.
Polarizer/Analyzer
To photometer
Laser
 Set retarder so that either the slow or the fast axis is at a 45 angle with respect
to the laser polarization.
 Vary the orientation Q of polarizer(used as an analyzer).
 Measure I(Q) and plot I versus Q on polar graph paper.
 Write down exactly how the retarder was oriented with respect to the laser
polarization and figure out whether the light got left- or right-polarized in your
setup.
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Optical Activity
Left circularly
polarized light
n
Right circularly
polarized light
n
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Linearly Polarized Light = Superposition of
Circularly Polarized Light (left and right)
+
=
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
…and if one circular polarization is phase shifted…
+
=
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Rotation Due to Optical Activity
b
n and n
d
Linearly polarized light
Linearly polarized light – different orientation
Polarization rotated by angle b
Rotation angle : b 
d
nleft  nright 
l
Modern Optics Lab
Lab 5 Part 2: Experiments involving Light Polarization
Dextrorotary versus Levorotary
(“Looking” against the direction of light propagation.)
Dextrorotary
Levorotary
b
b
before
after
Optical activity causes a rotation of
polarization to the right.
after
before
Optical activity causes a rotation of
polarization to the left.
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