Polarization

Polarization is a characteristic of all transverse waves.

Oscillation which take places in a transverse wave in
many different directions is said to be unpolarized.
In an unpolarized transverse wave oscillations may
take place in any direction at right angles to the
direction in which the wave travels.


Direction of
propagation
of wave
Linear Polarization
If the oscillation does take place in only one
direction then the wave is said to be linearly
polarized (or plane polarized) in that direction.
Direction of oscillation
Direction of travel
of wave
Polarization of Electromagnetic Waves
Any electromagnetic wave
consists of an electric field
component and a magnetic field
component.
The electric field component is
used to define the plane of
polarization because many
common electromagnetic-wave
detectors respond to the electric
forces on electrons in materials,
not the magnetic forces.
Polarization by Selective Absorption
Polarization of light by
selective absorption is
analogous to that
shown in the diagrams.
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/polarizedlight/filters/index.html
Polaroid
A Polaroid filter transmits 80% or more of the
intensity of a wave that is polarized parallel to a
certain axis in the material, called the polarizing
axis.
Polaroid is made from long chain molecules
oriented with their axis perpendicular to the
polarizing axis; these molecules preferentially
absorb light that is polarized along their length.

Polarizing axis
Explanation of Polarization at the
Molecular Level (1)
An electric field E that oscillates parallel to the
long molecules can set electrons into motion
along the molecules, thus doing work on them
and transferring energy. Hence, E gets absorbed.
http://www.colorado.edu/physics/2000/index.pl

Explanation of Polarization at the
Molecular Level (2)
An electric field E perpendicular
to the long molecules does not
have this possibility of doing
work and transferring its energy,
and so passes through freely.
When we speak of the axis of a
Polaroid, we mean the direction
which E is passed, so a polarizing
axis is perpendicular to the long
molecules.
Intensity of Light transmitted through a
Polarizer (1)
An ideal polarizer passes 100% of the incident
light that is polarized in the direction of the
polarizing axis but completely blocks all light
that is polarized perpendicular to this axis.
When unpolarized light is incident on an ideal
polarizer, the intensity of the transmitted light is
exactly half that of the incident unpolarized light,
no matter how the polarizing axis is oriented.
Intensity of Light transmitted through a
Polarizer (2)
If a beam of plane-polarized light strikes a polarizer
whose axis is at an angle of to the incident polarization
direction, the beam will emergy plane-polarized parallel
to the polarizing axis and its amplitude will be reduced by
cos .

o
E
cos
o
E
Incident beam of
Amplitude
Vertical
Polaroid
Transmitted
wave
cos
o
E E
Intensity of Light transmitted through a
Polarizer (3)
A Polaroid passes only that component of
polarization that is parallel to its axis.
As the intensity of a light beam is proportional to
the square of the amplitude, and cos
o
E E
Hence the intensity of a
plane-polarized beam
transmitted by a polarizer
is

2
cos
o
I I
Transmission of Polarized Light
through an Analyzer
Polarization by Reflection
Unpolarized light can be polarized, either
partially or completely, by reflection.
The amount of polarization in the reflected beam
depends on the angle of incidence.
http://www.microscopy.fsu.edu/primer/java/polarizedlight/brewster/index.html
Brewsters law
It is found that experimentally when the reflected
ray is perpendicular to the refracted ray, the
reflected light will be completely plane-polarized.
Reflected
ray
Incident
ray
o
90
p

1
n
2
n
Polarizing angle (Brewsters angle)
The angle of incidence at which the reflected
light is completely plane-polarized is called
the polarizing angle (or Brewsters angle).
By Snells law, r p
n n sin sin
2 1

Since
and
p p
o
r
cos ) 90 sin( sin
Then we get
1
2
tan
n
n
p

90
r p

Polarization by Scattering (1)
When a light wave passes through a gas, it will be
absorbed and then re-radiated in a variety of
directions. This process is called scattering.
Unpolarized
sunlight
Gas molecule
Light scattered at right angles
is plane-polarized
O
y
z
x
http://background.uchicago.edu/~whu/intermediate/Polarization/polar1.html
Polarization of Scattered Sunlight
x
y
z
O


E
Polarization by Scattering (2)
Consider a gas molecule at point O. The electric
field in the beam of sunlight sets the electric
charges in the molecule into vibration.
Since light is a transverse wave, the direction of
the electric field in any component of the sunlight
lies in the yz-plane, and the motion of charges
take place in this plane.
There is no electric field, and hence no motion of
charge in the x-direction.
Polarization by Scattering (3)
The molecule reemits the light because the
charges are oscillating. But an oscillating charge
does not radiate in the direction of its oscillation
so it does not send any light to the observer
directly below it.

Therefore, an observer
viewing at right angles to the
direction of the sunlight will
see plane-polarized light

Polarization by Scattering (4)


Lunar eclipses appear orange and red
Polarization by Refraction
When an incident
unpolarized ray enters
some crystals it will be
split into two rays
called ordinary and
extraordinary rays,
which are plane-
polarized in directions
at right angles to each
other.
http://www.microscopy.fsu.edu/primer/java/scienceopticsu/birefringence/index.html
Double Refraction
When light is refracted into two rays each
polarized with the vibration directions
oriented at right angles to one another, and
traveling at different velocities. This
phenomenon is termed "double" or "bi"
refraction.

Applications of Polarizations (1)
Polaroid sunglasses
The glare from reflecting surfaces can be
diminished with the use of Polaroid
sunglasses.
The polarization axes of the lens are vertical,
as most glare reflects from horizontal surfaces.
Applications of Polarization (2)
Stress Analysis
Fringes may be seen in the parts of a
transparent block under stress, viewing
through the analyser.
The pattern of the fringes varies with the
stress.
Stress Analysis

Stress patterns appear in

transparent materials
sandwiched between crossed
polarizers.

Some manufacturing
processes can introduce
permanent stress. This
protractor will most likely
break where the stress lines
are most dense (at 60

on the
arc or 4 1/4 inches on the
base).

Applications of Polarization (3)
Liquid Crystal Display (LCD)
Applications of Polarization (4)
VHF and UHF antennas (aerial)
Radio waves can be detected either through
their E-field or their B-field.
Stations transmitted radio waves which are
plane-polarized.
Applications of Polarization (5)
Electric field of EM wave produces a
current in an antenna consisting of straight
wire or rods.
Applications of Polarization(6)
Changing magnetic field induces an emf
and current in a loop antenna.
Blue Sky
The blue color of the sky is caused by the
scattering of sunlight off the molecules of the
atmosphere. This scattering, called Rayleigh
scattering, is more effective at short wavelengths

Sunset
As incoming sunlight passes through a more dense
atmosphere, shorter wavelengths of light (violet and blue)
are efficiently scattered away by particles suspended in
the atmosphere. This allows predominantly yellow and
red wavelengths of light to reach the observer's eyes,
producing a yellowish-red sunset.

Blue Skies and Red Sunsets
As the path which sunlight takes through our
atmosphere increases in length, ROYGBIV
encounters more and more atmospheric particles.
This results in the scattering of greater and greater
amounts of yellow light.

Polaroid Sunglasses
Liquid Crystal
Liquid crystal is a substance that behaves
something like a liquid and something like
a solid.
The shape of its molecules are long and
thin.
Properties of LCD

Their orientations can be aligned with one
another in a regular pattern.
A particular sort of liquid crystal, called
twisted nematics, (TN), is naturally
twisted. Applying an electric current to
these liquid crystals will untwist them to
varying degrees, depending on the current's
voltage.

Twisted Nematics
They can rotate the plane of oscillation of
polarized light passing through them.
Light passes through the cell
with its plane of polarization
turned through 90
Light cannot pass through
since the line does not
rotate the plane of
polarization

Liquid Crystal Display
LCD
The liquid crystalline phase exists in a ground state
that is termed cholesteric, in which the molecules are
oriented in layers, and each successive layer is slightly
twisted to form a spiral pattern (Figure 9).
When polarized light waves interact with the liquid
crystalline phase the wave is "twisted" by an angle of
approximately 90 degrees with respect to the incident
wave.
The exact magnitude of this angle is a function of the
helical pitch of the cholesteric liquid crystalline phase,
which is dependent upon the chemical composition of
the molecules (it can be fine-tuned by small changes to
the molecular structure).