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Polarization
Electromagnetic Waves
February 2005
Electromagnetic Wave
EM wave is …
• Light is an electromagnetic wave.
• It consists of vibrations of electric field and
magnetic field.
• The electric field and magnetic field are
perpendicular to each other and in phase.
• EM wave is a transverse wave.
• The speed of EM wave is 3 x 108 ms-1.
Electric Field Vector
Polarized Light
Polarized Light
Vibrations lie on one single
plane only.
Unpolarized Light
Superposition of many beams,
in the same direction of
propagation, but each with
random polarization.
Representation . . .
E
Unpolarized
E
Polarized
Representation . . .
Unpolarized
Polarized
Polarization of Light
Selective Absorption
Unpolarized
Light
Vertical
Component
being
Transmitted
Horizontal Component
being Absorbed
Selective Absorption - Explanation
Polarizing Material
A Polarizing material will
only allow the passage of
that component of the
electric field parallel to
the polarization direction
of the material
I = I0 cos2q
Polarizer & Unpolarized Light
• Each wave is attenuated by factor cos2q.
• Average attenuation is < cos2q > = 1/2
Crossed Polarizers
• The first polarizer reduces the intensity by half.
• The second polarizer reduces the intensity by another factor of
cos2q.
• The second polarizer projects the electric field onto a new axis,
rotated by q from the axis of the first polarizer
Crossed Perpendicularly
Crossed at different angles . . .
Crossed at different angles . . .
Reflection
Brewster’s Angle
i  θ P
r  90 - θP
sin i
n
sin r
sin θ P
sin θ P
n

sin (90 - θ P ) cos θ P
n  tan θ P
Glares
Sunglasses – Glare Reduction
• Polarized lenses have the added benefit of
filtering out reflected light, or glare, off surfaces
such as water or pavement
• Ideal for boating, fishing, driving or any other
activity associated with intense glare
• Reduces eyestrain and fatigue, while increasing
contrast and visual acuity
How Do Polarized Lenses Work?
• Light reflected from surfaces
like a flat road or smooth
water is generally horizontally
polarized. This horizontally
polarized light is blocked by
the vertically oriented
polarizers in the lenses.
• The result: a reduction in
annoying and sometimes
dangerous glare.
Action of Polaroid Sunglass
Light reflected from
surfaces like a flat road or
smooth water is generally
horizontally polarized.
This horizontally
polarized light is blocked
by the vertically oriented
polarizers in the lenses.
Vertically Polarized Light
from Objects
Unwanted glares are usually
horizontally polarized light
Glare and Polarization
http://regentsprep.org/Regents/physics/phys04/cpolarize/glare.htm
Glare and Polarization
The polaroid absorbs most of the polarized light reflected from the water’surface,
allowing the dimmer light from the bottom of the river, and any fish swimming there,
to be seen more readily.
Glare and Polarization
www.johnsonwindowfilms.com/cutglare.htm
Glare and Reduction
Scattering
Incident Light Ray
Scattered
Small Particle
Light Ray
Scattering of Sunlight
When unpolarised sunlight impinges
on the molecules, the electric field of
the EM wave sets the electric charges
within the molecules into motion. The
EM wave is absorbed.
The molecules then quickly re-emit
light in all directions since oscillating
electric charges produce EM waves.
Because of the transverse nature of
light, the electric field of the re-emitted
waves should be in the plane that
includes the line of oscillation, the
scattered light is completely plane
polarized with its electric vector in the
direction shown.
Scattering of Sunlight
Polarization by Scatterings
Polarized Light by Scattering
Question ?
• Why is the sky blue in the day and red at
sunsets?
The sky is blue because . . .
• The tiny particles in the atmosphere (dust,
clumps of air molecules, microscopic water
droplets) are better at scattering shorter
wavelength blue light than the longer
wavelength red light.
• As sunlight passes through the
atmosphere, the scattered blue light give
the atmosphere an overall blue glow.
The sunset is red because . . .
• At sunrise and sunset, sunlight enters our
atmosphere at a shallow angle and travels
a long distance before reaching our eyes.
• During this long passage, most of the blue
light is scattered away and virtually all that
we see coming to us from the sun is its
red and orange wavelengths.
Blue Sky and Red Sunset
Optical Activity
Photograph of a plastic
model of a crane hook
Optical Activity
• When polarized light is passed through some materials,
the plane of polarization is rotated.
• Examples : plastic ruler, sellotape, sugar solution
• Applications :
– Stress Analysis : The optical activity of plastic and glass
depends on the stress in them. When the stress is greatest,
the rotation of the vibration is greatest. Dark and colour
bands are produced when polarized light is shone through
the material. Engineers build plastic models of structures to
help them to analysts the stress distribution.
– LCDs (Liquid Crystal Displays)
Application
• Used in LCD display
• Electrical voltage on a liquid
crystal diode turns on and off
polarizing filter effect.
Seven-Segment LCD
Aerial
In a vertical transmitting
aerial, vertical electron
oscillation results in a
vertical polarized radio
wave.
Maximum electron oscillation in
the receiving aerial is achieved
when the aerial is aligned to be
parallel to the wave’s electric field.
Links
• http://micro.magnet.fsu.edu/optics/lightandcol
or/polarization.html
• http://dpfwiw.com/polarizer.htm#unwanted
• Blue Skies and Red Sunsets
http://science.howstuffworks.com/framed.htm?
parent=question39.htm&url=http://www.glenb
rook.k12.il.us/gbssci/phys/Class/light/u12l2f.h
tml