Polarization, types of polarization, mechanisms to produce polarization, Applications of polarization, precautions with polarizing sunglasses, ophthalmic uses of polarization

1. Polarization and applications
Raju Kaiti, M. Optom
Consultant Optometrist
Nepal Eye Hospital

4. Polarized Lens on a Camera
ReduceReflections

6. EM Waves
 Light is an electromagnetic wave. EM waves are
transverse. Thus, the electrical field can vibrate in any
direction perpendicular to the direction of propagation.
Most light sources (candles, incandescent light bulbs etc..)
emit light that is unpolarized – the electric field has all
possible directions of vibrations.

7.  However it is possible to have polarized light.
Polarized light occurs when the vibrations of the
electrical field are confined to one plane.

8. Introduction
 Transforming unpolarized light into polarized
light
Restriction of electric field vector E in a particular
plane so that vibration occurs in a single plane
Characteristic of transverse wave
Longitudinal waves can’t be polarized;
direction of their oscillation is along the direction
of propagation

9. Polarization
 Restriction of the
electric field
vectors of a light
in a particular
plane at a distinct
point of time.

10. 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.

11.  Why only electric field vector is considered
in polarization and not magnetic field vector?
 Maxwell’s Equation E=c × B
 c is velocity of light (c = 3 × 108
m/s),very large value
 E>>>B i.e. Electromagnetic wave is predominantly an
electric wave (3*108
times magnetic wave)
 To change any characteristics of Electromagnetic wave
wave, including polarization, Electric part should be
affected

12. Explanation of Polarization at the Molecular
Level
 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

13. Explanation of Polarization at the Molecular
Level
 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.

15. 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

16. CIRCULAR POLARIZATION
Consists of two perpendicular plane Em waves with
equal amplitude and 900 phase difference
Plane of oscillation rotates around the propagation axis
May be right circularly polarized(clockwise) or left
circularly polarized(counterclockwise)

17. ELLIPTICAL POLARIZATION
Consists of two perpendicular waves of unequal
amplitude that differ in phase by 90 degrees
The tip of the resultant electric field vector describes an
ellipse in any fixed plane intersecting and normal to the
direction of propagation
Circular and linear polarization: special cases of elliptical
polarization

18. Production of polarized light
Four physical mechanism can produce
polarized light.
 Dichroism
 Reflection
 Scattering
 Birefringence

19. Production of polarized light
 All four mechanisms have one factor in
common.
 The system involved must have some form of
asymmetry in order to select one state of
polarization and remove all others from the
incident natural light.
 The asymmetry may be related to the incident or
viewing angle or it may be an anisotopiy in the
polarizer itself.

20. Reflection
 When an unpolarized light wave reflects off a non-
metallic surface, it can be completely polarized, partially
polarized or unpolarized depending on the angle of
incidence. The amount of polarization depends upon
the angle.

21.  Complete polarization occurs when the reflected beam
and the refracted beam are 90o
to each other.
Reflected
ray
Incident
ray
o
90
pθ
rθ
pθ
1n
2n

22. Polarizing angle (Brewster’s angle)
 The angle of incidence at which the reflected
light is completely plane-polarized is called
the polarizing angle (or Brewster’s angle).
By Snell’s law, rp nn θθ sinsin 21 =
Sinc
e
o
r p90 = +θ θ
and
pp
o
r θθθ cos)90sin(sin =−=
Then we get
1
2
tan
n
n
p =θ

23. Scattering
 The basis of reflection, refraction and
diffraction.
 A plane polarised light is always polarised
in the same plane after being scattered by
a molecule in any direction.

24. Polarization by Scattering
 When light(such as that from the sun) shines on
particles, they can absorb and then reradiate part
of the light.
 The absorption and reradiation of light is called
scattering.
 Unpolarized light will be partially plane polarized
after scattering from small particles of dust etc
 The scattered light will be completely plane
polarized if scattered light is 90.0o
from incident
light.

25. 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.

26. 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.

27. Dichroism
 Dichroism is selective absorption of the two orthogonal
p-states in incident natural light.
 It is physically anisotropic.
 Wire grid polariser
 In this parallel electrically conducting copper wires are
stretched between two insulating boards.
 Here the x- component perpendicular to the wire is
transmitted while y-component parallel to the wire is
absorbedby the grid.

29.  Dichroic crystals
 Because of their anisotropic structure. For eg tourmaline
(boron sillicate crystal compounded with elements such
as Al, Fe, Mg, Na, Li or K)
 Optic axis of the crystal is the specific direction through
which
 The electric field component perpendicular to the axis is
strongly absorbed
 Only parallel electric field component is transmitted.
 Dichroic means of two colors (transmitted light has two
color)

30. Birefringence
 A material which displays two different speeds of
propagation in fixed and orthogonal directions
and therefore displays two refractive indices
 Optical anisotropy
 In some crystals the forces binding an electron
to the atomic nuclei is stronger in one direction
than another.
 When an electromagnetic wave act upon them
the electrons set into vibration in one plane and
hence emit the wave with same energy and
frequency in that plane at the end.

31.  But in another plane the wave will
be damped gradually and energy
is dessipated as heat.
 The optic axis in these medium is
a direction and not a particular
discrete line
 E.g Calcite crystal or calcium
carbonate is a fairly typical
birefringent crystal.

32. A calcite crystal

33. O ray and e ray
 Following the speed of these two rays there are
two different index planes in the medium.
 ne-no= dn, which is a positive value in a positively
uniaxial medium.
 Considering the fact, birefringent biprisms such
as nicol, glan foucalt and wollaston are used to
produce polarized light.

34. O ray and e ray
 A narrow beam of natural light incident normally to a
cleavage plane of calcite crystal emerges as two parallel
beams displaced laterally.
 One beam passes through the parallel sided crystal
undeviated as expected or in ordinary manner, so called
O-rays.
 The other beam is displaced sideays in an extraordinary
fashion so called e-rays.
Crystal no ne
calcite 1.6584 1.4864
Ice 1.309 1.313
Quartz 1.5443 1.5534
Tourmaline 1.669 1.638

35. Glan foucalt
prism

37. The most common method of polarization involves the use of polaroid
Have long chain of molecules that are aligned within the filter in a
particular direction
When an unpolarized light falls on a polaroid:
The electric vector E oscillating in the direction of the alignment of
molecules of the polaroid is absorbed
Electric field vector oscillating perpendicular to the direction of the alignment
of molecules pass through the polaroid
Polaroids

38. How are Polaroids made?
 To reduce the intensity of reflective glare more than that
of the surrounding objects, a filter that absorbs the
horizontally vibrating components of light would be
useful.
 Polaroids are made up of sheet of polyvinyl acetate
(PVA).
 PVA is heated and stretched to 5 times in one direction.
 Later dipping it into iodine solution provide iodine
attached to the chain like hydrocarbon molecules.

39.  Thus, producing an iodine wire grid (Polarizing filter).
 This filter may be sandwiched between 2 layers of CAB
making plano polarizing lenses.
 For prescription lenses, polarising sheet can be mounted
on one layer of CAB and molded directly into plastic lens
during the lens casting process.
 An ideal H sheet would transmit 50% of the incident
natural light.

40. Dual Filter: Polarizer + Analyzer
If the transmission axes of polarizer and analyzer
are perpendicular, no light is transmitted
The light transmitted at other angles follows the
Law of Malus
Polarizer and analyzer relation can be best
described by picket fence analogy:

41. Polarization by Selective Absorption
 Polarization of light by
selective absorption is
analogous to that shown
in the diagrams.

42.  A polarizing filter has an absorption and transmission
axis.
 If an ideal polarizing filter is oriented with its absorption
axis along 180 degrees, it will extinguish all horizontally
polarized light.
 This means the transmission axis of the same filter will
be at 90 degrees and will allow all vertically polarized
light to pass through.
 When the filter is tilted somewhere between these two
position, certain % of horizontally polarized light comes
through the filter.
 MALUS LAW is a predictor of how much polarized light
will be transmitted by an obliquely oriented polarizing
filter.

43. Mathematics of Polarization
 • Two consecutive polarizers.
 – The first polarizer reduces the intensity by half.
 – The second polarizer reduces the intensity by another factor of cos2
θ. This
is called Malus’s Law.

44. Malus law
 Intensity emerging from polarizer is I α E2
 Intensity emerging from analyser is I θ α E2
cos2
 Dividing 2 by 1
 We get I θ = I cos2
θ
 So when θ=90 , transmission axis of polariser and
analyser is perpendicular
I θ = 0.

45. Detection of p-state light
 Each component p-state in a natural light can itself be
resolved parallel and perpendicular to the polariser’s
transmission axis.
 Those components parallel to this axis will emerge
from the polariser whilst those perpendicular to it will
be extinguished.
1. Take two plano polarizing lenses and by holding one
before the other with their polarizing axes crossed at 90
degrees, eliminate all incoming visible light rays. What
one polarizing lens doesn’t extinguish, the other will.

46. 2. Use a pair of glasses with
polaraizing lenses to view glossy
magazine. With the glossy
magazine in between you and
the light, the magazine will show
reflective glare. Move around
until glare is maximal. Now rotate
the lenses until the glossiness
decreases and totally disappear.

47. 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.

48. 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.

49. Applications of Polarization (3)
 Liquid Crystal Display (LCD)

50. 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.

51. 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

52. 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

53. Applications
 As sun glasses to
cut of unwanted
reflected light best
utilized by
fisherman,
motorist, skiers,
sportsman etc.

54. To identify
thermally
tempered lenses.

55. Application of polarization by Dichorism
 In Titmus stereo test
 Makes use of vectograph
 RE and LE pictures are polarized at 45 and 135
degrees respectively.
 The pictures are viewed through a correspondingly
oriented spectacle analysers.
 In normal eye, a perception of depth i. e. stereopsis is
produced when the brain fuses two images.

56. Application of polarization byBirefringence
 In birefringence biprisms
 In birefringent biprisms such as nicol, glan-focault
and wollastone are used to produce polarized light.
 In Liquid Crystal Displays (LCDs)
 There are some crystals that become aligned when
an electric field is put across them. When this
happens they act as polarizing filter.
 In Retina diagnosis
 Polarization sensitive OCT (PS-OCT) is used to
measure the thickness and birefringence of the
RNFL
 Birefringence change of the RNFL canserve as an
early indicator of Glaucoma

57. Application of polarization byBirefringence
 In polarized Snellen’s eye charts
 Special polarizing glass is used. RE polarized at 90 and LE
polarized at 180 degrees
 Test one eye at a time though patient viewing binocularly
 Alternative lines of optotype are also polarized at 90 and 180
degrees
 Use to detect malingering
 To detect defect in Intraocular lenses
 Birefringence is detected by placing the lens between two linear
polarizers at right angles to each other.
 Any light transmitted appears as a readily recognizable bright
spot
 The bright spot indicates a possible defect in the strength of
lens
 In polarized light Microscopy
 Used extensively in optical mineralogy

58. Other uses
 Haidinger’s Brush :Entoptic phenomenon
 Yellowish bow tie shaped
 Always positioned in macula, so visible in centre of visual field
 Viewed while facing away from sun, bright background, eg LCD screen
 Due to dichroism of xanthophyll pigment of macula
Used in Eccentric Fixation: utilized to train people with strabismus to look at
objects with their fovea rather than their eccentric retinal zone
 In 3D films
 Two films shown at a time through two projectors
 Projected through polarizing filters with axes perpendicular to each other
 Viewers wear glasses with 2 polaroid filters with axes perpendicular
 RE sees the movie projected from left
 LE sees the movie projected from Right

59. Other uses
Photoelasticity: stress analysis
In saccharimetry: measurement of
concentration of sugar solution
In slit lamp and Ophthalmoscopes
Control unwanted reflections e.g. that from the front
of cornea
Red filter, blue filter, green filter etc.

60. Recommended for:
1. To decrease driving fatigueness and increase driving
safety.
2. For fishing and boating on the water
3. For more visual comfort on the beach
4. So that colors are not bleached out
5. So that bright, snowy days are not as blinding
6. To block UV radiation
7. As a good sunglass
8. Photographers use filters to cut glare and get better
pictures

61. Precautions with Polarizing lenses
1. Since windshields are tempered, the tempering process
induces intentional stress into the materials. The stress
may be visible through polarizing lenses distracting the
observer.
2. Some skiers believe polarizing lenses make snow
conditions harder to judge. Tilting heads will change the
amount of transmission/absorption causing an ongoing
change in intensity of the reflected light.
3. Similar problems in golfers
4. An LCD display is polarized. If it is horizontally
polarized, polarizing sunglasses will extinguish the
display. LCDs are widely used in instrument panels in
cars.

62. Precautions with Polarizing lenses
5. Pilots experience a number of adverse situation.
 Polycarbonate windshields in many aircrafts have stress
patterns. These patterns become visible and may distract.
 Some airplane cockpits like in car instrument panels may have
polarized numbers or images that can disappear when viewed
through polarizing lenses.
 Much of light coming from oncoming aircrafts that make it visiblr
is reflected from the metallic surface of plane. Much of this
reflected is horizontally polarized. When viewed through the
polarized lenses, these reflected lights may be eliminated
making oncoming aircrafts invisible.