Polarization of light and its application

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POLARIZATION OF LIGHT
ITS APPLICATION
Maharajgunj Medical Campus, Nepal

• BIKASH SAPKOTA
• Bachelor of Optometry
• 16th Batch

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PRESENTATION LAYOUT

• Concept of Polarization
• Types of Polarization
• Methods of achieving Polarization
• Applications of Polarization

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ORDINARY LIGHT

• Electromagnetic wave
• Electric field E and magnetic field B are
• Perpendicular to each other
• In phase
• Also perpendicular to the direction of
• propagation

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Electric field vector



Magnetic field vector
Em wave
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ORDINARY LIGHT

• Unpolarized Light
• Contains large no.of atoms producing
• waves with particular orientation of
• electric vector E
• Resultant waveunpolarized
• wavesuperposition of waves vibrating in
• all possible directions

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POLARIZATION

• 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 cant be polarized direction
  of
• their oscillation is along the direction of
• propagation

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.
Polarization
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• Plane of vibration
• A plane including the direction of light
  propagation
• and the direction of electric field
• Plane of polarization
• The plane perpendicular to the plane of
  vibration

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Why only electric field vector is considered in
polarization and not magnetic field vector?

• Maxwells Equation
• Ec B
• c is velocity of light(c3 108 m/s),very
  large value
• EgtgtgtB i.e. Em wave is predominantly an electric
• wave
• To change any characteristics of Em wave,
  including
• polarization,E should be affected

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TYPES OF POLARIZATION

• Linear Polarization
• Circular Polarization
• 3. Elliptical Polarization

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LINEAR POLARIZATION

• Plane polarized wave
• Electric field vector oscillates along a
• straight line in one plane

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Resultant wave is linear in vertical plane

• Resultant wave is linear in 450 plane

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Superposition of plane polarized wave

• Two plane polarized waves are added according to
• the rules of vector addition
• Results in a linear,elliptical or circular
  polarized wave
• depending on the amplitude and the phase
  shift
• between two waves

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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)

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Blue wave is resultant circular polarized wave

• .

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.

• Superposition of oppositely polarized waves
  results in to plane polarized wave

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ELLIPTICAL POLARIZATION

• Consists of two perpendicular waves of unequal
  amplitude that differ in phase by 900
• 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 polarizationspecial cases of
  elliptical polarization

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Blue wave is resultant elliptical polarized wave

• Green wave is resultant elliptical polarized wave

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METHODS OF ACHIEVING POLARIZATION

1. Reflection
2. Scattering
3. Dichroism
4. Birefringence

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POLARIZATION BY REFLECTION

• Unpolarized light can undergo polarization by
• reflection off of non metallic surfaces like
  snow, glass
• Incident angle is such that angle between
  reflected
• and refracted ray is 900
• Such incident angle is k/a polarizing angle or
• Brewsters angle
• Reflected ray is linearly polarized parallel to
  the
• reflecting surface

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BREWSTERS LAW

• When light is incident at polarizing angle
• The tangent of polarizing angleRefractive
  index of
• material
• i.e, tan ? µ
• For Sapphire, µ1.77
• So, ?tan-1(1.77)60.5350
• If the angle of incidence
  
  is not exactly the Brewsters
  
  angle the reflected ray will
  
  only be partially polarized

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Ano polarizer usedBvertical polarizer
usedChorizontal polarizer used
B
A
C
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POLARIZATION BY SCATTERING

• Polarization also occurs when light is scattered
• When light strikes the atoms of a material,
  electrons
• are set into vibration
• Vibrating electrons produce new Em waves radiated
  
• in all possible directions
• Newly generated waves strike neighbouring atoms,
  
• thereby continuing the process
• Absorption re emission ?scattered light

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• Polarization by scattering occurs in atmosphere
  leading to
• blue sky
• According to Rayleighs law
• Amount of scattering ? 1/?4

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.

• Light scattering off atoms is
• Unpolarized if the light keeps traveling in the
  same
• direction
• Linearly polarized if it scatters in a direction
• perpendicular to the path it was travelling
• Somewhere between linearly polarized and
• unpolarized if it scatters off at any other
  angles

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POLARIZATION BY BIREFRINGENCE

• Polarization due to double refraction
• A double refracting crystals like Iceland spar,
  calcite
• refracts incident light into two different
  paths
• So if an object is viewed by looking through the
  crystal,
• two images are seen
• Polarizing filter can be used to completely block
  one
• image
• Two rays are formed because they have different
• speeds due to two index planes in the medium

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O-raypasses undeviated,ordinary waveE-wavebeam
displaced sideway,extraordinary wave

• Both beams thus formed are polarized
• One parallel to the surface
• Other perpendicular to the surface

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POLARIZATION BY DICHROISM

• Polarization by selective absorption
• Such crystals are used which transmits wave whose
• electric field vibrates in a particular plane
  and absorbs
• electric field vibrating in other planes
• Eg. Tourmaline polaroid

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Polaroids

• 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
• Transmitted light is plane polarized

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Dual FilterPolarizer 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

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Law of Malus

• When a beam of completely plane polarized light
  is
• incident on an analyzer, the resultant
  intensity of light
• (I) transmitted from the analyzer varies
  directly as the
• square of the cosine angle (?) between plane
  of
• transmission of analyzer and polarizer
• i.e ,I ? cos2?
• I I0cos2?
• Where, I0 is the intensity of polarized
  light transmitted
• through a polarizer
• Mind It!! I0 is half the intensity of unpolarized
  light
• incident on the polarizer

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Intensity is maximum if the transmission axes are
parallel and intensity is zero if the
transmission axes are perpendicular to each other
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APPLICATIONS OF POLARIZATION OF LIGHT

• .

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Application of polarization by reflection

• In polaroid sunglasses
• Light reflected off a pool of still water is
  partially
• polarized parallel to water surface
• This gives rise to glare
• The transmission direction of polaroid sheet in
  sun
• glasses is vertical which blocks horizontal
  components
• of light
• Hence reduce intensity and glare

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Fishermen use polaroid sun glasses to locate fish
under water

• .

Without polaroid sun glasses
With polaroid sun glasses
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Polaroid sun glasses are also used to reduce head
light glare of car

• .

Without polaroid sun glasses
With polaroid sun glasses
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In Photographic Filters

• Glare caused by reflected light off water surface
  makes
• it harder to see behind water surface
• So photographers often use filters to cut out
  glare and
• get better pictures

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Any specimens in glass cases can be well
photographed by using filters
No polarizer
Vertical polarizer
Horizontal polarizer
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• Application of Polarization by Dichroism
• In Titmus Stereo Test
• Makes use of victograph
• The right eye and left eye pictures are polarized
  at
• 450 and 1350 respectively
• The pictures are viewed through a correspondingly
• oriented spectacle analysers
• In normal eye, a perception of depth i.e. stereo
  is
• produced when the brain fuses the two images

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Titmus Fly Test
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Application of Polarization by Scattering

• Photographic secret of capturing a vivid blue sky
• using polaroid filter

No polaroid filter has been used
Horizontal polarizer used Deep blue sky
Vertical polarizer used No significant difference
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Application of Polarization by Birefringence

• In birefrigent biprisms
• Birefrigent biprisms such as nicol,
  glan-foucault and wollaston are used to produce
  polarized light

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Glan foucault prism
Wollaston prism
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In Liquid Crystal Displays(LCD)

• There are some crystals that become aligned when
• an electric field is put across them
• When this happens they act as polarizing filters

LCD
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In Retinal Diagnosis

• Polarization Sensitive Optical Coherence
  Tomography
• (PS-OCT) is used to measure the thickness and
  
• birefringence of the Retinal Nerve Fibre
  Layer(RNFL)
• Birefringence change of the RNFL can serve as an
  early
• indicator of glaucoma

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In Polarized Snellen Eye Chart

• Special polarizing glass is usedOD polarized at
  900
• and OS polarized at 1800
• Test one eye at a time though patient viewing
• binocularly
• Alternative lines of optotype are also polarized
  at 900
• and 1800
• Use to detect
• malingering

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To detect defect in Intra Ocular 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 the lens

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In Polarized Light Microscopy

• Use of polarized light to illuminate birefrigent
• sample
• Directly transmitted light can, optionally, be
  blocked
• with a polarizer oriented at 900 to the
  illumination
• Polarized light interacts strongly with the
  sample
• and so generating contrast with the
  background
• It is used extensively in optical mineralogy

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Mineral concentration
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Other Applications of Polarization

• .

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Haidingers Brush

• Yellowish bow tie shaped
• Entoptic phenomenon
• 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 Fixationutilized to train
  people with
• strabismus to look at objects with their
  fovea rather
• than their eccentric retinal zone

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IN 3D Films

• Two films shown at same time through two
  projectors
• Projected through polarizing filters with axes
• perpendicular to each other
• Viewers wear glasses with 2 polaroid filters with
  axes
• perpendicular
• Left eye sees the movie projected from right
• Right eye sees movie projected from left
• This gives viewers a perception of depth

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PhotoelasticityStress Analysis

• When light passes through some materials its
  plane
• of polarization is rotated i.e optical
  activity
• The thicker the material the more it is rotated
  and
• different colours are rotated by different
  amounts
• To investigate the stress in an engineering part
  a
• model is made in plastic,pass light through
  and put
• it under stress
• The deformed spot is located by analysing the
• coloured pattern produced

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stress analyser

• .

Stress analysis
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In Saccharimetry

• Measurement of concn of sugar in solution
• Due to molecular structure of sugar,these
  solution
• rotate the plane of polarization as light
  passes
• through them
• rotation may be right-handed(dextro) or left-
• handed(laevo)

Saccharimeter
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In Slit Lamp and Ophthalmoscope

• Control unwanted reflections eg. that from the
  front of
• cornea
• Red filter,blue filter,green filter etc.

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Multiple choices questions

• 1.Optically active substances are those
  substances which
• Produce polarized light
• Produce double refraction
• Convert a plane polarized light into circular
  polarized light
• Rotate the plane of polarization

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.

• 2.An unpolarized light wave is traveling along
  positive X-axis. The electric field vector in the
  beam vibrates in the direction of
• Positive Y-axis definitely
• Negative X-axis
• Positive X-axis
• Y or Z-axis

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.

• 3.A calcite crystal is placed over a dot on a
  piece of paper and rotated. On seeing through the
  calcite, one will see
• Two rotating dots
• Two stationary dots
• One dot only
• One dot rotating about the other

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.

• 4. An unpolarized beam of transverse waves is one
  whose vibrations
• Are confined to a single plane
• Occur in all directions perpendicular to the
  direction of propagation
• Are confined to a single plane perpendicular to
  the direction of propagation
• Occur in all directions

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.

• 5.Two polaroids are placed with their
  transmission
• axis at 900 to each other and transmitted
• Intensity of light is zero. If one more polaroid
  is
• placed between these two, bisecting the angle
• between them, then the ratio of transmitted
• intensity of light to the intensity of
  unpolarized light
• will be
• 1/4
• 1/8
• 1/2
• 0

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REFERENCE

• Optics by Tunnacliffe
• Optics and Refraction by A.K. Khurana
• Principle of Physics, Ayam Publication
• Internet

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Palanchowk Bhagawati