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Polarization, scattering and absorption of light

Medical Campus
Bachelor of Optometry
16th Batch
 Polarization of Light: Types, Methods &
 Absorption of Light: Types & Application
 Scattering of Light: Types & Application
Electromagnetic wave
Electric field E and magnetic field B are:
oPerpendicular to each other
oIn phase
oAlso perpendicular to the direction of propagation
Electric field vector
Magnetic field vector
Em wave
Unpolarized Light
oContains large no.of atoms producing
waves with particular orientation of
electric vector E
oResultant wave: unpolarized
wave: superposition of waves vibrating in
all possible directions
Transforming unpolarized light into polarized
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
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
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 × 10 m/s),very large value
 E>>>B i.e. Em wave is predominantly an electric
 To change any characteristics of Em wave, including
polarization,E should be affected
1. Linear Polarization
2. Circular Polarization
Elliptical Polarization
Plane polarized wave
Electric field vector oscillates along a
straight line in one plane
Resultant wave is linear in vertical plane
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
Resultant wave is linear in 450 plane
Resultant wave is linear in 900 plane
 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)
 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 polarization: special cases of
elliptical polarization
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
Brewster’s angle
Reflected ray is linearly polarized parallel to the
reflecting surface
When light is incident at polarizing angle:
The tangent of polarizing angle=Refractive index of
i.e, tan θ= µ
For Sapphire, µ=1.77
So, θ=tan-1(1.77)=60.5350
 If the angle of incidence
is not exactly the Brewster’s
angle the reflected ray will
only be partially polarized
A:no polarizer used
B:vertical polarizer used
C:horizontal polarizer used
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
Polarization by scattering occurs in atmosphere leading to
blue sky
According to Rayleigh’s law
Amount of scattering ἀ 1/λ^4
Light scattering off atoms is:
•Unpolarized if the light keeps traveling in the same
•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
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
Two rays are formed because they have different
speeds due to two index planes in the medium
Both beams thus formed are polarized:
One parallel to the surface
Other perpendicular to the surface
O-ray:passes undeviated,ordinary wave
E-wave:beam displaced sideway,extraordinary wave
Polarization by selective absorption
Such crystals are used which transmit wave whose
electric field vibrates in a particular plane and absorbs
electric field vibrating in other planes
Eg. Tourmaline polaroid
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
Transmitted light is plane polarized
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:
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 (θ) between plane of
transmission of analyzer and polarizer
i.e ,I ἀ cos2θ
I = I0cos2θ
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
Intensity is maximum(I=I₀) if the transmission axes are
parallel and intensity is zero if the transmission axes are
perpendicular to each other
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
Fishermen use polaroid
sun glasses to locate fish
under water
Without polaroid sun glasses
Polaroid sun glasses
are also used to
reduce head light glare
of car
With polaroid sun glasses
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
Application of Polarization by
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
Titmus Fly Test
Application of Polarization by Scattering
Photographic secret of capturing a vivid blue sky
using polaroid filter
Horizontal polarizer used
Deep blue sky
Vertical polarizer used
No significant difference
No polaroid filter has
been used
Application of Polarization by Birefringence
In birefrigent biprisms
Birefrigent biprisms such as nicol, glan-foucault and wollaston
are used to produce polarized light
Nicol prism
Glan foucault prism
Wollaston prism
In Liquid Crystal Displays(LCD)
There are some crystals that become aligned when
an electric field ,are put across them
When this happens they act as polarizing filters
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
In Polarized Snellen Eye Chart
Special polarizing glass is used: glass over OD polarized at 900
and OS polarized at 1800
Test one eye at a time though patient viewing
Alternative lines of optotype are also polarized at 900
and 1800
 Used to detect
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
In Polarized Light Microscopy
Use of polarized light to illuminate birefrigent
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
Mineral concentration
Other Applications of Polarization
Haidinger’s 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 Fixation: utilized to train people with
strabismus to look at objects with their fovea rather
than their eccentric retinal zone
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
Left eye sees the movie projected from right
Right eye sees movie projected from left
This gives viewers a perception of depth
Photoelasticity: Stress 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 colors 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 analyzing the
colored pattern produced
Stress analysis
stress analyzer
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 lefthanded(laevo)
In Slit Lamp and Ophthalmoscope
Control unwanted reflections eg. that from the front of
Red filter, blue filter, green filter etc.
 Deflection of a ray from a straight path, for example by
irregularities in the propagation medium, particles, or in
the interface between two media
 It is a consequence of the interaction of light with the
electric field of scattering particle
 It is the primary mechanism of physical observation
Scattering of light occurs as follows:
 An incident photon induces oscillation of electron cloud
of the particle which results in periodic separation of
charge within the particle
 This separation of charge is called induced dipole
 The oscillation of this induced dipole is manifest as a
source of electromagnetic radiation thereby resulting
scattering of light
Radiation scattered from a particle depends on:
 Size of the particle
 Shape of the particle
 Index of refraction of particle
 Wavelength of radiation
Types of scattering
I. Elastic Scattering
II. Inelastic Scattering
Elastic scattering
 The energy of the incident photon is conserved
 Light scattered by the particle is emitted at the identical
frequency of the incident light
 Types of elastic scattering:
 Rayleigh Scattering
 Mie Scattering
 Nonselective Scattering
Inelastic scattering
 The energy of the incident photon is not conserved.
 Inelastic scattering includes:
Brillouin scattering
Raman scattering
Inelastic X-ray scattering
Compton scattering
Rayleigh Scattering
 It occurs as a result of radiation being scattered by a
particle which is smaller than the wavelength of the
incident light
 It is very weak scattering & depends very strongly on
 Scattering produced by such small particles is isotropic i.e.
equal in all direction
 Scattering efficiency(Kλ) is inversely proportional to the
fourth power of the wavelength of light(λ)
i.e. Kλ α 1/ λ⁴
 Nitrogen and oxygen in atmosphere are smaller than
wavelength of UV and Visible light. So sunlight
undergoes Rayleigh scattering in atmosphere
Why is the sky blue
o As sunlight moves through the atmosphere, longer
wavelengths(eg.red) pass straight through
o However, shorter wavelengths(eg.blue) interact with gas
molecules and scatter in the atmosphere
Secret of red sunset
o As the sun approaches the horizon during sunsets,
sunlight travels longer distance to reach our eyes
o Hence, light with shorter wavelengths(eg.blue) are
scattered more before reaching to our eyes and
thus sunsets appear red
Mie Scattering
 It occurs when the size of the particle becomes
equivalent to or greater than the wavelength of the
incident light
 Scattering changes from being isotropic to a distortion in
forward scattering direction
 White glare around the sun is also due to Mie scattering
 Cloud droplets being larger scatter all wavelengths of
visible light. So the cloud appears white
Attenuation in optical fiber
o Involves scattering of light: due to change in
local refractive index
o Also involves absorption of light: UV
absorption, infrared absorption & ion
resonance absorption
Nonselective Scattering
 Occurs when the particles are much larger than the
wavelength of the radiation
 Caused by water droplets and large dust particles
 Also known as geometrical scattering
E.g. Rainbows
Particle Size (in
Kind of Particles
<< 0.1
Air molecules
Mie Scattering λ^˚ to λ^‾⁴
0.1 to 10
Smoke, cloud
Larger dust
droplets, etc
Ocular Scattering of light
 When light enters the eye, it is scattered as a result of
optical imperfections in the eye(like various proteins,
lipid particles, lamellar bodies, etc).
 This scattering can be sub-divided into:
a) Forward scatter: Light scattered toward the retina
b) Backward scatter: Light scattered backward
The scattering material interferes with vision in
two ways
Glare Effect: When a light from a source reaches the
eye, a fraction of the light scattered within the ocular
media falls on the retina. That light which falls in the
foveal area lowers the contrast in the image of interest
ii. Light Reduction Effect: When the scattering is very
strong, there occurs a reduction in the light available to
form the image on the retina
Scattering of light occurs in various
pathological conditions:
Corneal haze in corneal edema
o Corneal edema: caused by excess water in the stroma;
disrupts the very regular close-packed collagen structure
of stroma; loss in corneal transparency
Corneal haze
Normal cornea
Age Related Nuclear Cataract
 Light scattering from micrometer sized particles
surrounded by lipid shells: multilamellar bodies(MLBs)
 MLBs are the major source of forward light
scattering:reduces contrast of fine details, particularly
under dim light in ARNC
Due to ARNC
Flare In Anterior Chamber
 It is caused by scattering of light by the proteins in
the aqueous humour
 Sclerotic Scatter Illumination
 It is an indirect illumination technique in slit lamp
 Light beam is focused mainly to the temporal sclera
(mainly at the limbus)
 Total internal reflection occurs within the cornea. So the
light pass through the substance of cornea and illuminate
the opposite side of limbus
 If there is any pathology like corneal opacity, corneal
scarring, etc it becomes visible as it scatters the ray of
 It is a process by which radiant energy is taken up
internally by a substance or the medium through which it
 Light energy is transformed in to internal energy of the
absorber such as thermal energy
Types of absorption
 Neutral Absorption: all wavelengths are equally absorbed
 Selective Absorption: some wavelengths are absorbed
and others are transmitted; in colored glass, dyes, etc
 A substance which absorbs all radiations is called a black
Black Hole
 The amount of absorption mainly depends on:
a) the properties of the material
b) the thickness of the material
Absorption factor:
o It is the ratio of the absorbed luminous flux
to the incident luminous flux
 Absorption is usually expressed in optical density(OD)
Where T=Transmittance
An OD of 1 represents transmittance of 10%
An OD of 2 represents transmittance of 1% and so on
 It is a property by which substance absorbs light of a
given wavelength and re-emits it as radiations of a longer
E.g. Fluorescein
imaging of
in a dividing
cancer cell
o It is a weak dibasic acid of molecular wt. of 330
o It is a yellowish-red compound which fluoresces a
brilliant yellow-green under ultraviolet or blue
Fundus photograph
in FFA
Fluorescein spectrum
Application of absorption of light
 Colors of Objects
o The color of an object is
determined by the
wavelengths of light that
the object absorbs,
transmits and reflects
 Atmospheric Absorption of Radiations
o Ozone, water vapour, carbon dioxide, oxygen, nitrogen,
etc present in the atmosphere absorb the specific
wavelengths emitted from the sun
o Green house effect
Photosynthetic absorption of light
o Chlorophylls absorb particular wavelengths of light
and converts into chemical energy: basis of food
 X-Ray
o X-rays are absorbed by different extends by
different tissue,bone in particular, which is the
basis for X-ray imaging
 Radiation Absorption By Ocular Tissues
o Tears and cornea: Far UV ( 180-315 nm)
Far IR ( 1400 nm- 1 mm)
o Aqueous humor absorbs very little radiation
o Lens: Near UV (315-390 nm)
IR > 2500 nm
UV absorption by lens increases with the increasing age
o Vitreous body: UV < 290 nm
IR > 1600 nm
Effects: Cataract, macular degeneration
 Absorption of light by photoreceptors
o The photochemical reactions occurred in the
photoreceptors by the absorption of light forms
the basis of the visual system
 Light Filters
o Material used to absorb or transmit light of all
wavelength equally i.e. neutral density filter or
selectively such as the colored filters
E.g. green filters, blue filters
 Absorptive Lenses
o Absorption may be uniform or selective
o Some lenses absorb mostly in the IR region of spectrum.
E.g. Calobar, Ray Ban
o Other absorb in UV region. E.g. Spectacle Pink, UV 400,
UV 530
o Colored contact lens,tinted lens
•Optics by A. H. Tunnacliffe
•Optics and Refraction by A. K. Khurana
•Clinical Optics (section 3) AAO 2011-2012