POLARIZATION OF LIGHT

& ITS APPLICATION

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➢Concept of Polarization

➢Types of Polarization

➢Methods of achieving Polarization

➢Applications of Polarization

PRESENTATION LAYOUT

ORDINARY LIGHT

❑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

ORDINARY LIGHT

❑Unpolarized LightoContains 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

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 can’t be polarized; direction of

their oscillation is along the direction of

propagation

.

Polarization

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 × 108

m/s),very large value

▪E>>>B i.e. Em wave is predominantly an electric

wave

▪To change any characteristics of Em wave, including

polarization,E should be affected

TYPES OF POLARIZATION

1. Linear Polarization

2. Circular Polarization

3. Elliptical Polarization

LINEAR POLARIZATION

➢Plane polarized wave

➢Electric field vector oscillates along a

straight line in one plane

Resultant wave is linear in vertical plane

Resultant wave is linear in 450 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

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)

Blue wave is resultant circular polarized wave

.

.

Superposition of oppositely polarized waves results in to plane polarized wave

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 polarization:special cases of elliptical polarization

Blue wave is resultant elliptical polarized wave

Green wave is resultant elliptical polarized wave

METHODS OF ACHIEVING POLARIZATION

1. Reflection

2. Scattering

3. Dichroism

4. Birefringence

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

Brewster’s angle

➢Reflected ray is linearly polarized parallel to the

reflecting surface

BREWSTER’S LAW

➢When light is incident at polarizing angle:

The tangent of polarizing angle=Refractive index of

material

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

A

B

C

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

.

➢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

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

O-ray:passes undeviated,ordinary waveE-wave:beam displaced sideway,extraordinary wave

➢Both beams thus formed are polarized:

▪One parallel to the surface

▪Other perpendicular to the surface

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

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

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 if the transmission axes are parallel and intensity is zero if the transmission axes are perpendicular to each other

APPLICATIONS OF POLARIZATION OF LIGHT

.

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 With polaroid sun glasses

Polaroid sun glasses are also used to reduce head light glare of car

.

Without polaroid sun glasses 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

❖Any specimens in glass cases can be well photographed by using filters

No polarizerVertical polarizerHorizontal polarizer

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

Titmus Fly Test

Application of Polarization by Scattering

➢Photographic secret of capturing a vivid blue sky

using polaroid filter

No polaroid filter has been used

Horizontal polarizer usedDeep blue sky

Vertical polarizer used No significant difference

Application of Polarization by Birefringence

❖In birefrigent biprismsBirefrigent biprisms such as nicol, glan-foucault and wollastonare used to produce polarized light

Wollaston prism

Glan foucault prism

❖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

❖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:OD 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

❖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

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

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

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

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 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

stress analyser

.

Stress analysis

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

In Slit Lamp and Ophthalmoscope

➢Control unwanted reflections eg. that from the front of

cornea

➢Red filter,blue filter,green filter etc.

Multiple choices questions

1.Optically active substances are those substances which

a. Produce polarized light

b. Produce double refraction

c. Convert a plane polarized light into circular polarized light

d. Rotate the plane of polarization

.

2.An unpolarized light wave is traveling along positive X-axis. The electric field vector in the beam vibrates in the direction of

a. Positive Y-axis definitely

b. Negative X-axis

c. Positive X-axis

d. Y or Z-axis

.

3.A calcite crystal is placed over a dot on a piece of paper and rotated. On seeing through the calcite, one will see

a. Two rotating dots

b. Two stationary dots

c. One dot only

d. One dot rotating about the other

.

4. An unpolarized beam of transverse waves is one whose vibrations

a. Are confined to a single plane

b. Occur in all directions perpendicular to the direction of propagation

c. Are confined to a single plane perpendicular to the direction of propagation

d. Occur in all directions

.

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

a. 1/4

b. 1/8

c. 1/2

d. 0

REFERENCE

•Optics by Tunnacliffe

•Optics and Refraction by A.K. Khurana

•Principle of Physics, Ayam Publication

•Internet

Palanchowk Bhagawati