POLARIZATION OF LIGHT

& ITS APPLICATION

BIKASH SAPKOTABachelor of Optometry16th Batch

Maharajgunj Medical Campus, Nepal

Concept of PolarizationTypes of PolarizationMethods of achieving PolarizationApplications of Polarization

PRESENTATION LAYOUT

ORDINARY LIGHT

Electromagnetic wave Electric field E and magnetic field B are:oPerpendicular to each otheroIn phaseoAlso 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 EoResultant wave:unpolarized wave:superposition of waves vibrating in all possible directions

POLARIZATION

Transforming unpolarized light into polarized lightRestriction of electric field vector E in a particular plane so that vibration occurs in a single planeCharacteristic of transverse waveLongitudinal waves can’t be polarized; direction of their oscillation is along the direction of propagation

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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 waveElectric 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 additionResults 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

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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. Reflection2. Scattering3. Dichroism4. Birefringence

POLARIZATION BY REFLECTION

Unpolarized light can undergo polarization by reflection off of non metallic surfaces like snow, glassIncident angle is such that angle between reflected and refracted ray is 900

Such incident angle is k/a polarizing angle or Brewster’s angleReflected 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 vibrationVibrating electrons produce new Em waves radiated in all possible directionsNewly generated waves strike neighbouring atoms, thereby continuing the processAbsorption + re emission →scattered light

Polarization by scattering occurs in atmosphere leading to blue skyAccording to Rayleigh’s law Amount of scattering ἀ 1/λ^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

POLARIZATION BY BIREFRINGENCE

Polarization due to double refractionA double refracting crystals like Iceland spar, calcite refracts incident light into two different pathsSo if an object is viewed by looking through the crystal, two images are seenPolarizing filter can be used to completely block one imageTwo 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 surfaceOther perpendicular to the surface

POLARIZATION BY DICHROISM

Polarization by selective absorptionSuch 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 polaroidHave long chain of molecules that are aligned within the filter in a particular directionWhen 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 polaroidTransmitted light is plane polarized

Dual Filter:Polarizer + Analyzer

If the transmission axes of polarizer and analyzer are perpendicular, no light is transmittedThe light transmitted at other angles follows the Law of MalusPolarizer and analyzer relation can be best described by picket fence analogy:

Law of MalusWhen 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 polarizerMind 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

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Application of polarization by reflection

In polaroid sunglassesLight reflected off a pool of still water is partially polarized parallel to water surfaceThis gives rise to glareThe transmission direction of polaroid sheet in sun glasses is vertical which blocks horizontal components of lightHence reduce intensity and glare

Fishermen use polaroid sun glasses to locate fish under water

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Without polaroid sun glasses With polaroid sun glasses

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

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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 surfaceSo 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 victographThe right eye and left eye pictures are polarized at 450 and 1350 respectivelyThe pictures are viewed through a correspondingly oriented spectacle analysersIn 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 usedHorizontal polarizer used

Deep blue sky

Vertical polarizer used No significant difference

Application of Polarization by Birefringence

In birefrigent biprismsBirefrigent biprisms such as nicol, glan-foucault and wollaston are 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 themWhen 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 ChartSpecial polarizing glass is used:OD polarized at 900

and OS polarized at 1800

Test one eye at a time though patient viewing binocularlyAlternative 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 otherAny 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 sampleDirectly transmitted light can, optionally, be blocked with a polarizer oriented at 900 to the illuminationPolarized light interacts strongly with the sample and so generating contrast with the backgroundIt is used extensively in optical mineralogy

Mineral concentration

Other Applications of Polarization

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Haidinger’s Brush

Yellowish bow tie shapedEntoptic phenomenon Always positioned in macula, so visible in centre of visual fieldViewed while facing away from sun,bright background,eg LCD screenDue to dichroism of xanthophyll pigment of maculaUsed 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 projectorsProjected through polarizing filters with axes perpendicular to each otherViewers wear glasses with 2 polaroid filters with axes perpendicular Left eye sees the movie projected from rightRight 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 activityThe thicker the material the more it is rotated and different colours are rotated by different amountsTo 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

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Stress analysis

In Saccharimetry

Measurement of concn of sugar in solutionDue 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 corneaRed filter,blue filter,green filter etc.

Multiple choices questions

1.Optically active substances are those substances whicha. Produce polarized lightb. Produce double refractionc. Convert a plane polarized light into circular polarized

lightd. 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 a. Positive Y-axis definitelyb. Negative X-axisc. Positive X-axisd. 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 a. Two rotating dotsb. Two stationary dotsc. One dot onlyd. One dot rotating about the other

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

propagationc. Are confined to a single plane perpendicular to the

direction of propagationd. Occur in all directions

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5.Two polaroids are placed with their transmissionaxis at 900 to each other and transmittedIntensity of light is zero. If one more polaroid isplaced between these two, bisecting the anglebetween them, then the ratio of transmittedintensity of light to the intensity of unpolarized lightwill bea. 1/4b. 1/8c. 1/2d. 0

REFERENCE

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

Palanchowk Bhagawati