Polarization, scattering &absorption of light

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Transcript of Polarization, scattering &absorption of light

POLARIZATION OF LIGHT & ITS APPLICATION

POLARIZATION, SCATTERING AND ABSORPTION OF LIGHT & APPLICATIONBIKASH SAPKOTABachelor of Optometry16th Batch Maharajgunj Medical Campus,Nepal

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Polarization of Light: Types, Methods & Application

Absorption of Light: Types & Application

Scattering of Light: Types & Application

PRESENTATION LAYOUT

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POLARIZATION OF LIGHT

ORDINARY LIGHTElectromagnetic wave Electric field E and magnetic field B are:Perpendicular to each otherIn phaseAlso perpendicular to the direction of propagation

Em waveElectric field vector Magnetic field vector

ORDINARY LIGHTUnpolarized LightContains large no.of atoms producing waves with particular orientation of electric vector EResultant wave: unpolarized wave: superposition of waves vibrating in all possible directions

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 cant 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 fieldPlane of polarization The plane perpendicular to the plane of vibration

Why only electric field vector is considered in polarization and not magnetic field vector Maxwells 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 POLARIZATIONLinear Polarization

Circular Polarization

3. Elliptical Polarization

LINEAR POLARIZATIONPlane polarized waveElectric field vector oscillates along a straight line in one plane

Resultant wave is linear in vertical plane

Superposition of plane polarized waveTwo 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

Resultant wave is linear in 450 plane

Resultant wave is linear in 900 plane

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)

ELLIPTICAL POLARIZATIONConsists of two perpendicular waves of unequal amplitude that differ in phase by 900The tip of the resultant electric field vector describes an ellipse in any fixed plane intersecting and normal to the direction of propagationCircular and linear polarization: special cases of elliptical polarization

METHODS OF ACHIEVING POLARIZATIONReflectionScatteringDichroismBirefringence

POLARIZATION BY REFLECTIONUnpolarized 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 900Such incident angle is k/a polarizing angle or Brewsters angleReflected ray is linearly polarized parallel to the reflecting surface

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BREWSTERS LAWWhen light is incident at polarizing angle: The tangent of polarizing angle=Refractive index of material i.e, tan = For Sapphire, =1.77 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

A:no polarizer used

B:vertical polarizer used

C:horizontal polarizer used

ABC

POLARIZATION BY SCATTERINGPolarization 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 Rayleighs law Amount of scattering 1/^4

.Light scattering off atoms is:Unpolarized if the light keeps traveling in the same directionLinearly polarized if it scatters in a direction perpendicular to the path it was travellingSomewhere between linearly polarized and unpolarized if it scatters off at any other angles

POLARIZATION BY BIREFRINGENCEPolarization 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 waveBoth beams thus formed are polarized:One parallel to the surfaceOther perpendicular to the surface

POLARIZATION BY DICHROISMPolarization by selective absorptionSuch 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

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 + AnalyzerIf 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 () 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 polarizerMind It!! I0 is half the intensity of unpolarized light incident on the polarizer

Intensity is maximum(I=I) 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.

Application of polarization by reflectionIn polaroid sunglasses

Light 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.

Without polaroid sun glassesWith polaroid sun glasses

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

In Photographic FiltersGlare 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

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 ScatteringPhotographic secret of capturing a vivid blue sky using polaroid filter

No polaroid filter has been used

Horizontal polarizer usedDeep blue skyVertical polarizer used No significant difference

Application of Polarization by BirefringenceIn birefrigent biprismsBirefrigent biprisms such as nicol, glan-foucault and wollaston are used to produce polarized light

Glan foucault prismWollaston prismNicol prism

In Liquid Crystal Displays(LCD)There are some crystals that become aligned when an electric field ,are put across themWhen this happens they act as polarizing filters

LCD

In Retinal DiagnosisPolarization 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: glass over OD polarized at 900 and OS polarized at 1800Test one eye at a time though patient viewing binocularlyAlternative lines of optotype are also polarized at 900 and 1800 Used to detect malingering

To detect defect in Intra Ocular LensesBirefringence 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

Haidingers BrushYellowish 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

Other Applications of Polarization

In 3D FilmsTwo 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 AnalysisWhen 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 colors 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 analyzing the colored pattern produced

stress analyzer .

Stress analysis

In SaccharimetryMeasurement 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 OphthalmoscopeControl unwanted reflections eg. that from the front of corneaRed filter, blue filter, green filter etc.

SCATTERING OF LIGHT

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 moment

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 Elastic Scattering

II. Inelastic Scattering

Elastic scatteringThe 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 ScatteringMie ScatteringNonselective Scattering

The energy of the incident photon is not conserved.

Inelastic scattering includes:

Brillouin scattering

Raman scattering

Inelastic X-ray scattering

Compton scattering

Inelastic 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 wavelength

Scattering produced by such small particles is isotropic i.e. equal in all direction

Scattering efficiency(K) is inversely proportional to the fourth power of the wavelength of light() i.e. K 1/

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 blueAs sunlight moves through the atmosphere, longer wavelengths(eg.red) pass straight through

However, shorter wavelengths(eg.blue) interact with gas molecules and scatter in the atmosphere

Secret of red sunsetAs the sun approaches the horizon during sunsets, sunlight travels longer distance to reach our eyes

Hence, light with shorter wavelengths(eg.blue) are scattered more before reaching to our eyes and thus sunsets appear red

Mie ScatteringIt 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

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Cloud droplets being larger scatter all wavelengths of visible light. So the cloud appears white

Attenuation in optical fiberInvolves scattering of light: due to change in local refractive indexAlso 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

Scattering ProcessWavelength DependenceParticle Size (in m)Kind of ParticlesRayleigh Scattering^ 2500 nm UV absorption by lens increases with the increasing ageVitreous body: UV < 290 nm IR > 1600 nm Effects: Cataract, macular degeneration

Absorption of light by photoreceptors The photochemical reactions occurred in the photoreceptors by the absorption of light forms the basis of the visual system

Light FiltersMaterial 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

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Absorptive LensesAbsorption may be uniform or selectiveSome lenses absorb mostly in the IR region of spectrum. E.g. Calobar, Ray BanOther absorb in UV region. E.g. Spectacle Pink, UV 400, UV 530Colored contact lens,tinted lens

REFERENCEOptics by A. H. TunnacliffeOptics and Refraction by A. K. KhuranaClinical Optics (section 3) AAO 2011-2012Internet

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