polarization, scattering &absorption of light
TRANSCRIPT
POLARIZATION, SCATTERING AND ABSORPTION OF LIGHT
& APPLICATION
BIKASH SAPKOTABachelor of Optometry16th Batch
Maharajgunj Medical Campus,Nepal
Polarization of Light: Types, Methods & Application
Absorption of Light: Types & Application
Scattering of Light: Types & Application
PRESENTATION LAYOUT
POLARIZATION OF LIGHT
ORDINARY LIGHTElectromagnetic wave Electric field E and magnetic field B are:oPerpendicular to each otheroIn phaseoAlso perpendicular to the direction of propagation
Em wave
Electric field vector Magnetic field vector
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
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
.
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
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
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
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 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
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
.
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 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 + 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(I=I₀) if the transmission axes are parallel and intensity is zero if the transmission axes are perpendicular to each other
I₀
I
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 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 glasses With polaroid sun glasses
Polaroid sun glasses are also used to reduce head light glare of car
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
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 usedHorizontal polarizer used
Deep blue sky
Vertical 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 prism Wollaston 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 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: glass over 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
Used 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
Haidinger’s 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 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 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 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.
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
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
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(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 blueo 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 sunseto 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 fibero Involves scattering of light: due to change in
local refractive indexo 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
Scattering Process
Wavelength Dependence
Particle Size (in µm)
Kind of Particles
Rayleigh Scattering
λ^‾⁴ << 0.1 Air molecules
Mie Scattering λ^˚ to λ^‾⁴ 0.1 to 10 Smoke, cloud droplets
Nonselective Scattering
λ^˚ 10 Larger dust particles, water droplets, etc
Comparison
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
i. 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 edemao 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 light
ABSORPTION OF LIGHT
It is a process by which radiant energy is taken up internally by a substance or the medium through which it passes
Light energy is transformed in to internal energy of the absorber such as thermal energy
Incident
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 body
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) OD=log(1/T) Where T=Transmittance An OD of 1 represents transmittance of 10% An OD of 2 represents transmittance of 1% and so on
Fluorescence It is a property by which substance absorbs light of a
given wavelength and re-emits it as radiations of a longer wavelength
E.g. Fluorescein
Fluorescent imaging of three components in a dividing human cancer cell
Fluorescein
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 illumination
Fundus photograph in FFA
Fluorescein spectrum
Colors of Objects
o The color of an object is determined by the wavelengths of light that the object absorbs, transmits and reflects
Application of absorption of light
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 cycle
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 radiationo Lens: Near UV (315-390 nm) IR > 2500 nm UV absorption by lens increases with the increasing ageo 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 selectiveo Some lenses absorb mostly in the IR region of spectrum.
E.g. Calobar, Ray Bano Other absorb in UV region. E.g. Spectacle Pink, UV 400,
UV 530o Colored contact lens,tinted lens
REFERENCE
•Optics by A. H. Tunnacliffe•Optics and Refraction by A. K. Khurana•Clinical Optics (section 3) AAO 2011-2012•Internet
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