Polarization and its application

PABITA DHUNGEL

B.OPTOMETRY

Institute of Medicine

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PRESENATION LAYOUT

• Introduction• Types of polarized light• Few related terms• Few laws related to polarization• Applications

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Introduction

• Light is form of energy which gives us the sensation of vision

• Different properties of light are:• Reflection• Refraction• Interference• Diffraction• Polarization

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What is polarization?

• The vibrations are limited to just one direction in a plane perpendicular to the direction of propagation, the wave is said to be plane polarized

• Thus, the phenomenon of the restriction of the vibration to a particular direction is called polarization

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Contd…

• Unpolarized/ natural wave

Wave nature vibrating in all direction

Mixture of various plane polarized beam

e.g. sun, thermal, gaseous discharge lamp, flame,etc

• polarized beam

Contains vibration in only one plane

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• Partial polarized light

Mixture of polarized and non polarized light

• The plane which contains the vibrations of polarized light is called plane of vibration

• The plane normal to the plane of vibration is called the plane of polarization

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Mathematically

• If direction of propagation is z, E field oscillates in x direction, B field in y then:

Ex = E0 cos(ωt – 2πz/λ + δ) By = B0 cos(ωt – 2πz/λ + δ)

E0, B0 = amplitude of waves (related by optical properties of transmission medium)

ω = angular frequency λ = wavelength δ = phase

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Electromagnetic radiation

• EM field emitted by different atoms will have varying orientations and phases

• Resultant radiation may be described by components of E field oscillating in xz and yz planes with different amplitudes and phases:

Ex = Ex0 cos(ωt – 2πz/λ + δx) Ey = Ey0 cos(ωt – 2πz/λ + δy)

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Types of polarized light

• 1. plane/linear:• Vibration restricted to only one direction In

single meridian• Linearly polarized light can be obtained from

the polariser such as Dichoric polarisers

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Linear polarization

• The two orthogonal components are in phase so that the ratio of the strengths of two components is constant

• The direction of the electric vector (vector sum of these two components) is constant so tip traces out as single line in pane

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Dichoric polarisers

• Wire grid• H- sheet (made of PVC)• K- sheet – heat and humidity resistant (used in

sun glasses)• J- sheet

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2. Elliptical polarized

• Tip of the electric vector describes an ellipse about the direction of propagation

• Equivalent to two plane polarised of unequal magnitude (phase difference 90⁰)

• Vibration in an elliptical orbit

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Elliptical polarized

• Two components are not in phase and either do not have same amplitude or are not 90⁰ out of phase, though amplitude ratio are constant

• Called so because their electric vector traces out an ellipse in the plane

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3.Circular polarized

• Wave in which the tip of the electric vector describes a circle around the direction of propagation

• Equivalent to two plane polarized waves of equal magnitude moving together (out of phase by 90⁰)

• Vibration is circular orbit

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Circular polarized

• One component is zero while other is max. or minimum amplitude

• Either x comp. can be 90⁰ ahead of y comp. or it can be 90⁰ behind the y comp.

• These cases are called right handed circular polarization and left hand circular polarization, depending upon way electric vector rotates

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Few terms

• Polariser an instrument by which

the light gets polarized• AnalyserAn instrument which

detects whether a light ray is polarized or not

→polariser along with an analyser constitute polariscope

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Few related laws

• Malus law• States that : when a beam of completely plane

polarized light is incident on an analyser, the resultant intensity of light (I) transmitted from the analyser varies directly and the square of the cosine angle (θ) betn plane of transmission of analyser and polariser

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Malus law

• Intensity emerging from polarizer is IαE2

• Intensity emerging from analyser I θ α E2 cos2 • Dividing 2 by 1

• We get I θ = I cos2 θ• So when θ=90 , transmission axis of polariser and

analyser is perpendicular I θ = 0.01/03/15 18

Brewster’s law

• States that when an unpolarized light is incident at polarising angle φ on an interface separating a rarer medium from a denser medium such that the refractive index of the denser medium is μ w.r.t the rarer medium , then the reflected light in the rarer medium is completely polarised

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Brewster’s law

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Production of polarized light

1. Dichorism

2. Double reflection

3. Scattering

4. Reflection

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Dichorism

• Dichroism is selective absorption of the two orthogonal incident natural light.

• It is physically anisotropic.• Wire grid polariser• In this parallel electrically conducting copper

wires are stretched between two insulating boards.

• Here the x- component perpendicular to the wire is transmitted while y-component parallel to the wire is absorbed by the grid.

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• Dichroic crystals because of their anisotropic structure. For eg tourmaline.

• Optic axis of the crystal is the specific direction through which the electric field component perpendicular to the axis is strongly absorbed

• Only parallel electric field component is transmitted.

• Dichroic means of two colours..

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Birefrenges/double reflection

• Optical anisotropy• In some crystals the forces binding an electron

to the atomic nuclei is stronger in one direction than another.

• When an electromagnetic wave act upon them the electrons set into vibration in one plane and hence emit the wave with same energy and frequency in that plane at the end.

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Contd…

• Eg. Calcite/CaCO3

Symmetrical atoms arrangement about an axis perpendicular to the carbonate plane

• Other birefrigent crystals

Ice land spar, Wallaston prism

• Birefringent polarisers

Nicol prism, Glan Foucalt prism

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A calcite crystal

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Scattering

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• Sky acts as a partial polariser due to the scattering properties of air molecules

• Demonstration: viewing with crossed polaroid• Scattering α λ-4 so blue wavelength scattered

more

Reflection

• When angle of incidence is equal to the angle of polarization in any boundary, the reflected ray is plane polarised with it’s electric field vector parallel to the surface of reflection.

• When series of such reflecting surfaces are aligned as in microscopes (slit lamp) the reflected waves of all surfaces will form plane polarised beam intense enough to produce glare.

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Contd…

• So polaroid glasses with their transmission axis perpendicular to the reflecting surface would easily prevent the polarized intense beam from reaching eye.

Brewester’s law: n= tan Ip

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Applications

1. To differentiate longitudinal and transverse wave

2. As sun glasses to cut of unwanted reflected light best utilized by fisherman, motorist, skiers, sportsman etc.

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Effect of polarizing filter in sky(right pic)

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• Similarly in driving ,the light reflected from the road surface and from the painted or glass surface of other automobiles is also partiallly polarised,usually horizontally

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Applications

3. Geology

Frequently exploited using polarization microscopes, for identifying minerals

photograph of a volcanic sand grain: upper pic is plane polarized light, bottom pic is cross polarized

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4.Chemistry

• To check the chirality of the organic compound

• Polarization is also used in infrared spectroscopy

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5. Astronomy

• For providing information on sources of radiation and scattering, polarization probes the interstellar magnetic field via Faraday rotation

• Polarization of cosmic microwave background is being used to study the physics of the early universe

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6. 3D movies

• Polarization is used for 3D movies ,in which the images intended for each eye are either projected from two different projectors orthogonally oriented polarizing filters or, more typically , from a single projector with multiplexed polarization

• Polarized 3D glasses with suitable polarized filters ensure that each eye receives only the intended image

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7. Communication and radar applications

• All radio transmitting and receiving antennas are intrinsically polarized, special use of which is made in radar

• Vertical polarization is most often used when it is desired to radiate a radio signal in all directions such as widely distributed mobile units

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Contd…

• AM and FM radio use vertical polarization, while television uses horizontal polarization

• Alternating vertical and horizontal polarization is used on satellite communications to allow the satellite to carry two separate transmission on given frequency

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8.Material science

• In engineering , the relationship between strain and birefringence motivates the use of polarization in characterizing the distribution of stress and strain in protypes

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9. Navigation

• Sky polarization has been exploited in the “sky compass”, which was used in the 1950s when navigating near the poles of the Earth’s magnetic field when neither the sun nor stars were visible

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10. To identify thermally tempered lenses

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11.Haidinger’s brushes phenomenon

• It is a entopic phenomenon perceived by a normal eye while observing in a surface illuminated by plane –polarized white light

• It appears as faint yellow &blue-brush like pattern radiating from the fixation point with their long axis oriented at rt.angle to the transmission plane of polarization

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12. Ophthalmic instruments

• Eliminate strong reflection from the patients cornea

• Polariser 90⁰ to the polarisation of the incident light

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13.Polaroid vectograph(BV test)

• Stereogram picture : part of the picture polarised at 900 to the other part by the plane polariser placed in front of each eye – retinal disparity – measure of stereopsis

• Other sensitive testing

Fixation disparity

Anisokonia

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References

• Optics by tunaclife• Clinical optics• Clinical visual optics• Clinical refraction by Borish• Clinical procedures in optometry• Internet

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Thank you