PolarizationPolarization

Electromagnetic WavesElectromagnetic Waves

February 2005February 2005

Electromagnetic WaveElectromagnetic Wave

EM wave is …EM wave is …

• Light is an electromagnetic wave.Light is an electromagnetic wave.• It consists of vibrations of electric It consists of vibrations of electric

field and magnetic field.field and magnetic field.• The electric field and magnetic field The electric field and magnetic field

are perpendicular to each other and are perpendicular to each other and in phase.in phase.

• EM wave is a transverse wave.EM wave is a transverse wave.• The speed of EM wave is 3 x 10The speed of EM wave is 3 x 1088 ms ms-1-1..

Electric Field VectorElectric Field Vector

Polarized Light

Polarized LightVibrations lie on one single plane only.

Unpolarized LightSuperposition of many beams, in the same direction of propagation, but each with random polarization.

Representation . . . Representation . . .

Unpolarized

Polarized

EE

Representation . . . Representation . . .

Unpolarized

Polarized

Polarization of LightPolarization of Light

Selective AbsorptionSelective Absorption

Light

Unpolarized

Horizontal Component being Absorbed

Vertical Component being Transmitted

Selective Absorption - Selective Absorption - ExplanationExplanation

Polarizing MaterialPolarizing Material

A Polarizing material will only allow the passage of that component of the electric field parallel to the polarization direction of the material

I = I0 coscos22

Polarizer & Unpolarized LigPolarizer & Unpolarized Lightht

• Each wave is attenuated by factor cos2.• Average attenuation is < cos2 > = 1/2

Crossed PolarizersCrossed Polarizers

• The first polarizer reduces the intensity by half.• The second polarizer reduces the intensity by another

factor of cos2.• The second polarizer projects the electric field onto a

new axis, rotated by from the axis of the first polarizer

Crossed PerpendicularlyCrossed Perpendicularly

Crossed at different Crossed at different angles . . .angles . . .

Crossed at different angles . .Crossed at different angles . . . .

ReflectionReflection

Brewster’s AngleBrewster’s Angle

Pθ i

rsin

isin n

P

P

P

P

θ cos

θsin

)θ-(90sin

θsin n

Pθ-90 r

Pθ tan n

GlaresGlares

Sunglasses – Glare Sunglasses – Glare ReductionReduction

• Polarized lenses have the added benefit Polarized lenses have the added benefit of filtering out reflected light, or glare, of filtering out reflected light, or glare, off surfaces such as water or pavement off surfaces such as water or pavement

• Ideal for boating, fishing, driving or anIdeal for boating, fishing, driving or any other activity associated with intense y other activity associated with intense glare glare

• Reduces eyestrain and fatigue, while inReduces eyestrain and fatigue, while increasing contrast and visual acuity creasing contrast and visual acuity

How Do Polarized Lenses How Do Polarized Lenses Work?Work?

• Light reflected from suLight reflected from surfaces like a flat road or rfaces like a flat road or smooth water is generasmooth water is generally horizontally polarizelly horizontally polarized. This horizontally pold. This horizontally polarized light is blocked barized light is blocked by the vertically oriented y the vertically oriented polarizers in the lenses.polarizers in the lenses.

• The result: a reduction The result: a reduction in annoying and sometiin annoying and sometimes dangerous glare.mes dangerous glare.

Action of Polaroid Action of Polaroid SunglassSunglass

Unwanted glares are usually horizontally polarized light

Vertically Polarized Light from ObjectsLight reflected from

surfaces like a flat road or smooth water is generally horizontally polarized. This horizontally polarized light is blocked by the vertically oriented polarizers in the lenses.

Glare and PolarizationGlare and Polarization

http://regentsprep.org/Regents/physics/phys04/cpolarize/glare.htm

Glare and PolarizationGlare and Polarization

The polaroid absorbs most of the polarized light reflected from the water’surface, allowing the dimmer light from the bottom of the river, and any fish swimming there, to be seen more readily.

Glare and PolarizationGlare and Polarization

www.johnsonwindowfilms.com/cutglare.htm

Glare and ReductionGlare and Reduction

ScatteringScatteringIncident Light Ray

Scattered

Light RaySmall Particle

Scattering of SunlightScattering of SunlightWhen unpolarised sunlight impinges on the molecules, the electric field of the EM wave sets the electric charges within the molecules into motion. The EM wave is absorbed.

The molecules then quickly re-emit light in all directions since oscillating electric charges produce EM waves.

Because of the transverse nature of light, the electric field of the re-emitted waves should be in the plane that includes the line of oscillation, the scattered light is completely plane polarized with its electric vector in the direction shown.

Scattering of SunlightScattering of Sunlight

Polarization by Polarization by ScatteringsScatterings

Polarized Light by ScatterinPolarized Light by Scatteringg

Question ?Question ?

• Why is the sky blue in the day and red at sunsets?

The sky is blue The sky is blue because . . . because . . .

• The tiny particles in the atmosphere The tiny particles in the atmosphere (dust, clumps of air molecules, (dust, clumps of air molecules, microscopic water droplets) are better at microscopic water droplets) are better at scattering shorter wavelength blue light scattering shorter wavelength blue light than the longer wavelength red light. than the longer wavelength red light.

• As sunlight passes through the As sunlight passes through the atmosphere, the scattered blue light give atmosphere, the scattered blue light give the atmosphere an overall blue glow.the atmosphere an overall blue glow.

The sunset is red The sunset is red because . . . because . . .

• At sunrise and sunset, sunlight At sunrise and sunset, sunlight enters our atmosphere at a shallow enters our atmosphere at a shallow angle and travels a long distance angle and travels a long distance before reaching our eyes.before reaching our eyes.

• During this long passage, most of the During this long passage, most of the blue light is scattered away and blue light is scattered away and virtually all that we see coming to us virtually all that we see coming to us from the sun is its red and orange from the sun is its red and orange wavelengths.wavelengths.

Blue Sky and Red SunsetBlue Sky and Red Sunset

Optical ActivityOptical Activity

Photograph of a plastic model of a

crane hook

Optical ActivityOptical Activity

• When polarized light is passed through some matWhen polarized light is passed through some materials, the plane of polarization is rotated.erials, the plane of polarization is rotated.

• Examples : plastic ruler, sellotape, sugar solutionExamples : plastic ruler, sellotape, sugar solution• Applications :Applications :

– Stress Analysis : The optical activity of plastic and glass Stress Analysis : The optical activity of plastic and glass depends on the stress in them. When the stress is greatdepends on the stress in them. When the stress is greatest, the rotation of the vibration is greatest. Dark and cest, the rotation of the vibration is greatest. Dark and colour bands are produced when polarized light is shonolour bands are produced when polarized light is shone through the material. Engineers build plastic models e through the material. Engineers build plastic models of structures to help them to analysts the stress distribuof structures to help them to analysts the stress distribution.tion.

– LCDs (Liquid Crystal Displays)LCDs (Liquid Crystal Displays)

ApplicationApplication• Used in LCD display• Electrical voltage on a

liquid crystal diode turns on and off polarizing filter effect.

Seven-Segment LCDSeven-Segment LCD

AerialAerial

In a vertical transmitting In a vertical transmitting aerial, vertical electron aerial, vertical electron oscillation results in a oscillation results in a vertical polarized radio vertical polarized radio wave.wave.

Maximum electron Maximum electron oscillation in the oscillation in the receiving aerial is receiving aerial is achieved when the aerial achieved when the aerial is aligned to be parallel to is aligned to be parallel to the wave’s electric field.the wave’s electric field.

LinksLinks

• http://micro.magnet.fsu.edu/optics/lighttp://micro.magnet.fsu.edu/optics/lightandcolor/polarization.htmlhtandcolor/polarization.html

• http://dpfwiw.com/polarizer.htm#unwanted

• Blue Skies and Red SunsetsBlue Skies and Red Sunsets

http://science.howstuffworks.com/frahttp://science.howstuffworks.com/framed.htm?parent=question39.htm&urlmed.htm?parent=question39.htm&url=http://www.glenbrook.k12.il.us/gbssc=http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l2f.htmli/phys/Class/light/u12l2f.html