PolarizationPolarization

Interference and diffraction proved that light is a wave motion

It does not tells about character of wave

In 1817 thomas young postulated that light has transverse

Polarization property is a direct consequence of light being a transverse wave.

Light is a Electromagnetic WaveLight is a Electromagnetic Wave

EM wave is EM wave is ……• Light is an electromagnetic wave.Light is an electromagnetic wave.• It consists of vibrations of electric field and It consists of vibrations of electric field and

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

perpendicular to each other and in phase.perpendicular to each other and in phase.• EM wave is a transverse wave.EM wave is a transverse wave.

• Traditionally light wave is described by electric Traditionally light wave is described by electric field vector Efield vector E

Electric Field VectorElectric Field Vector

Liht coming from an ordinary source towards us , then we would observed that the direction of E is randomly varying with time. as a result the direction of oscillation of E vector occurs in all possible planes

A light in which E-vector oscillating in all possible directions is referred to as unpolarised light

It is produced naturally

The light emitted from the sun, from the bulbs are unpolarised light

Polarized Light

Polarized Light

Vibrations lie on one single plane only.

Unpolarized Light

Superposition 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

LightUnpolarized

Horizontal Component being Absorbed

Vertical Component being Transmitted

Selective Absorption - ExplanationSelective Absorption - Explanation

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 coscos22θθ

Polarizer & Unpolarized LightPolarizer & Unpolarized Light

• Each wave is attenuated by factor cos2θ.

• Average attenuation is < cos2θ > = 1/2

Crossed PolarizersCrossed Polarizers

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

cos2θ .• The second polarizer projects the electric field onto a new axis,

rotated by θ from the axis of the first polarizer

Crossed PerpendicularlyCrossed Perpendicularly

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

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

ReflectionReflection

BrewsterBrewster’’s Angles 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 Sunglasses –– Glare Reduction Glare Reduction

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

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

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

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

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

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

Action of Polaroid SunglassAction of Polaroid Sunglass

Unwanted glares are usually horizontally polarized light

Vertically Polarized Light from Objects

Light 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.

PolarizationPolarization

Glare and PolarizationGlare and Polarization

www.johnsonwindowfilms.com/cutglare.htm

Glare and ReductionGlare and Reduction

ScatteringScattering

Incident 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 ScatteringsPolarization by Scatterings

Polarized Light by ScatteringPolarized Light by Scattering

Question ?Question ?

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

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

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

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

• During this long passage, most of the blue During this long passage, most of the blue light is scattered away and virtually all that light is scattered away and virtually all that we see coming to us from the sun is its we see coming to us from the sun is its red and orange wavelengths.red and orange 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 materials, When polarized light is passed through some materials, the plane of polarization is rotated.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 greatest, depends on the stress in them. When the stress is greatest, the rotation of the vibration is greatest. Dark and colour the rotation of the vibration is greatest. Dark and colour bands are produced when polarized light is shone through the bands are produced when polarized light is shone through the material. Engineers build plastic models of structures to help material. Engineers build plastic models of structures to help them to analysts the stress distribution.them to analysts the stress distribution.

– 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