16.1 PolarizationPolarized LightPolarization by absorptionPolarization by reflectionPolarization by scatteringWave Optics• Geometric Optics – Light rays move in straight lines.– Okay for interaction with objects much larger than the wavelength• Wave Optics – Light propagates as spherical waves. – Describes interactions with objects with the same size as the wavelength.Wave Properties of LightWave optics or Physical optics is the study of the wave properties of light. Some wave properties are:Interference, diffraction, and polarization.These properties have useful applications in optical devices such as compact discs, diffraction gratings, polarizers. Polarization• Polarized light has it E field in one direction.• Light is normally unpolarized• Light can be polarized by several different processes– Absorption – Polaroid filter– Reflection – Brewster’s angle– Scattering – Light from the sky• Polarized light has many applications– Polaroid sunglasses, Polarization microscopy, liquid crystal display.Light is a transverse waveA plane wave with Electric field in the y directionPolarized and un-polarized lightUnpolarized Light has E field at any instantcan have E in any direction.Polarized Lighthas E field in a certain direction2Polarization by absorptionOriented molecules absorb light with E along y directionPolaroid filmEyEzEzun-polarizedpolarizedfor an ideal polarizer the intensityis reduced by 1/2 12polarized unpolarizedII=Polarized light passing through a polarizer at angle θθEoEocosθEsinθEocosθparallel componenttransmittedBut I ∝ E2Therefore transmitted intensity I = Iocos2θPolarized light passing though a polarizerIoIocos2θDecrease in intensity when polarized light passesthrough a polarizer I= Iocos2θThe angle of polarization changesLaw of MalusTwo polarizersθ=0θ=45oθ=90o“Crossed-polarizers”I=Iocos2θExampleUn-polarized light is incident upon two polarizersthat have their polarization axes at an angle of 45o. If the incident light intensity is I what is the final intensity?Io2oI2cosθoI2o21cos 452224⎛⎞===⎜⎟⎝⎠oooIIII45oPolarization by reflectionUn-polarized light can be polarized by reflection at a specific polarization angle θp(Brewster’s angle)θpθ2θpn1n2Fully polarizedUn-polarized21tanpnnθ=3Polarization by reflectionReflected beam isPartially polarizedReflected beam isFully polarizedE is⊥to planeof incidenceThis E componentis absentExampleSuppose you wanted to have fully polarized light by reflection at the air water interface. What conditions would you use? What would be the direction of the polarized E field? θpAngle of incidence equalto the polarizing angle2p1opntan 1.333n53θ= =θ=n1=1.00n2=1.333E would be ⊥ to the plane ofincidence.••Polarization by reflectionno filterpolarizing filterThe reflected light is polarized -Polarization by scatteringScattering particle has oscillations partially polarizedin the plane ⊥ to the direction of propagationA transverse wave has no E field in the direction of propagationobserverscattered light is partially polarized with E field ⊥ to the direction of propagation of the incident lightPolarization of light by air Polarization of scattered lightno filter polarizing filterLight from the sky is partially polarized4Applications- Crossed PolarizersCrossed polarizers used to detect materials that rotatethe plane of polarized light (optically active materials) includingmany biological materials and materials under mechanical stressApplications – Liquid crystal display (LCD)Oriented moleculesrotate the plane of the polarized lightWhen an electric fieldis applied the moleculesreorient so that thelight is not