1Modern Physics: QuantizationThis Lecture!Finish polarization!Quantization in general!Light quantization!photonsFrom previous Lecture!EM waves and their properties!Poynting vector!PolarizationRadiation properties!This is the power per unit area (J/s.m2 = W/m2)! Iav=PavA= uavcI " E2E/B=c! prad=FA=Power / Ac=Ic!Complete absorption on a surface: total transported energy U in time interval #t $ total momentum p = U / c and prad=Sav/c!Perfectly reflecting surface: momentum of incoming and reflected light p = U/c $ total transferred momentum p = 2U/c and prad = 2Sav/cPolarization of Light Waves (34.8)!Linearly polarized waves: E-field oscillates at all times in the plane of polarization!Any two waves can be superposed to make a third, or a single wave decomposed into two.Linearly polarized lightE-field has one spatial orientationUnpolarized lightE-field in random directionsIt’s a superpositionof waves with E vibrating in many different directionsSun lightPolarization by selective absorptiontransmission axisPolaroid sheetLong-chain hydrocarbon moleculesIf linearly polarized light of intensity I0 passes through a polarizing filter with transmission axis at an angle ! along yEinc = E0sin! i + E0 cos! jAfter the polarizer Etransm = E0cos! jSo the intensity transmitted isItransm = E02 cos2! = %0cos2!yx!E0cos!If upolarized light of intensity I0 passes through it the intensity becomes I0/2Polarization by reflection!If reflected and refracted beams are orthogonal complete polarization occurs !Brewster’s law: angle of incidence is given by tan!p=n (n = index of refraction)UnpolarizedIncident lightReflection polarized with E-field parallel to surfaceRefractedlight!Unpolarized light reflected from a surface becomes partially polarized!Degree of polarization depends on angle of incidence nReducing glareA polarizer can substantially reduce intensity of reflections, since the reflections are partially polarized. Sunlight reflected from water, glass,snow is partially polarized. If surface is horizontal the E-field vector of reflected light has strong horizontal component. Polarized glasses: vertical transmission axis absorbs strong horizontal componentReflected light can be eliminated!Transmission axishttp://polarization.com/index-net/index.htmlQuick Quiz!Which pair of polaroid glasses can eliminate the glare from a road surface? Polarization axes are shown as straight linesAnswer: the glare is the result of a large concentration of light aligned parallel to the road surface. To block such plane-polarized light, a filter with a vertically aligned polarization axis must be used.Polarization by scatteringLooking far from the Sun we see mainly scattered light $blue skyLooking towards the Sun the light that survives is weighted towards red because most of the blue light has been scatteredSo different directions relative to Sun have different polarizations. Some insects can detect this polarization and use it to navigate.Vikings may have used polarization of sky to navigateWhen light hits a material electrons absorb and reradiate part of the light. The sky appears blue due to scattering of light on air and resulting partially polarized light. Short wavelengths (blue) are scattered more intensely than red.Polarization by scatteringThe light from blue sky is polarized and the direction of polarization at each point depends on the position of the Sun. Nobel prize to Frisch 1973: bees use the Sun as a compass, orienting their dance angle to the plane of polarization of the sunlight. Even when the Sun is obscured by clouds, bees can detect its position from polarization pattern of blue sky.The direction and duration of waggle runs are closely correlated with the direction and distance of the patch of flowers being advertised by the dancing bee. Optically active materials (your lab)!!They rotate the plane of polarization of incident light!Solution with sugar: when polarized light passes through the Karo syrup, the direction of its polarization is changed. !The rotation angle is proportional to depth and concentration of syrup.!The rotation angle depends on the wavelength or color of the light. Blue light, with its shorter wavelength, rotates more than the longer-wavelength red light.11Modern Physics!Physics changed drastically in the early 1900’s!New discoveries — Relativity and Quantum Mechanics!Relativity!Changed the way we think about space, time, and gravity!Quantum mechanics!Changed our conceptions of matter.Origins of quantum mechanics!1600-1800s classical description of the laws of the world we can see (Galileo, Newton, …) !Late 1800s: Maxwell’s equations describe propagation of EM waves in exquisite detail.• Early 1900s:—Further Investigations into light, and interaction of light with matterPhotoelectric effect, Matter wavesEnergy is quantized in discrete units!!Light is made of ‘quanta’ called photons13Quantum mechanics!The quantum mechanical world is VERY different!!Energy not continuous, but can take on only particular discrete values.!Light has particle-like properties, so that light can bounce off objects just like balls. !Particles also have wave-like properties, so that two particles can interfere just like light does.!Physics is not deterministic, but events occur with a probability determined by quantum mechanics.Photoelectric effect!A metal is a bucket holding electrons!Electrons need some energy in order to jump out of the bucket.Energy transferred from the light to the electrons.Electron uses some of the energy to break out of bucket.Remainder appears as energy of motion (kinetic energy).Light can supply this energy.Unusual experimental results!Not all kinds of light work!Red light does not eject electronsMore red light doesn’t eitherNo matter how intense is the light: until the light wavelength passes a certain threshold, no electrons are ejected.16Wavelength dependenceLong wavelength: NO electrons ejectedShort wavelength: electrons ejectedHigh-energy photonsLow-energy photonsThreshold depends on materialEinstein’s explanation!light is made up of photons, each with energy E = hf h=Planck constant f = frequency. !One photon collides with one electron - knocks it out of metal if photon has enough energy to transferred to the electron!Power ( = # photons / sec= Nhf/s) doesn’t change this.Minimum frequency (maximum wavelength) required to eject electron.Need UV!Nobel prize192118Summary of Photoelectric effect!Explained by quantized light.!Red light is
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