光（電磁波）的輻射與光譜Wheredoeslightcomefrom?We’veseenthatMaxwell’sEquations(i.e.,thewaveequation)describethepropagationoflight.Butwheredoeslightcomefrominthefirstplace?Somemattermustemitthelight.Itdoessothroughthematter’s“polarization”:whereNisthenumberdensityofchargedparticles,qisthechargeofeachparticle,andisthepositionofthecharge.Here,we’veassumedthateachchargeisidenticalhasidenticalmotion.Notethatmatter’spolarizationisanalogoustothepolarizationoflight.Indeed,itwillcausetheemissionoflightwiththesamepolarizationdirection.Theinducedpolarization,,containstheeffectofthemediumandisincludedinMaxwell’sEquations:Maxwell'sEquationsinaMediumNoticethattheinducedpolarization,andhence,getsdifferentiatedtwice.Butisjustthechargeacceleration!

Soit’sacceleratingchargesthatemitlight!Thisextratermalsoaddstothewaveequation,whichisknownasthe"InhomogeneousWaveEquation:”Thepolarizationisthe“sourceterm”andtellsuswhatlightwillbeemitted.SourcesoflightLinearlyacceleratingchargeVibratingelectronsornuclei

ofanatomormoleculeSynchrotronradiation—

lightemittedbycharged

particlesdeflectedbya

magneticfieldBremsstrahlung("Brakingradiation")—

lightemittedwhenchargedparticles

collidewithotherchargedparticles+AcceleratingchargesemitlightThevastmajorityoflightintheuniversecomesfrommolecularvibrationsemittinglight.Electronsvibrateintheirmotionaroundnuclei highfrequency:~1014-1017cyclespersecond.Nucleiinmoleculesvibratewithrespecttoeachother intermediatefrequency:~1011-1013cyclespersecond.Nucleiinmoleculesrotate lowfrequency:~109-1010cyclespersecond.Molecules(andeverythingelse)haveenergylevelsandcanemitlightonlybymakingatransitionfromoneleveltoanother.Atypicalmolecule’senergylevels:Groundelectronicstate1stexcitedelectronicstate2ndexcitedelectronicstateEnergyTransitionLowestvibrationalandrotationallevelofthiselectronic“manifold”ExcitedvibrationalandrotationallevelTherearemanyothercomplications,suchasspin-orbitcoupling,nuclearspin,etc.,whichsplitlevels.Differentatomsemitlightatdifferent

frequencies.

Frequency(energy)Atomshavesimplersystems(andhencesimplerspectra)becausetheyhavenonuclearvibrations.Collisionsbroadenthefrequencyrangeof

lightemission.Acollisionabruptlychangesthephaseofthesine-wavelightemission.

Gasesatatmosphericpressurehaveemissionwidthsof~1GHz.Solidsandliquidsemitmuchbroaderrangesoffrequencies.TheElectromagneticSpectruminfraredX-rayUVvisiblewavelength(nm)microwaveradio105106gamma-rayThetransitionwavelengthsareabitarbitrary…TheElectromagneticSpectrumTheLong-WavelengthElectro-magneticSpectrumRadioµwaveregions(3kHz–300GHz)Itconsistsof24orbitingsatellitesin“half-synchronousorbits”(tworevolutionsperday).Foursatellitesperorbit,

equallyspaced,inclined

at55degreestoequator.Operatesat1.575GHz

(1.228GHzisareference

tocompensateforatmos-

phericwatereffects)4signalsarerequired;

onefortime,threefor

position.2-maccuracy

(100mforus).GlobalPositioningSystem(GPS)22,300milesabovetheearth’ssurface6GHzuplink,4GHzdownlinkEachsatelliteisactuallytwo(oneisaspare)GeosynchronouscommunicationssatellitesAnantennashouldbelargerthanthewavelength.MicrowaveAntennasMicrowaveConnectors&ComponentsMicrowaveovensSourcesofInfraredandMicrowaveLightinAstronomy30GHz300GHz3THz30THz300THzmmmmIRisusefulformeasuringthetemperatureofobjects.LindaHermans-Killam,outreach@OldFaithfulSuchstudieshelptoconfirmthatOldFaithfulisinfactfaithfulandwhetherhumanexistenceisinterferingwithit.IRLie-detectionIdon’treallybuythis,butIthoughtyou’denjoyit…ThemilitaryusesIRtoseeobjectsitconsidersrelevant.Jetenginesemitinfraredlightfrom3to5.5μmThislightiseasilydistinguishedfromtheambientinfrared,whichpeaksnear10mmandisrelativelyweakinthisrangeAtmosphericPenetrationdepth(fromspace)vs.WavelengthTheInfraredSpaceObservatoryStarsthatarejustformingemitlightmainlyintheIR.Usingmid-IRlaserlighttoshootdownmissilesTheTacticalHighEnergyLaserusesahigh-energy,deuteriumfluoridechemicallasertoshootdownshortrangeunguided(ballisticflying)rockets.Wavelength=3.6to4.2mmThankstoMichaelGuraforthisreference!LaserweldingNear-IRwavelengthsarecommonlyused.VisibleLightWavelengthsandfrequenciesofvisiblelightDyelaserscovertheentirevisiblespectrum.Dyesareresponsibleformuchofthebeautifulcolorsweseeinthevisible.Fluorescentlights“Incandescent”lights(normallightbulbs)lacktheemissionlines.Theeye’sresponsetoLightandColorTheeye’sconeshavethreereceptors,oneforred,anotherforgreen,andathirdforblue.Intermediatecolors,suchasyellowandorange,areperceivedbycomparingrelativeresponsesoftwoormoredifferentreceptors.Theeyeispooratdistinguishingspectra.Becausetheeyeperceivesintermediatecolors,suchasorangeandyellow,bycomparingrelativeresponsesoftwoormoredifferentreceptors,theeyecannotdistinguishbetweenmanyspectra.Thevariousyellowspectrabelowappearthesame(yellow),andthecombinationofredandgreenalsolooksyellow!TheUltravioletTheUVisusuallybrokenupintothreeregions,UVA,UVB,andUVC.Youcangetsunburnedbyallthree.IR,Visible,andUVLightandHumansUVfromthesunTheozonelayerabsorbswavelengthslessthan320nm,andcloudsscatterwhatisn’tabsorbed.ButmuchUVpenetratestheatmosphereanyway.Theveryshort-wavelengthregionsSoftx-rays5nm>l>0.5nmStronglyinteractswithcoreelectronsinmaterialsVacuum-ultraviolet(VUV)180nm>l>50nm

Absorbedby<<1mmofairIonizingtomanymaterialsExtreme-ultraviolet(XUVorEUV)50nm>l>5nmIonizingradiationtoallmaterialsSynchrotronRadiationSynchrotronX-raysourceandusesatLBLEUVAstronomyThesolarcoronaisveryhot(30,000,000degreesK)andsoemitslightintheEUVregion.EUVastronomyrequiressatellitesbecausetheearth’satmosphereishighlyabsorbingatthesewavelengths.Microwavescanbeusedtogeneratex-raysandtreatcancerMicrowavesaccelerateelectronstohighvelocity,whichthenimpactametal.Electronsdisplaceelectronsinthemetal,whichthenemitsx-rays.Thefastertheelectrons,thehigherthex-rayfrequency.X-raysforphoto-lithographyYoucanonlyfocuslighttoaspotsizeofthelightwavelength.Sox-raysarenecessaryforintegrated-circuitapplicationswithstructureasmallfractionofamicron.1keVphotonsfromasynchrotron:2micronlinesoverabaseof0.5micronlines.High-HarmonicGenerationandx-raysgasjetx-raysAmplifiedfemtosecondlaserpulseAnultrashort-pulsex-raybeamcanbegeneratedbyfocusingafemtosecondlaserinagasjetHarmonicorders>300,photonenergy>500eV,observedtodateHHGisahighlynonlinearprocessresultingfromhighlynonharmonicmotionofanelectroninanintensefield.Ionelectronx-rayThestrongfieldsmashestheelectronintothenucleus—ahighlynon-harmonicmotion!Howdoweknowthis?Circularlypolarizedlight(orevenslightlyellipticallypolarizedlight)yieldsnoharmonics!Smashingahigh-intensityultrashortlaserpulseintoagasyieldshighharmonicsinthesoftx-rayregion.Suchshorthigh-intensitypulsessmashtheelectronintothenucleus.Thecollisionresultsinrapiddecelerationoftheelectronandhigh-energy-photonemission.Uptothe299thharmonichasbeengenerated.Atomsabsorbstronglyinthex-rayregionBlackbodyRadiationBlackbodyradiationisemittedfromahotbody.It'sanythingbutblack!Itresultsfromacombinationofspontaneousemission,stimulatedemission,andabsorptionoccurringinamediumatagiventemperat