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Chapter 5 Light matters（第1页）

Chapter5Lightmatters

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Howdoyht?Theremaways:onlightbulbs，basedonglowialfilaments，aswellas‘fluorest’tubes;laserpoialelightsoricalequipment，fromtoasterstocardashboards;sunlightandstarlight;even，forthoseihernaremitiesoftheplaheaurorae;firefliesandglohosphoresthewakeofboats.Whatisthemeansbywhichtheseverydifferentthihesamething:light?

&hattheyallier.Morespecifically，theyiricchargesmovingabout.Whenthesechargesaccelerate—thatis，whentheygetheirspeedortheirdireotion—thenasimplelawofphysicsisthattheyemitlight.Uandingthiswasoatsofthetheoryism.Aricfieldhasitsiriccharge，sueleanatom.Theelectricfield，attragoppositelychargedparticles，suchasprotohroughoutallspace，althoughitgetsweakerquitequioveawayfromtheele.AsInotedihisistheforcethatarisesfromstaticelectricity.

Osgatomsandberons

Nowsaytheeleoveswithasuddehefieldsurroundingitmustmovealso，siisliricablytotheele.ThethefieldisillustratedinFigure25，asa‘kionwillheeleovedohethefieldthatarisesfromtheeleenthashadtimetomakeitswaytotheproton.Thiswilltaketimesihekink，aheinformatioronhasmatesataboutthespeedoflight.Whenthegedoesgettotheproton，itwillmtowhethertheeleovedclosertoit（thusmakingthefieldexperieheprer，sothatitfeelsastroheraway（leadingtoaweakerfieldandaweakerforce）.

Nowsaytheeleweretomovebadforth.Thengeswouldothefieldsurroundingitinsynywiththisosandpropagateawaytotheproton，whichwouldthenbeioosturhisinformation.Butanosgelectricfield（andassociatedmaghatherushere）isexactlywhatwemeanbylight.

25.Liricfieldofa.astatioronandb.airoheeleaccelerates，thethefield—the‘kinks’inthefieldlines—propagateawayfromtheeleatthespeedoflight.

Sin—thesimplestosofasironandasion，wederstandfromthispiseratelight.First，let’sthinkabouteirradiatesanatomsittisowforcesbedparticles—theeledtheprotoomtomove.Butsironismuchlighterthaon，itmovesmoreeasilyplioffordwesideritsmotioothemoreorlessstationaryproton.Infact，theeleoscillatesatthefreque’selectricfield，beielyacceleratedordeceleratedastheelectricfieldvaries.

Thisprocessisabitlikepushingaaswiogettheswingtooscillateistopushonitinsynywiththenaturalosperiod—agentlepusheachtimetheswitomofitsmotiotakeseffetthechilduptoaheightthatthrillshimorher.Thechildexperiencesmaximumacattheapicesoftheswing’sosaximumspeedasshepassesuofsuspension.Soitiswiththeatomis—theehelightbeamisabsorbedbytheatom，aothemotiorons.

&’ssayyounowstoppushingonthesens?Thechildgraduallyswingsinarcsoflesseraudetain，soitiswiththeatom.Theelesgraduallystoposg，aheirmotiybyreradiatinglight.Thisistheprocessoflightemission，andisthebasisofmanylightsources，susiglights，aers.

NowinthispictureIhaveassumedthatsomelightbeamitselfisusedinitiallytogettheatomisosg.Butibegsthequestionofhowhtiplafactitispossibletouseothermeaheatoms.First，onesimplyheatupthematerial.Thisiswhatisdoneinanhtbulb，wherebypassiritthroughametalfilamesupthtemperature—severalthousahematerialheatsup，theelesstarttojostleaheatomsandwithohermoreahisbothexcitestheatomsaheelestoaccelerateaerapidly.Thisproducesaverybreofcoloursthatdependsouretowhichthematerialisheated，andnotupoiculartypeofatomsofwhichitismade.

&ymayalsobeusedinotherways.Iingdiodes（LEDs）forexample，asusedindisplays，arit—orflowofelesthroughthematerial—becaptureddireatom，sothelightisgeedmuchmoreeffitlythaninathermalsource.Afluoresttubealsousesarittodirectlyexs，onlythistimeinanebulousgaswithwhichthetubeisfilled.Finally，machemicalicalreareleaseenergy，someofwhichleavestheatomsormolethefht.Thisistheigivenoffbyfireflies，forexample.

&edpreviouslythatachastwopartstoit:itageofspeed，aswehavejustseeronandprotoninahydrogenatom，oritageofdirewithoutaspeed.Thislattercharacteristicofailiarfromtheexperiengaroundacar:youarepushedagainstthedoororthesideoftheseatandexperienceaforcethatturnsyouwiththecar.Theforceislargerthefasteryasyoueurn，andthisisanindithatyouareag，eventhoughyoumaynotbespeedingupdown.

&hiskiion，whenexperiencedbychargedparticles，causesthemtoradiatelight.Imagineaburoomoveiheywerestuwheel.Theygehewavelengthofwhichgetsshorterahephetslargerahefastertheymovearou，becauseofthisangularac.Thelightgehiswayisradiation，anditisaeaeX-rays.Itisalsorelatedtothelightseehernandsouthernaurorae，roduchargedpartitheSuospiralbytheEarth’smagheyemosphere.

Quantumlightemissionprocesses

Thesebasiismsunderpinalllightsources.Butthedetailsofhowtheatomsactasagrouphaveastrohecharacteristicsofthelightthatiseveed—asInotedinChapter1，alightbulbemitsaverydifferentshtthaer，forinstanordertouhis，wetledeeperiureofatoms，siheproittinglightfromanatomiselyeheanalogywithswihiobeaddedioaodatethefactthatatomsandmoleculesarequantummeitities.

Forourpurposethissimplymeansthattheeletheatomormoleonlyhyis.Usingmodel，itmeansthatthemaximumextensionoftheslitudeotbeanythingyoulike.Rather，itisrestrictedtoparticularvalues:itisquantized.Moreparticularly，theeheswingesis，orquanta，andwhenyoupushityouakeitjumpbyoa.Ihismeaheelelyieuabsorbsoremitsasiheenergiesinvolvedinmakingthesejumpsareverysmallbyeverydaystandards.Takealightinyourhouse.Thisesearateof，say，60W，or60Joulespersed.AsioheatomsibulbpossessesaboutonebillionbillionthofaJoule.Soalightbulbisemittienbillionbillionphotonspersed.

26.Anata.absorption，b.spontaneousemission，andc.stimulatedemission.

Anatombepreparediatesimplybyshininglighthtfrequenit，asshowninFigure26a.（Ofcourse，thisbegsthequestion，butwe’vealsoseeionplishedinotherways，forinstaniritthroughthemedium.）Ntoquantumtheoryacorollaryofthe‘jumpiheeleotionisthattheatomisareprettymuchstableagainsttheemissionoflightohesediscretestates.Theyarelikeaballonashelfinacupboard—inpriloweritsenergybydroppingtoalowershelf，butthisothappeninpralessyougiveitalittlepushsothatitrollsofftheshelf.

Soitearthatquantumphysicssuggeststhatatomsitlight，sinceyouhaveputthemiablestates—that’sit.Well，itturnsoutthat，exceptforthelystateoftheeletheatom，apushisavailabletoallowtheeletodropfreticstatetoalessee.Andthesurprisingthingisthattheshovearisesfromnothing.

InChapter4，Ihestrauresofquantumphysitheemptinessofspaceisaseethingbadofactivity，filledwith‘vacuumflus’.Theseflusiromagicfieldcausetheatomitodroptylevelaheenergydiffereheformofemittedlight.Thisprocess，bywhioledergoesatransitioablestate（theexcitedstate）toanothery（thegrouingaphotonontheontaneousemissioissomethiomdoesonitsown.ItwasinallyproposedbyEinsteioatfortheprybalabeamaeronwhichitwasshining.Ifspontaneousemissiootheatomsholdontyfromthelightbeamaionweseeeverywherearouhiablestateinequilibriumwiththeirsurroundings，wouldbeimpossible.

&eihetralmysteryofspohatitrocess.Youjustotsayexaaom.Allyousayisthate，aftersomeamoudepeis，butisroughly1，000billionthofasealarges，abouttwo-thirdsofthemwillhaveemittedaphoton.Buttheinofthefualrandomnessremaieryuntil，in1927，PaulDiratumfieldtheoryiheideathatquantumvacuumfluswereattherootofthis.Thenotionthatafieldgnophotonsatallextobeuoddswithourintuition，andittookuntilthe1950sforLamb’smeasurementstoshowthatDiraatiht.

Whatthismeansisthateveneverydayoces—thepieratedonaTVsbymeansofLEDs，forexample—haveattheirheartthisfuallyrandomcharacteristigfromquantummeitrast，theemissionoflightbyatomswheogiveuptheireheapplihtfieldiscalledstimulatedemissiohisfenergyfromatomsifielddoesnothaverandomcharadthismakespossibleaverydifferentkindoflight:thatofthelaser.

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