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Chapter 5 Light matters（第1页）

Chapter5Lightmatters

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。

galltogether

&omsaiclesbehaveindividually，‘doihiheyemitwhentherearemanyofthemisasortofunatedsetofwaves。EvenaverysmallLED，withasizeofmuchlessthanamillimetre，savastoms，sothisisaonsituation。