马文蔚,英语讲课ppt,10-BaseofQuantumtheory(2009V1)

Chapter10BaseofQuantumtheory-----HuangQiongMichelson-Morleyexperiment

Blackbodyradiation

PhotoelectricEffectAtomicSpectrumofHydrogen

ComptonEffect

QuantummechanicSpecialRelativityDarkCloudsinStretchesofBlueSky(1899)§1BlackbodyRadiation

1.BlackbodyRadiationThebodythatonlyabsorbsincidentelectromagneticwaveanddoesn’treflectiscalledtheblackbody.Theblackbodyisanidealmodel.thesmallholeisjustapproximatelyablackbody.DevicetoMeasuretheDistributionofEnergyinBlackbodyRadiation:acavitywithsmallhole,thetemperatureinthecavityiscontrolled.lensethelightbeamradiatedfromtheholeaparallellighttubeadisperseprismathermocouplemonochromaticlightTwoFeaturesoftheDistributionIntensity:themonochromaticenergyradiatedfromunitareaonthesurfaceoftheblackbodyforunittime.IntensityTheamountofradiationemitted(theareaunderacurve)increases(Stefan’slaw)Thepeakinthedistributionshiftstowardshorterwavelengths.(Wien’slaw)IntensityTheenergyofdifferentwavelengthsradiatedfromunitareaonthesurfaceoftheblackbodyforunittimeconformstotheequationinthefollowing:Stefan-BoltzmannlawIfrepresentsthewavelengthcorrespondingtothegreatestmonochromaticenergyfluxdensityIntensityWienDisplacementLawWiththeincreaseoftemperaturethevalueofmovesleft.Whatistheexpressionequationconformingtothefollowfigure?Thisisahotspot.0123456(μm)1700K1500K1300K1100KAnyattemptstouseclassicalideastoexplaintheshapesofthecurveisfailed.ExperimentaldataWienformula

Rayleigh-Jeansformula

UltravioletcatastropheinfiniteenergyPlanckFormulaAfterrepeatedthoughtPlanckbelievesthat,thekeyofsolvingthatquestionistorecognizetheenergyis

quantized.Wherenisapositiveintegercalledaquantumnumber,hisPlanck’sconstant.ThekeypointofPlanck’stheoryistheradicalassumptionofquantizedenergystates.Energy01234n0PlanckFormulaofBlackbodyRadiation

ForlowerfrequencythereisRayleigh-JeansformulaForhigherfrequencythereisWienformulaExperimentdataRayleigh-JeansformulaWienformulaT=1646kPlanck’stheoreticaldataAtallwavelengthsPlanckequationforthewavelengthdistributionthatisincompleteagreementwiththeexperimentalresults.Planckintroducedtheconceptofa“quantumofaction”inanattempttoexplainthespectraldistributionofblackbodyradiation,whichlaidthefoundationsforquantumtheory.In1918,hewasawardedtheNobelPrizeinphysicsforthediscoveryofthequantizednatureofenergy.(1858-1947)

Theabsorbedorradiatesenergyis

quantized.Thisdevelopmentmarkedthebirthofthequantumtheory.WhenPlanckpresentedhistheory,mostscientist(includePlanck!)didnotconsiderthequantumconceptrealistic.Itwasfelttobeamathematictrickthathappentopredictthecorrectresults.

Subsequentdevelopments,however,showedthatatheorybasedonthequantumconcept(ratherthanonclassicalconcepts)wasrequiredanumberofotherphenomenaattheatomlevel.在十年之后，1915年，不相信光量子的米立肯(1868—1953年)宣布他的实验无歧义地证实了爱因斯坦的光电效应理论和1922年康普顿(1892—1962年)发现Ｘ射线散射效应必须由光量子论解释之后，人们才正确评价了光量子论，宣布爱因斯坦由于“在理论物理学方面的成就，特别是光电效应定律的发现”而授予他1921年度的诺贝尔物理学奖。爱因斯坦和普朗克不同，当时就坚信自己的光量子论是“非常革命的”。的确，光量子论并不是简单地复活光微粒说，而是揭示了光的波粒二象性。对统计平均现象光表现为波动，对瞬时涨落现象光表现为粒子。光量子论第一次确认了光的波粒二象性这个最基本的性质。

1.PhotoelectricEffectmetalExperimentsshowthatlightincidentoncertainmetallicsurfacescauseselectronstobeemittedfromthesurfaces.Thisphenomenoniscalledthephotoelectriceffect,andtheelectroniscalledthephotoelectron.§2PhotoelectricEffectOOOOOOOOLightASchematicDiagramofaPhotoelectricEffectApparatus.Whentheevacuatedglasstubeiskeptindark,theammeterreadszero,indicatingthatnocurrentisflowinginthecircuit.WhenlightoftheappropriatewavelengthshinesonplateE,however,acurrentisdetectedbytheammeter,indicatingaflowofchargesacrossthegapbetweenAandK.HighintensityLowintensitycurrentStoppingpotentialAppliedpotentialdifferencePhotoelectriccurrentversusappliedpotentialdifferencefortwolightintensity.Thecurrentincreaseswithintensitybutreachesasaturationlevelforlargevalues.2.Experimentallawofphotoelectriceffectanddifficultyofclassicaltheory

Independenceofphotoelectronkineticenergyonlightintensity.HighintensityLowintensitycurrentStoppingpotentialThisisshowninfigurebythefactthatbothcurvesfalltozeroatthesamenegativevoltage.Accordingtoclassicalprediction,electronsshouldabsorbenergycontinuouslyfromtheelectromagneticwaves.Amoreintensitylightshouldtransfermorekineticenergytotheelectrons,sothestoppingpotentialshouldbelarger.HighintensityLowintensityCurrentStoppingpotentialSupposetheinitialkineticenergyofphotoelectronescapedfromtheplateKisThelowestvoltagetomakethecurrentdropsjusttozerobetweentwopolesisiscalledtherestrictedvoltage,whichmakesthephotoelectronescapedfromthepoleKcouldnotjustarriveatthepoleL.restrictedvoltage(Stoppingpotential)v0CSKCUv01v02v03frequencyStoppingpotential

Dependenceofphotoelectronkineticenergyonlightfrequency.

Theinitialkineticenergyofphotoelectronisrelatedwiththefrequencyofincidentlightinsteadoftheintensity.Accordingtotheclassicaltheory,thegreaterintensityofincidentlightis,thegreaterinitialkineticenergyofphotoelectron.Theinitialkineticenergyofphotoelectronshouldberelatedwiththeintensityofincidentlightinsteadofthefrequency.

DependenceofejectionofelectronsonlightfrequencyAccordingtotheclassicaltheory,nomatterhowmanythefrequencyofincidentlightis,ifonlytheintensityisgreatenough,theelectroncanobtainenoughenergytoescapefromthemetal,existingnothecutofffrequency.Foronekindofmetalthereexistslowestfrequency,whichiscalledthecutofffrequency.Ifthefrequencyofincidentlightissmallerthanthecutofffrequency,nomatterhowmanytheintensityofincidentlightis,thephotoelectriceffectdoesn’tappear.

>0

TimebetweenincidenceoflightandejectionofphotoelectronsIfonlythefrequencyofincidentlightisgreaterthanthecutofffrequency,photoelectronsescapedrapidly,thehysteretictimedoesn’texist.Accordingtotheclassicaltheory,fromthestartofshiningtopoleKtotheescapeofphotoelectronsthereshouldexistthelongerorshorterhysteretictimeinwhichtheelectronabsorbsenergyofincidentlight.t(s)0

10-9I3.EinsteinPhotoelectronTheory(in1905)EinsteinextendedPlanck’sconceptofquantizationtoelectromagneticwaves.Heassumedthatlightoffrequencycanbeconsideredtobeastreamofquantawhichiscalledphotons.EachofthephotonshasanenergyEgivenbythefollowingequations:E=hEachphotonmoveswithavelocityC.EinsteinPhotoelectricEquationtheinitialkineticenergytheescapeworkofthemeta

Dependenceofejectionofelectronsonlightfrequency

Thecutofffrequencyis

Dependenceofphotoelectronkineticenergyonlightfrequency.v0CSKCUEinsteinreceivedthe1921NobelPrizeinPhysics"forhisservicestoTheoreticalPhysics,andespeciallyforhisdiscoveryofthelawofthephotoelectriceffect.§3ComptonEffect

AmericanphysicistCompton(A.H.Compton,1892-1962)startedtoresearchXray(10-10m)athimselfyouth.1896,RoentgenHediscoveredinexperimentthat,afterthroughthegraphitethewavelengthofsomescatteredwavesdoesn’tchange,andthewavelengthofanotherscatteredwavesincreases,theincrementofwavelengthincreaseswiththeincreaseofscatteringangle.ThisiscalledComptoneffect.AfterexplorationformanyyearshepublishedthearticleinMay1923,whichclaimsthatexperimentalphenomenoncouldbeexplainedonlybyuseofthequantumtheory.X-raysourcediaphragmCarbontargetIonizationchamber........................................................................................(a)(b)(c)(d)0.7000.7501.Twopeaks:0X-rayYXYXIftheelectroncollidedwiththephotonisinexternallayerofatom,sincetheboundactiontotheelectroninexternallayerisweaker,theelectronmaybeconsideredasafreeelectron.Sothemomentumofthephotonischangedafterthecollision,namelythewavelengthoflightisincreased.Butiftheelectronisininternallayerofatom,sincetheboundactiontotheelectronininternallayerisstranger,thecollisionofthephotonwiththeelectronmatchesuptothecollisionwithwholeatom,andthemassofatomisgreaterfarthanthemassofphoton,sothemomentumofthephotondoesn’tchangeafterthecollision,namelythewavelengthoflightdoesn’tchange.Thus,byuseoftheenergyconservationlawandthemomentumconservationlawexplainedperfectlyComptonEffect.Namely,theenergyconservationlawandthemomentumconservationlawarebothstricttenableinmicroscopicworld.Comptonwavelengthofelectron:Comptonshiftequation:ÅComptoneffectisonlydetectableforwavelengthsthatareveryshort.§4BohrTheoryofHydrogenAtom1.AtomicSpectrumofHydrogenTheatomicspectrumofhydrogenisnotcontinuous.Rydbergformulareflectsreallytheatomicspectrumlawofhydrogen.Rydbergformula:AtomicSpectrumofHydrogen6562.794861.334340.474101.741215.681025.83972.5418.7540.50Lymanseries

Balmerseries

Paschenseries

Brackettseries

continuousn=4n=3n=2n=1r=a1r=4a1r=9a1r=16a1Lymanseries

BalmerseriesPaschenseries

Theorbit,velocity,energyofelectronarequantized.Niels

HendrikDavidBohr(1885~1962)receivedthe1922NobelPrizeinPhysics.

2.BohrAtomicTheoryofHydrogenBohrguessesthatorbitofelectronoutofnucleusisnotarbitrary,andshouldsatisfycertaincondition.AccordingtothistrainofthoughtBohrputforwardtwosuppositionsin1913.Theorbitofelectronoutofnucleusisnotarbitrar