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光通讯基础.pptx

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Light:6.InterferenceThereisinterferencebetweenlightwavesofbotharms.Iflightwavesofbotharmshavethesamephase,theywillinterfereconstructivelyanddestructivelyiftheyarereverse.1.2BEAMS(RAYS)RefractiveIndexSnell'slaw:θ1=θ3n1sinθ1=n2sinθ2(1.3)1.3ASTREAMOFPHOTONSAPhotonWhathappensifanatomjumpsfromanupperleveltoalowerlevel,say,fromlevelE3tolevelE2?Thereisanenergygapbetweenthesetwolevels,ΔE=E3–E2,andthisdifferencewillbereleasedasaquantumofenergy,whichiscalledaphoton.Aphotonasaparticle,anelementaryparticlethatcarriesaquantumofenergy,EP,andthattravelswiththespeedoflight,cAphoton'senergy,EP,isdefinedasfollows:EP=h(1.4)WherehisPlanck'sconstant(h=6.62610-34J.s)andisthephoton'sfrequencyThehigherthephoton'sfrequency,themoreenergyitcarries.Lightisastreamofphotons.RadiationandAbsorptionAphoton'senergy,EP=handaphotonwascreatedwhenanatomjumpedfromE3toE2andreleasedenergy(E3–E2).Therefore,EP=ΔE=E3–E2(1.5)ButEP=h;hence,h=E3–E2and=(E3–E2)/h.Ontheotherhand,=c/,Therefore,=ch/(E3–E2)(1.6)Conclusion:thewavelength(thecolor)ofradiatedlightisdeterminedbytheenergylevelsoftheradiatingmaterial.Theseenergylevelsaregivenbynatureandwecannotcontrolthem.Butwecanchooseanothermaterialtoachievedifferentcolorsofradiatedlight.Atomswanttoexistatthelowestpossibleenergylevels.Toraisethemtohigherlevels,wemustenergizethemfromanexternalsource.Whenatomsabsorbexternalenergy,theyjumptothehigherenergylevelsandthendroptothelowerlevels,radiatingphotons---thatis,light.Theprocessofmakingatomsjumptothehigherlevelsbyfeedingthemexternalenergyiscalledpumping.Whathappensifanexternalphoton(light)strikesamedium?Ifitsenergy,EP=h,isequaltotheenergygap,ΔE,thephotonwillbeabsorbedbyanatomandtheatomwilljumptotheappropriatehigherlevel.IfEPisnotequaltoΔE,thephotonwillpassbythematerialwithoutinteraction.TheEnergybandtheoryofsemiconductor1.ThevalueofEgdeterminestheconductive(resistive)propertiesofthematerial.Goodconductorshavenogapbetweenthevalenceandconductionbands,goodinsulatorshaveabigenergygap,andsemiconductorshaveagapsomewher