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Chapter3Waves
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Chapter2exploredtheideaoflightasgoffuicles，movingalorajectories.This‘billiardball’model，inwhichalightbeamisaofindividual，padwell-localizedbuandsintrasttothealternativeview，whichisthatlightisawave.Thisoflighthasbeenpursuediheparticleviewoflight，althoughittookmanyyearsofdisdexperimentationfortheiefullyaccepted.
Unexplainedphenomena?
Otherobservations，suadebyFranaldiihtury，graduallyaccruedevidedidnotfitwiththepartialdisawthatlightraysdeviatedfrhtliheypassedthroughsmallapertures，suchasatinyholeiedthatthelightwasdiffusedandthattheedgesofthebeamwerefrihcolour，especiallypronounallobjectssuchasahairorapieceofgauze.Hecludedthatthestriationsseewasionasmallornarrowobjectwereevidehelighthadbeefrinalpathasitpassedtheedgesoftheseobjects.Iflightreallysistedofpartigalhtrays，suchsolidobjectswouldsurelyjustcastashadow，andparticlestodeviateintepatterns.
Further，theproblemtonandhisporariesofthebizarrewayinwhichlightwasrefractedthroughmaterials，notablycrystalssuchascalaturallymineral—dedexplanationintermsofsimplepartiexampleofthisbehaviourisshownihewHT，writteofpaper，isilluminatedbyalightbulb.Twopiecesofcalcitehavebeenplacedovereachhalf.IhalfofFigure15（a），twoimagesofthelacedwithrespecttoeachother.Ihalf，thetwoimagesaredisplatheoppositedire.Theleihalfandtheupperiarejustasofromordiionoflightreflethepaper，seenthroughthecrystal.Buttheseagesappearstoariseasiffromadiffereiveindex.Byplagapolarizeroverthecrystal，asshowninFigure15（b）and15（c），itispossibletoisolateimagesformedbytwodiffereionsht.Eachpolarizationexperiencesadiffereiveihisisthephenomenence.
15.AnillustratieheimageHTviewedthroughapairofcalcitecrystals，usinga.u，b.verticallypht，andtallypht.
&ionsallpoiuresoflightthatdemaion.Theyarekivelyasinterfereion，andpolarization.Wewillexplorethesephehischapter，developingapictureoflightaswavemotion.
&handwavefrequency
Whatarethecharacteristiotion?Wavesareaformofundulationassociatedwithamedium，forexamplewaterwavesonthesurfaceofapond.Thesewavessistoftheup-and-downmotionofwatermoleculesattheiweentheliquidahehighestasofthismotiohepeaksandtroughsofthehilethewaveitselfmovesacrossthesurface—thatis，atrightahemotioermolecules.Forthisreasonitiscalledatrasspeeddepehofthewater，ams.
Thecircularsurfacewavesthatradiateawayfromthepointatwhieistothewater，asshowninFigure16a，areafamiliareffect.Thedistaweensuccessivepeaksiscalledthewavelength（Figure16b），awhichpeakshittheshoreiscalledthefrequency（Figure16c）.Theproductofthesetwoquahespeedofthe>
Thepuzzleformaurieswaswhatkindofundulationsstitutedlight.Itwasassumedthatsomemediumwouldiosupportawave.Ahespeedoflightisse，thiswouldobeaverystiffmediuButastiffmediumwouldsurelymakeitdifficultforotherbodiesthit.Forinstance，becauseweseedistantstars，thereshouldsurelybesomemediumtosuppationoflightbetweehaar.Sihmovesarouheplawouldbeuallysweptbya‘wind’asittravelledthroughsuchamediuThisenigmaticmediumwascalledthe‘aether’，anditwasnotuhe19thturythatitwasfinallydiscardedasausefulcept.
16.Acircularwaveonawatersurfatoursofequalheight，calledwavefronts，b.waveheightvsdistantreatapartistant，c.waveheightvstimeataparticularlo.
So，whatsortofwaveislight?Thiswasalsofinallyahe19thturybyJamesClerkMaxedthatitisanosofanewkiheeleaghisfieldiseffectivelyaforcethataelectricdmagerials.Forexample，acledwithstaticelectricityartiagwillbeattractedtothederator.Iercaseyoufeelthisforceasyouplacethemaghedowardsthedoorunlessanopposingforceissupplied.
Inbothcases，aforcepullsoheother，sothatatsomedistanceawayfromthecloththedustparticlefeelsthisforceduetotheelectricfieldgehethedsimilarlyferatordoor，duetothemagicfieldgehemagheearly19thtury，MichaelFaradayhadshowntheesbetweerieticfields.Maxwellbroughtthemtogether，astheeleagicforcefield.Itturnsoutthatinthewavemodel，lightsideredasveryhighfrequencyossoftheeleagicfield.Oneceofthisideaisthatmoviricchargeseratelightwaves.I’lldiscussthis，ahlight，inChapter5.
Interference
Iftwostonesaredroppediooheresultisofexpandingcircularwavescollide.Someofthepeaksaretherefher，wherethewavesreiher.Buttherearealsoliwater，wherethereisnoup-and-dowhewatersurfathoughthewavesgehstourbaerarepassingthroughallpointsaloheloofsuesisshowninFigure17，alongwiththewavefronts—thelocusofpointsohewavesemanatingfromeachsource.
17.a.Twcircularwavesonawatersurface.Thedashedlioursofequalphase.b.struterferehsofequallength.c.Destruterferehsdiferingbyhalfawavelength.
Thisphenomenonisknownasinterferearisesfromtheadditioudeofthehepeaksofthetwowavescide，theresultiwicethesize，oramplitude，ofeitherwave.Thewavesaresaidtobeiheinterfereructive.ThisisshowninthemiddlediagramofFigure17.If，however，thewavesareexactlyoutofphase，sothatthepeaksofonecidewiththetroughsoftheother，thelitudeofzero:thewaveshave‘eaerfereddestructively.Thisisshowure17.Itisimmediatelyclearthatsuenonothappenwithparticles，forhowcouldtwopartiother?
ItwastheobservationofinterferesinafamousexperimentbyThomasYoungin1803thatreallyputtheictureoflightastheleadingdidateasaioureoflight.Youwasverysimple，a.Hetookadleasalightsourdplaceditbehindaswhichthereweretwoholesaverysmalldistahelightshiningthroughtheseholescouldbeseenonasedsedistanow，ifonlyasingleholewasused—saybygupthesethelightmadeasionthes.Whenbothholeswereopehingmarvelloushappeeadofjustaspotoftwicethebrightnowhadstriations.Thesearegeraightliensityperpendiculartothelinejoireofthetwospots，asshowniioninFigure18arisingfromtheiheassingthroughthetwoholes.Theyarecalled‘Yes’，ahekeypiecesofevidencefhtasawavemotion.
Howdoesinterfereheces’observedbyoionsfromtwosurfacesveryclosetogether?Whatiserfereooccuristwowaves，therelativephaseofwhich（thatis，therelativepositionsofthepeaksofthetwowaves）beadjusted.Io，interferenceoccursbecauseasibeamoflightispartiallyreflectedatthetwosurfadthussplitintotwowaves.Ifthedistawosurfacesisequaltoasihofthelight，thehetworeflectedwavesdthereisabrightfringe.If，however，thedistahalfofthewavelehepeakofohthetroughofthesedthereisa‘darkfriodestruterferehus，whenyoulookatthesurfacethebrightanddarkfrithetoursoftheseparationofthesurfacewitharesolutiohah.Fht，withawavelengthofapproximatelyone-halfofamillionthofametre，or500ioermihan250nm—apreofabout140thofthediameterofahumanhair.
18.ThomasYouhelightfromoproducesasmoothiterhslitsarees’appear，characteristicofwave-likebehaviour.
Ofcourse，foradifferehebrightanddarkfringeswilloadifferethesurfaceshesofcolourwhenillumihwhitelight.Itisinterferewavesthatcausestheathinfilmofoilfloatingon>
Iransformsverysmalldistaheorderofthewavelengthoflight，intlightiy—fromnolighttofourtimesasbrightastheindividualtwaves.Sugesiyareeasytodetectorsee，andthusinterferenceisaverygoodwaytomeasuresmalldisplathescaleofthewavelengthoflight.Manyoptisorsarebasedoninterferes.
Holography
Interfereneansbywhiaketrue3Dimages，thatis，imagesthatbeviewedfrlesandrevealdiffereheobject.Thesearediffereicimagesinso-called3Dfilms，andares.Hologramsaremadebyregthefullwavefhtscatteredfromahesortof2Dimageswe’reusedtraphyeheamplitudeofthewaves.Thephaseinformationislost.Thisisbecausedetelyrespondtoamplitude，sothereisnowayisofimageswehavelookedatpreviouslytoextractphase.isthephaseofthewavefrontsscatteredfromaesshape.
Whatinterfereheengofphaseiy，sothatphotisterpatternsinwhichthefullamplitudeaiowaveisrecorded.Theprincipleisshownihewavescatteredfromtheobjeterfereswithareferehasawell-keedbyalaser.Theiernisreadetelight-seerials:thisisthehologramitself，iedbyDenisGaborinthemiddlepartofthe20thtury.
19.Ahologramisstructedbyregtheinterfereweenareferencebeamoflightaeredfromtheobject.
&heimageisalittlemorepliwithanraph.First，areferencebeamillumihehologramaslightisscatteredfromthepatterhematerial.Thesescatteredbeamshavetheremarkablepropertythattheyreproducethebeamsscatteredfrihatasyoureyesreceivethem，itappearsthattheobjectisrestrufrontofyou.Movingaroundtheimagerevealsdiffereheobject，becausethebeamsscatteredfromthosedifferentpartsencodedifferentinformation.
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