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Hologramsputergeedaalsorothermaterials.Theideaisthatthesurfaceshapemimiterferehereferendobjectwaves，sothatraisedpartsdthtfringesandlowpartstain，illuminatingthiswithareferencewavecausesscatteredlighttoimitatethewavefrontsoftheobject.Thiskindofhologramisusedasasecuritydeviebanknotes（suchastheUK￡20note，ofholographicimagesofthe18thturyScottisheSmithoheyarehardtomakeandrequireadvaneeringtocopy.
Limitstoimaging，redux
Theictureoflightalsoexplaiseeinfinitesimallysmallobjegamicroscope，justasAbberealized.Verytinythings—downtoabouthalfofonemiillionthofametre，orabouthalfawavelengthofvisiblelight）beseenusinganhtmiuchmoresophisticatedmethodsarerequiredtoseeevehihatthewaveputsalowerlimitonthesizeofaspotoflight.
Ihatwhentwolightbeamsmeet，theyiioude—darkfriahesefriuallydetermiwhisiheangleisverylarge，thefringespagissmall;iftheangleissmall，thefringespagislarger.Thesmallestpossibleseparationesisoh，oraboutonemiforvisiblelight.
Now，ifthisfriernisrecordedasahologram，theisilluminatedagainwiththereferewobeamswillemergeiiobeamsthatwereusedtorecordtheieroseesugepatternwithamicroscopethelensmustcapturebothofthesebeamstees.Ifthisdidn’thappehelensly，thentherewouldbenointerfereheimage.
ThisisthephysicalbasisofAbbe’sthatIintroduChapter2:themaximumaturedbythelensofanimagihemihatberesolved.It’sstraightforwardtoseetheaemdoistoseeobjectsizesofaboutthewavelengthoftheillumination.Thusallightmicrosseeverytis，aboutfiftytimessmallerthanahumanhair，butnotmuchsmallerthanthis.Theybeusedtolookatbiologicalcells，butnottheuclei，forinstance.
&ionimaging
Optitistsandengineershavefoundanumberofverycleverwaystogetarousizelimitsposedbyicroscopes，sothattheysidecells，orviewobjectsthataremorethanoimessmallerthahoflight.Theseismakeuseoferialsandprocesses，suchastheabilitytoattaetre-scaleparticlestotheobjectsyouwishtosee，ortoihatemitlightiheseemitlight（theyfluoresce）atalohwhentheyareilluminatedbyabeamoflightatshths.Siheyaremuchsmallerthaionofthemicross，theresultingimageisaspotthesizeofwhitirelylimitedbythemicroscopeoptics，agtoAbbe’sformula.Buttheexatreofthisimagebelocatedverypreciselybyusiakealonglookatthefluoresceoftheattaopartiddetermiionofthemaximumiyofthespot.Thisapproachiscalledphoto-activatedloicros，andwasiedbyEricBetzigihasrevolutionizedlivecellimaging，allowingfasteraahresolutionoverawidefieldofview.
Ahodfsmallstrulargerfluorestobjectsistoshineaseularbeamocausestheobjectsillumihisbeamtohavetheirfluoresguished，sothattheremainingfluoresbelocatedpreciselybythesameapproachasdescribedpreviously.Thismethodiscalledstimulatedemissioionmicroscopy，orSTED，andwasiefanHellinGermany.Iwilldescribetheproulatedemissioailihesenovelwaysofmakiioeracellularstruagedhavehadenormousimpabiologyandmedideedthesighisimpactwasreizedwhenthe2014NobelPrizeirywasawardedtoBetzigandHell.
TheprotifiedbyAbbeworksioo.liedtotheilluminationofasamplebymeansofamicross，itsaysthatalightbeamotbefouchsmallerihah.Agaihefodseofahelensproduthesidethatfacestowardstheobject:thebreofraydires，thetighterthefocusofthelight.
&ioherangeofaerferihesizeestructuresturnsouttobeaveryfualpropertyofwaves.ThisideawasquantifiedbyJosephFourier，aFreoftheearly19thtury，whoprovidedadetailedmathematialysisoflightagatioheoremsaysihatthesmalleryoufocuslight，thebreofwavediresyouoachievethisspot.
Diffra
Thisexplaiureoflightbeams—theygraduallydivergeastheypropagate.Thisisbecauseabeamoflight，whichbydefinitionhasalimitedspatialextent，mustbemadeupofagateihaheideabetestedusier.Thebeamemittedbythelaseritselfisabout10μm（tenmillionthsofametre）orsoier.Whehes，itisabout1mm（athousandthofametre）ier.Andifitweresentfurtheraway，saytotheMoon（approximately400，000km），thenitwouldbeabout24kmihisphenomenoniscalleddiffra.
Diffraeiingapplideterminingtheshapeaructures.Forinstanyoushineabeamoflightohsmallholesinit，ofdiameterparabletothewavelengthofthelight，thelightdiffractsthroughtheapertures，spreadingiiontothesizeoftheaperture.Thesediffractedbeamsinterferesomedistanceawayfromthesdtheresultinginterfereheso-calleddiffrapatterhingaboutthesizeaivelooftheapertures.Foriheyareinaregulararray，theionpatternwillalsularities.Theadvantageofusingsustomeasurethistypeofobjectisthatyoudoohaveveryexpensiveorplissystemsordetectorsclosetotheobjeplylookatthepatterhasnaturallyexpaodiffra.
Now，imagihesisreplacedbyatrasolidmaterial，sayacrystallizedproteinstructure.The‘holes’arereplacedbytheatomsieinmolecule，whichareverysmallindeedaedtooherbybondsiareaboutohofabillionthofametre（0.1h.Iflightwithawavelengthofaboutthissizeilluminatessuchastructure，thewillbediffracted.Theactualstruoleculeitselfbedetermihediffrapattern.ThisisthebasisofX-raydiffra.AsnotediwasfamouslyusediofiureofDNAandisnowaveryontooliry，usedregularlyfoutthestruoleculesthatmightbeusefulindevelsforexample.ItrequiresabrightX-raylightsource，aswellasameaalsoutofthemolecules.Figure20showsadiffrapatternfromacrystalofbovierovirus.
20.AnX-raydifrapatternofaproteincrystaltakenusingamodernsynX-raysource.
It’sclearthatifyouwanttotransmitlightdistahendiffracouldbeaprobleItwillcausetheehelightbeamtospreadout，sothatyouwouldneedabiggeraicalsystemaocaptureallofit.Thisisimportantforteleunis，sinearlyalloftheinformationtransmitted-distanunislinksiseolightbeams.
Guided>
&omaionsothatlong-distanuniispossibleistousewaveguides，suchasopticalfibres.Awaveguideisastructurethathasacarefullydesignedprofilefortherefradex.Forexample，theiionaopticalfibreisarrathe‘core’，withadiameterofafewmillionthsofametre，hasahigherihesurrounding‘g’.Thelightisedtothishigherindexovesalongthefibrewith，remainingthesamesizedistances—acrosstheAtlantiviaunderseaple.Thismeanslightisthebasisofatiationinfrastruunistosensors.
Polarization
Thefinalimportahewavemodelisthecharacteristicofpolarizatiointraheundulationsoadireatrightahedireationoftheortawosuchdiresfortheseundulations.
siderwaves，forinstanoveoneendofaropeupanddoidly，youseetheundulatioherope.Asimilarthinghappensifyoumovetheeandrightequallyrapidly.Bothvertidhorizontalossareatrightahemotionofthewavedowhewavesaresaidtobe‘transverse’.
Lightwavesbehavesimilarly.Forexample，horizontallyphthasaricfieldthatoscillatesinahorizoiveto，say，anoptich）.Similarly，thefieldofaverticallyphtbeamoscillatesiie.（Therearemoreplisofpolarizationtoo，butthesewillsuffiow.）Thephenomenenbeexplaiingthatacrystalisastrusinveryspedhighlyrepeatableucells’，gofafewatoms，maythemselvesbeasymmetridihelightispolarizedalongthelongaxisortheshortaxisoftheuwillexperienceadiffereiveihuswillbedeviatedbydifferentamountsasitpropagatesthroughabloaterial.
&heaysinwhichpolarizationoflightisexploitedisinsunglasses.Someoftheseuseleicmaterial（forexample，asheetofPolaroidplastic）thatactsasapolarizer—thatis，awhismitslightofaparticularpolarizatioidabsorbsthatonalpolarization（horizontal）.Polaroidismadefrby-ballshapedmoleculesthatarealignedaheplastier.Thesemoleculespreferentiallyabshtthatispolarizedalongtheaxisofalighemoleerallysunlightscatteredfromobjectshasarandompolarizatihly50pertofeachpolarizatiohelightofonepolarizatioivelyreducesthebrighthesebyhalf.Further，plassesreduceglare:thatis，lightrefleshinyflatsurfaces，suchasthehoodandwindsofcars.Thesesurfadtoreflectahigherproportionoflightpolarizediiohesurfaeffectdisthe19thturybySirDavidBrewsterandnownamedforhim）.Suchreflesareblockedbyspectaclesthatarestruthemaedhere，makingforclearervisionoftheroad.
Trabirefrierialsalsogethepolarizationoflightwiththelight.Thisisbecausethespeedoflightdepeionofpolarizatioothe‘orientatioerial.Somematerials，suchaslass，havenospetation:yourotatethematerialwithheeffealightbeaAsnotedabove，atentmaterialsarearrangedinsuchawaythatthereisapreferehesymmetryaxis—alongoolight.Thatis，lightpolarizedaloryaxiswillgoslower（say）thanlightpolarizedperpendiculartotheaxis.Now，imagihelightispolarizedat45degreestothesymmetryaxis.Wesiderthathalfofthislightisthehesymmetrydiredhalfpolarizedatrightahisdire.Ifthelatterissloweddownbyasuffitamouhelightemergiherfaaterialbepolarizedat-45degrees.Thusthedireofpolarizatioed’by90degrees.
Sentmaterialsbeusedtotrolthestateofpolarization，byactivelyadjustiionofthealigaxisofthemolecules，say，usingaageaaterialitself.Ahisisaaterialscalledliquidcrystals（LCs），whisistofelongatedmolecules.TheorientationofthemoleanLbetrolledbyapplyingavoltagea.Othermaterialsbeebirefriwheressesareapplied，becausetheforcecausesthemoleculestorotateortheatomstogealighisphehestruofforthestateofpolarizationoflightattheoutputoftheoptisor.
Plagapieceewopolarizersalsoallowstrolofthelightiybymearie.Applyingavoltagereorientsthemolecules，thusgtherefradexseenbyaphtbeaIfapolarizerisplacedaftertheLdependingontheappliedvoltageagreaterorlesseramountoflightwillbetrahroughthefinalpolarizer.Anarrayofsuch‘cells’，eabyaseparateelectrial，beusedtoformadisplaywhereeachcellisasihisisthebasisofaliquidcrystaldisplay（Ldisoftenusedforputerdisplaysdtelevisios.
Infact，suchdisplaysbeusedtoshow3Dmovies.Theillusiohesemoviesderivesfromthestereographyofhumanvision.Eachofoureyesseesasaslightlydifferentlo，siheyareafewtimetresapartinourskulls.Thetwoimagesarebinediniveusaperofdepth.
Thisillusionisreprodug3Dglassesbymeansofpolarization.Trojetothedisplayors.Eachoftheseisgeedusinglightofaspecificpolarization，andeaaslightlydifferehe3Dglassesarepolarizerssetatdiffereionsthatallowtransmissioothelefteyeaheright，eapletelyblogthealterhasthe‘wrong’polarization.Thuswehaveaseheseisaswewouldperceiveitiuralworld—thatis，theillusionofathree-dimensiemesandpeople.
Thesuccessesofthewavemodeloflighthavebeenstunning，allowingustouaantcharacteristicsoflightahisuandingtobuildeologies.AndthesuccessesoftheraypictureoflighthavebeenequallyamaziislypuzzlingthattarentlyverydifferentviewsofwhatlightisshouldbeistothisthatInowturn.
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