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Thesearealltaiesthatsuggestlightisaphysitity.Butwhat，exactly，isit?
Wemaysiderwhatitmeansfhtthtbytakingaparticularsht-ahouseholdlightbulb.Theseeinvarioussizes，butallhavepowersofseveraltensofWatts（theunitinwhieasured，labelledW，andsignifyingtheenergyedpersedofoperation）.A50Wlightbulbgivessuitlightbywhichtoseethingsinsideahhtsaretypicallyofslightlygreaterporoximatelybetween60Wand100W.Thefloodlightsatafootballgroundareofmuchlargerpower，uptoseveralthousandWatts.Iwilldiscusslaterexactlyhowthesedifereelight，butthesepiveasehedinglighting.OfehtestsourcesistheSun.Ithasamassivepoweroutput，morethan1025W（1with25zeroesafterit），whichmakesitimpossibletolookatdirethoughitisaverygreatdistancea>
Thisbringsustotheassociatedwithhhtalightis.Lightofthekinddiscussedabovelooksdimmerthemoredistantitis.Sopoweraloheoerminingbrightisrelatedihefraofthepowerthatwerethelightsourple，alaserpoiypicallyhasmuch，muchlowerpoweroutputthanalightbulb，oftenohousandthsofaWatt（10-2Wor10mW）.Yetitappearsverybrightwhenitispoias.
ortaheiyofthelightgehesource-thepowerperuhereceiver.（Thisismoreproperlytheirradiaensityisperhapsamorefamiliarter）Theiyofalightsourceisrelatedtotheabilitytotratethelight.Alaserpoihtbecauseitslightbeamistratedonasmallspotonthes，whereastheSun’slightisdifusedoveravery>
&houghtheSupoweroutput，thelightitproduotasiofalaserpointer.
Theunderlyidescribesourabilitythtiscalledthesource.Thisisrelatedtothesource’spropensitytosendlightinaparticulardire.ForiheSunandalightbulbradiatelightintoalldires-theSunbeseenfromeveryplaEarth，andalightbulbfromwhereveryoustandinarooButalaserpointeronlyputsoutlightiion-thatinwhichyoupointit.Soyouotseethelaserbeamunlessyoulookatthesurfawhichitisi.Becauseofthepropertythatitsbeamhasawell-defihelaserpointerissaidtobeatlightsource，whereasthelightbulbisasource.
Ancharacteristicoflight，perhapsitsmostevidey，iscolour.Theraihefualideaofaspectrumofcolours-apaletteemergingfromtheofrainandsunlight-fromblueatooredattheother.Atralstrandioftheoriesoflighthasbeeofmodelsofcolourvision.Colourisioper，ashysiillustrationofthisisfoundisoureofdertakenbySirIsaa（Figure3），adominantfigureofearly18th-turysdwhosebookOptiedtheviewoflightfortwoturies，andbyJohannWolfgahe（Figure3），adominae18th-turyliterature，whoiifihiswritings，buthelessbelievedobeprofaboutthe.
&partofon’sfamousexperiment（similartooakeesandotherspreviouslytohim）wastoallowasmallbeamoflightfromtheSun，definedbyaholeinadarks，thaprism，aofallupohefamiliarrainbe.Goethewasfasatedbythisefedborrowedsomeprismsfromalocalaristocratwithwhientforhimself.Hecludedquicklythatwaswrohathisclaimoftheuyofcolourdissewhitelightwashehadhimselfdiscoveredaverydifereofcolours.
3.Isaa（left），JohannWolfgahe（middle），andRosalindFranklin（right）.
&Goetheuookwasththeprismatthemullionofawindow.Thatis，helookedatadarkliabrightbad，theveryoppositeofwhatohesawwasaverydifererumfromthered，green，aheoratheraeof，magenta，aheso-entarycoloursoftheorubion’scoloursproducesawhiteimage-bihe’sproducesablae.
&heespousedtheideathatcoloursarethethingsperewtohemasintriiesoflight.Theywerebht.Nowadayswearettoseparatethephysicalattributeofitsphysiologicalefects，thesensationofcolour.Weeachreatlytodeed，chtbeusedasatherapy.Fromanartistitofview，theihatouressrealizesfromthesensationsassociatedwithlightofaparticularcolourisacriticalmatter-perisvitallyimportafromaphysitofviewthereisanunderlyiwetothelabel‘biguously:itsfrequeilwegetintotherealmofquantumlight.
Thereachoflightextehevisiblespectrum，attheblueendintotheinvisiblerealmoftheultravioletaremeultraviolettoX-raysandγ-rays.Attheotherlietheinfrared，microwaves，radiowaves，a-rays（Figure4）.Toseethese，werumentsthanoureyesaloheless，weknowthatlightofthesecoloursexists.ForihewarmthoftheSuhtthatisabsorbedbyourskin.Atlowerfrequenciesmicrowavesareusedforcellularphoneunisaswellasf，byheatierinthefoodbeiheinvisiblecoloursatshthsarealsofamiliar.Sunburravioletlight，whileX-raysareusedroutinelyinmedig.
X-raysarealsousedinmanynon-medicalapplis.Forexample，thepatternsmadewhenX-rayssaregulararraomsinamoleculeorsolidearraobedetermihoughtheatomsarespacedvery，veryclosetogether，atdistahan10，000timessmallerthanahumanhair.X-raydifraagesrevealtheverystruoleculeorsolid，withprofoundimplis.PerhapsthemostfamousexampleisthestruoletifiedbyJamesWatsonandFrancisCrickmorethanhalfaturyago，basedestakenbyRosalindFranklin（Figure3）andMauris.Knowinghowmoleculesreplicaterevolutionizedbiomedie.
&ioiveoftheimporta-initsbroadestsense-inmakingpossibleourmodernactourabilitytoenjoyittoitsfull.Theyrestonthefualworkofanumberofstistsihtury-MichaelFaraday，
4.Thespectrumic>
HansOersted，André-MarieAmpère，CharlesAugustindeb，AlessandroVOhm，JamesClerkMaxwell，andHeithereexistsabetweeandotherapparentlydisjointthingslikemicrowavesandX-raysisremarkable，anditwasthetriumphofstifiquirybythesemenandotherswhomadeaheses.
&ofcolours，orspectrum，providesatoolforartandsce.aiistexploreswaysinwhiselvesarejuxtaposedorbiroscopistexploreswaysinondstodiferentcolours.Forexample，ihturyJosephvonFrauermiypesofatomsthatarepresentintheSunbylookitheparticularchtthattheSus.HecharacteristiccfromtheSurumahesecerprints’forpartis.Thestudyofspectraisthedomairoscopy，whichuseslighttoideomsormolecules.Itisavitalresearchactivitytoday，withimpayareasfromhealthmtoremotesensimosphereforpollutants.
Apartfromthesefamiliarpropertiesoflight，thereisoyIointout.It，too，issomethingthatweallknowaboutfromoureverydaylives，thoughperhapsierieherpropertiesoflight.Itispolarization.
Ifyouhavewatcheda3Dmovie，thehisprexploited.Watgsuchamovierequiresyoutoecialspectacleswithcardboardorplasticframesthathavepiecesofplasticforthe‘lenses’.Ifyoutaketwopairsofthesespedslidetheleftleherightlehenlhthematalightbulb，youwillseeavery，verydimimageofthebulb.Alternativelyifyourotateoaclesthrough90degreeswithrespecttotheotherahetwoleftlehetwhtlensesoveroheryouwillseesomethingverysimilar-almostnotransmissionoflightthroughthepairofglasses.
Thisbeexplainedbyassigniyof‘orientatio.Mostonlightsourcesemitlightreferredorientation.Whenyoulookatalightthroughthesespectaclesiteardimmer，indigthattheyhaveselectedaparticularorientatiolensallowsoopass，therightlehatisatrightahefirst.That’swhywhenyouputasesorierightahefirst，nothitedthroughit:thelightpassingthroughthefirstlenshasthe‘wrohthesed.Thischaracteristitationiscalledpolarization.Ittookagreatdealofcarefulexploratiohaheideaofpolarization，butitisanimportaeologiesbasedonlight，andindeedfwhatlightactuallyis.
Thesephysicalcharacteristitensity，dpolarizatioeobeusedtodiseasure，andtotrolpropertiesofmaterialsubstahereforetoformthebasisofahostoftandmanipulatingmatterandevensmallobjetheexamplesabove，lightalmostallaystheroleofaninformatioheritisganimageoraspectrumoraphoion，itactsasamesselightalsobeusedinanactiveway.Foriheheatiiesoflightbeusedtocutmetalaerialsveryprecisely.Thickplatesofmetal，uptoatimetreorso，bemaedusinghigh-powerlasersmorerapidly，withahigherqualityfinishae，thanusingasaw.Andlightisusedinmedivariousways，frvisihlasersurgerytoagdrugsforaherapies.
Lighteoviewthenaturalworldateverytimesddistancescaleimagiheearliestmomentsoftheuheunimaginablyfastmotioronsinatomsandmolethelarge-sgementalaxiesacrosstheuheatementofsiprovidesuswithinsightintotheveryfoundatiouralworld，fromtheweirdnessofquantumphysicstothestruolecules.
&oryofoptieinwhiewdiscoveriesaboutlighthaveeeologiesthathave，inturonewdiscoveriesaanyfieldsofsce.Ateachstage，fromtheiioothemostpreiodernteol，measuring，andunis，lighthasgivennelisthathaverevolutionizedhowwelive.Thiscycleofdisdinnovatioudyoflightavibrantdise，evenasitisamovenerable.Howweoderandingofwhatlightis，andthereforehowwecoulduseit，bothforandingoftheworldandforneabilitiesthatgetheworld，isthetalethatformstherestofthebook.
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