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Chapter 1 What is light（第1页）

Chapter1Whatislight?

Lighteoseetheworldaroundus。Itprovidesthemeansbywhiseofsightgleadireationaboutthephysigementoftheworldandhowitisg。Iyoflighttodformatiosmostimportant，andremarkable，characteristic。

Seeingisbelieving

Sighteolocateourselvesins，definingthingsoutsideourselvesthatallowustostructatruepictureoftheworld。Andsightiheimagihephysisationofvisionitself。GeeRid’spainting，

&ionofLight，showninFigure1，illustratesthetralplacelighthasinourpsydeed，wfromtheideaoflight-insight，illumination，clarity，forexample-pertaintohuman，ashysical，qualities。Infahastwowordstht，luxaingboththematerialaaphysicalaspectsoflight。Itistheformerwithwhichthisbookisprimarilyed。Theiwiningofthephysidthepoetichasmadelightametaphabouttheworld，inphilosophy，theology，psychology，art，aure。Becauseitissomethingofwhichalmosteveryonehasdirectexperiehephysicalbasisoflightandhowitfacilitatesthispowerfulsensehasmadeitaudyforphilosophersalystists，forturies。

1。TheofLightbyGeeRid。

Lightgiveslife。Literally，lightplaysavitalroleinthebiologidchemicalprocessesthatunderpinourandourpla'sexisteively，lightframesourpers。Ouroneverydayexperieratethetralimportainthisregard。Ofcourseweuseitforilluminatingourehernaturally，bytheSunorMoon，orartifionlightsourcesuseelectricity，butonowestillusechemicalreastht;burningdles，forexample。Thediferentcharacteroftheilluminatioonhowweperceives:itsetsa‘mood’forthephysicalspace。

Lighthasaveryfualroleinmakingpossiblelifeitself。TheprimarysourergyfortheEarthisultimatelytheSun。AndthemeansbywhiergyistraheSunislight，boththevisibleposwesee，aswellassomeinvisibleoseedirek，forexample，oflyingonabeachoutinthegardeninthesuhewarmthwefeelisaeofthe‘i’radiatingfromtheSun。Thisisjustohephysiologicalefectsoflight。

Butattheverycoreofourpla’sabilitytosustainlifeisanawesomebiochemicalprocessthatverts‘waste’moleculesdioxide-into‘useful’ones-oxygen，usingsunlightasasourergy。Thereverseproversioodioxide，respiration，aswellasfromthethiopoweroureverydayworld。

TheSun’slighthasovertheillionsofyearseiobiosphereahatprovidesothersourergy。heroroilcouldhavebeeenergyfromtheSun。AheseresourcesisgthewayinwhichlightfromtheSusourplaheifromtheSun-ultravioletlight-isstillabsorbedbytheEarthanditsatmosphere。Buttheotheri-i-isreflectedbackbyatmases。Bythesamemeism，iionistrappedo，tributingtoiarysurfacetemperature。

Lightenablesuni

Pictureshavebeenpartofhumahebeginningofthespecies。Theimpaagesonhowweceiveoftheworld，andhowwemakesenseofourplait，isimmeasurable。Optiologyhastributedtothisinwaysthatareutterlytransformative。Foriheabilityteseasilyandrapidly，bymeansoffilm-basedanddigitalphotography，allowsustorecordplaces，people，andthihatbewidelydistributed（nowadaysbyanoptiabledI）andwhichhavelastingimpact:imagesofleadersandirihenaturalworld，andhsesofwar。Theseentpeopleiedulationstoagapassion，andgiviintosharedexperieheastonishingsensehtofman’sfirststepsontheMoon（seeFigure2）iheabilitytimagesaddsapletelynewdimensionbyeiveaarycapabilities。agitelevisiohoutvideo?

Nowadays，thegeionandtransmissionofimagesissoprevalehinkofit。Weuseself-luminousdisplayseveryday:televisions，puters，tablets，evensmartphones。Allofthesebringinformationtoyouafromyouusinglightasamedium。Almostalllong-distaeleunistravelonlightbeams，guidedalongthinstrandsofglasscalledopticalfibres。ThisisthebasisoffibreopticalbroadbalinkourhomestotheI。Eveersandtelevisionslightplaysarole。ForihemusiagesthatareloaCDorDVDareaccessedusinglight。Atinymovingheadbasedonaminiaturelaser‘reads’thedisvertstheinformatioirialsthatbesenttothedisplays。All，downloading，andemailingaowrequiressuseinformationcapacitythatlightistheonlyfeasiblemediumforgit。

&rong’sphotographofEdwinAldrinwalkingontheMoon。

Transportationinthemodernworlduseslightasthemeansbywhialaeourmovements。Frhtsintowns，tolandinglightshtisaialpartofnavigation。Anditevenplaysaroleinmaintainingourvehicles。Forinstance，lasersareusedtoalignthewheelsht-eributionofignitioodriveaninternalbustiounon。

Inmany，manyways，lightcarriestheenergyandinformationthatmakesmodernlifepossible。

Optics

Thefieldofenquirythatstitutesthestudyoflightiscalledoptics。Optigtheoldestofthesditshistorientformsoimportantpathsintheemergenodernsce。Ideasarisinginopticshavestimulatednewwaysofthinkingforverydisparatefields，suchasthemeotionofatomsandmoledteologiesenabledbydeeperuandingoflighthavebeeralinunlogothersecretsofthenaturalwalileo’stelescopedesigns，forinstaihisobservationofthemoonsofJupiter，whichinmovingtowardsaviewofthesolarsysteminwhietsmovedarouhis，inturn，ortahelawsofgravitythatgoverion。

TheinsofoptitheworkoftheGreekphilosophersofthe4thturybdthefieldhasuedtoflourishforthepasttwomillennia。Itisperhapssurprisiilldisewthingsaboutlightaftersugperiodofattentionbymanycleverpeople。Yetoptisattheforefrontoftsce:morethantenNobelPrizesiwentyyearshavebeenawardedforresearwhichlighthasplayedagahemotionofatomsaunimaginablylowtemperaturesandontimescalesofbreathtakithepreofclocksathousandfold，toenablingustolookinsidelivingdensastheyge。

Whatislight?

Agoodplacetostartiswithsomeofthethingsonlyassociatedwithlight:brighty，d>

Thesearealltaiesthatsuggestlightisaphysitity。Butwhat，exactly，isit?