千米小说网

Chapter 1 What is light（第2页）

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。（Thisismoreproperlytheirradiaensityisperhapsamorefamiliarterm。）Theiyofalightsourceisrelatedtotheabilitytotratethelight。Alaserpoihtbecauseitslightbeamistratedonasmallspotonthes，whereastheSun’slightisdifusedoveravery>

&houghtheSupoweroutput，thelightitproduotasiofalaserpointer。

Theunderlyidescribesourabilitythtiscalledthesource。Thisisrelatedtothesource’spropensitytosendlightinaparticulardire。ForiheSunandalightbulbradiatelightintoalldires-theSunbeseenfromeveryplaEarth，andalightbulbfromwhereveryoustandinaroom。Butalaserpointeronlyputsoutlightiion-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-entarycoloursoftheorum。bion’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。