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Chapter 2 Light rays（第1页）

Chapter2Lightrays

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Takingaselfierequiresthatthephonecameraispoiowardsyou.It’sobviousthatthisisnecessaryifyouaretobeiure.Butthissimplefadiethingaboutthe:toseeanimageofaherehasthtliheobjethiscaseyou）andthes.Thisisusuallycalledthe‘li’.Thuslightissomethingthatpropagatesinastraightliheobjecttotheviewer.Ihisiswhatwemightexpeeoftypesoflightsource.Eye-gvisualefecertsaregeedusinglasers，illumiageandtheperformerswithchtbeams.Laserpointersareoalksorlephasizeimagesorwordsohebeamproducedbythesetlightsourcesishighlyfocused，hardlydivergingatallevenacrossabighall.Itgoesinastraightline-youpoihedirewhereyouwaogo，anditdoesso.

Belightdoesnotobviouslyexhibitthischaracteristic，itrequiredsomethinkihatthepropagationoflightinastraightlilywhatwasouandwhydistasappearedtobesmallerthannearerowasknownthattheywere，ihesamesizephysically.

5.Eustruofraysshowingwhyobjeesizelooksmallerwhehera>

&thattheceptht-liiohisefectisattributedtoEuclid，wreearound300bce.Hisidea，fromobooksonoptics，isillustratedinFigure5.Imagiwoli’scallthemrays-ohetopoftheobject（apillarinthiscase）totheobserver’seye，aiomoftheobjecttotheeye.Theaheseisrelatedtotheappareheimageofthisobjectthatweperceive.Amoredistantpillar，thesamephysicalsizeasthefirst，producestworayswhoseaerseattheobserverissmaller-hehepillarappearssmaller.Thisisetheimage.

Whatisthestufthattraversestherays?Euclid（buildingonearlierideas）thoughtthatitartitoutfromtheeyeitself（fromanimaginaryinterillumiaedbacktotheobserver.Butthissuggeststhatwewouldbeabletoseethiitwasdarkoutside.heideaofstufmovingaloweeandobserverremainedapowerfulcept.

ItwasmodifiedbyAl-HazehthatweisareilluminatedbyraysfromtheSuernalfire）aeredtowardstheobserver.Thereareseveralstoriesabouthowhecametothisidea-ingdoiwhelyattheSuermihepaiionheexperiencedwouldbethereallthetimeifthe‘internalfire’wereburnihus，heargued，thesourceofthelightogeheimagewasexternal.

&us，forthesakeumewhatmovesalongtheseraysareparticlesoflight:callthemphotohebeamisrelatedtothetherayinoneseordertouandhowanimageofaisformed，we’llosiderenswhehesephotoedofamirroredsurface，aswellasehiswillleadtothe‘lawsofoptics’thatareusedfningveryplexoptistrumentssuchassurgicalmicrosdcathetersfor‘keyhery，asticaltelescopesplaorbitabovetheEarthfdistaheimpactoftheseisonourlifeandoftheworldisimmense.

Whatsortofpropertiesdotheseparticlesoflightpossess?Theusualsortsofattributesassigoaparticleareitsposition，itsdireoftravel，anditsspeed.Fornow，assumethatitmovesatthe‘speedoflight’，withoutgoiailaboutwhatthatactuallyis.Thepositionofaphotonmightthehestartingpositionofthe‘ray’aioon’smotiohedireoftheray.Thephotonheadsoffromitsstartingpointinthisdireatthespeedoflightuersthesurfaceoftheobject.

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&stheobject，thelightisreflected.ensisthatthephotohesurfagingitsdire，butnotitspositiononthesurface.Thesituationisshownihemannerinwhichthedireisgedisspecifiedbythe‘lawofrefle’，discoveredbyHeroofAlexandriaiury.Itstatesthattheangleofiis，theaheiningraydiredthedireperpendiculartothesurfaceatthepointofince）isequaltotheaion（thatis，theaheoutgoingraydiredthedireperpendiculartothesurfaceatthepointofiherearescesofthisceptuallysimplearemelypo>

6.Raysofa.aflatmirrorandb.andc.acurvedmirror.

ThisbeexplaiirelybyHero’slawofrefle.Figure7showsheesareflewithoppositehaheclockwise-pointingarrowistheobject.Raysfromeatonthearrowrefleirrorandarerearrathearrowseeninthemirrorispointinganticlockwise.Youusethesamestrutoshoointingarrowisseeninthemirrht-pointingarrowaanup-pointingarroointingup，andadown-pointingarrowpointsdowion.

Imagingusiio

If，insteadoflookingatyouropposite-handedselfinaflatbathroommirror，youseeyourrefleapolishedspoon，theortedforme:maguresrounds.Thetsurfaceofthespooheobjedthevexbacksurfaifiesit.

7.Thegeofhandednessofobjectsrefleamirror.

Whyisthis?Siionofimagesisperhapsthemostimportantappliofoptistruments-rangingfromtasesforvisioospacetelescopesforstificdiscovery-it’swexactlyho>

Uptonow，I‘vesideredonlyasinglerayoflightfromoo.Infact，raysusuallysalldireeachobjet.sidera‘bundle’ofsuchraysallasio，fadtheinalray.Thiseofraysdivergesasitmovesawayfromtheobject，asshowniherayshitthecurvedmirrorsurfaceatdiferentpointsandthereforealslesofiheyrefledifereions，eaestillsatisfyingHero’slawofrefle.

8.Imageformationbyaraybundleusingacurvedmirror.Theraysfromooatointheimage.

Infaagdeventuallyallmeetatasihisisthe‘image’pointi.

Animageaswenormallysideritismadeupofmultiplesuchimagepfromdiferes.Thesizeoftheimageisdetermiafromthemirrorandthefogpowerofthemirror，specifiedbytheradiusofcurvature.amoreirrorhasasmallerradiusofsurfacecurvature）.Theimagebebiggerthaheobjectisirrorthaheratioofimagetoobjectsizeiscalledthemagnifi.

Theimage-magureofacurvedmirrorwasusedbyodesignatelescope，showninFigure9.Hisdesignhasaremarkableproperty:itfesofdistaarethesamesizeforeverycolour（saidtobefreefrom‘aticaberratiooncleverlyusedthepropertythattheangleofareflectedrayforafixedaheirayisthesamehecolourofthelight.Theimageofeachcolouristhereforeformedinthesameplace-allthecisterperfectly，guaranteedbyphysics.

&telesabledimageswithoutaticaberration.

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&ohisibecausethetelescopesusedbyporarypioneerssuchasGalileoGalileiandJohannesKeplersuferedseriouslyfromaticaberration.Theimagesformedbytheirtelescopesalwayshadablurredcolouredhaloarouheobject.Thereasonforthisisthattheyweredesignedusiprhtrays-refra:thephelightraysbeheygofromoramediumtoanother.

Itistherefraoflightthatproducesthe‘kink’observediiallyimmersedinabowlofwater.ThisisdescribedbythelawofrefraonlyknownasSnell’slaw，afterDutWillebrordSh-turypropohislawsaysthattheaingraymakeserpendiculartothesurfaceisrelatedtotheaheiningraybythepropertiesofthetwomediathatformtheinterfaplethesurfaceofthewater.Thisisillustratediheparticularpropertyofthemediathatisrelevahe‘refradex’.Therefradexbethoughtofasbeingameasureoftheoptiess’ofthemediumasexperiencedbyalightray.Solighttravelsmoreslowlyihalargerrefradexbecausethemoleediumareslightlymoreresistanttohaviomsarohelight.Itislikerunninginapoolofwater.Ifthedepthisverysmall，ysmoveeasilyandyoufast.Ifthetoyourk’sharder，becauseyouhavetaiaer.Infact，thelawofrefrabederivedfromthisanalogy.PierredeFermatshowedthatwhenlightgoesfromapoioapointinaseeksaraythattraversesashortertimeihhighrefradexaimeihalowerrefradex.Thisrequiresthelightraytobendatthisiweewomedia，a’spriurnsouttobeehesameasSnell’slaw.

10.Refraofarayattheiweenairand>

Imagingusinglenses