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Transcript
Refraction Chapter14 Refraction • Whenlightbendsattheboundarybetween twotransparentmedia,thisiscalled refraction. • Thisisbecauselighttravelsatdifferentspeeds throughdifferentmaterials. Refraction • Iflightgoesfromahigherspeedtoalower speed—suchastravelingfromairintoglass— therayoflightbendstowardsthenormal. • Iflightgoesfromalowerspeedtoahigher speed—suchastravelingfromglassbackinto air—therayoflightbendsawayfromthe normal. Refraction • Theindexofrefraction,n,isdefinedasthe ratioofthespeedoflightinavacuumoverthe speedoflightinaspecificsubstance. • Differentwavelengthsoflighthavedifferent speedsinavacuum,sorefractionaffectsthe differentwavelengthsinslightlydifferent ways. Refraction • Objectsappeartobeindifferentlocationsdue torefraction. • Snell’sLaw Sample492 Practice493 ThinLenses • Curvedsurfaceschangethedirectionoflight. Specifically,we’llbetalkingaboutlenses. • Whenasurfaceiscurved,thenormallineis pointingadifferentdirectiondependingon whereyouarealongthesurface.Remember, thewaylightbendsdependsontwofactors: *Whetheritisslowingdownorspeedingup, *anditsangleofincidence. ThinLenses • Lensesformimagesbyrefraction,ratherthan byreflection. • Lensescancreaterealorvirtualimages. • Arealimageisformedwhenraysoflight actuallyintersecttoformanimage. • Avirtualimageisformedwhenlightrays appeartocomefromapointthattheydon’t actuallycomefrom. ThinLenses • Lensesareusedinallmannerofoptical instruments: *glasses *telescopes *cameras *microscopes • Lenseshavetworefractingsurfaces,eachof whichisgroundintoeitherasegmentofa sphereoraplane. ThinLenses • Converginglensestakelightfromonesideof thelensandfocusitatapointontheother sideofthelens. • Diverginglensestakelightfromonesideof thelensandspreaditoutontheothersideof thelens. ThinLenses • Lenseshavetwofocalpoints,oneoneachside ofthelens. • Lightcomingfrominfinitywillbefocusedat thefocalpointontheothersideofthelens. • Thefocallengthforlensesisdeterminedby thedistancebetweenthefocalpointandthe centerofthelens(unlikemirrors,which measuretothesurfaceofthemirror). ThinLenses • WecanmodelTHINlenses(thatis,lenses whosethicknessissmallcomparedtoRorp) bydrawingraysthatpassthroughaline segmentdrawndownthecenterofthelens. • Modelaconverginglenswitharrowheadsthat pointawayfromeachother. • Modeladiverginglenswitharrowheadsthat pointtowardseachother. ThinLenses • Ourthreereferencerayswillbesimilartoour referenceraysforsphericalmirrors: (Someexamplescanbeseenonpage497) • Fromobjectthroughcenteroflens • Fromobject,paralleltoPA,theneitherpassing throughordirectedawayfromthefocalpoint (dependingonlenstype) • Fromobject,throughfocalpoint,thenmoving paralleltoPA ThinLenses • Converginglensescanproducerealorvirtual images. • Diverginglensesproducevirtualimages. • Ourequationsfromourworkwithmirrorswill alsoworkforthinlenses.Focallengthsare positiveforconverginglensesand SampleB500 negativefordiverginglenses. PracticeB501 ThinLenses • Theeyeitselfconsistsofacornea,whichacts asalens,andacrystallinelens,bothofwhich directlighttowardtheretinaonthebackof theeye. • Somepeoplerequirecorrectivelensesbecause theireyesdon’tfocuslightaccuratelyonthe retina. ThinLenses • Apersonwhoisfarsighted(hyperopia)cansee farobjectsclearlybutnearobjectsseem blurry.Thisisbecausethelensesfocuslightat apointbehindtheretinaratherthanonthe retina. • Apersonwhoisnearsighted(myopia)cansee nearobjectsclearlybutfarobjectsseem blurry.Thisisbecausethelensesfocuslightat apointthatisinfrontoftheretinaratherthan ontheretina. ThinLenses • Whenitcomestosystemsoflenses,theimage fromthefirstlensisgoingtobetheobjectfor thesecondlens,andsoforth. • Compoundmicroscopesandrefracting telescopesbothusetwoconverginglenses. OpticalPhenomena • Whenlightmovesonapathfromamedium withahigherindexofrefractiontowardsone withalowerindexofrefraction,it’spossibleto havetotalinternalreflection. • Atthecriticalangle,therefractedraymoves paralleltotheboundary.Ononesideofthat criticalangle,alllightwillbeentirelyreflected backintothehigher-indexmedium. OpticalPhenomena • WecanuseSnell’slawtofindthecriticalangle. • Sineof90°is1. SampleC507 PracticeC508 OpticalPhenomena • Otheropticalphenomenainclude: • AtmosphericRefraction.Lightbendsthrough theEarth’satmosphere.Oneprominent exampleofthisishowwecanseethesun evenifit’spassedbelowthehorizon.There’s alsoagreenflashsometimesvisibleatsea. OpticalPhenomena • Otheropticalphenomenainclude: • Mirages.Whenthegroundissohotthatthe airdirectlyaboveitishotterthanathigher elevations,ithasadifferentrefractiveindex thantheairdirectlyabove. OpticalPhenomena • Otheropticalphenomenainclude: • Dispersion.Differentwavelengthsoflight bendslightlydifferentlyastheypassthrough boundariesofvariousmediums.Thisisthe effectweseewithprismsandrainbows. OpticalPhenomena • Otheropticalphenomenainclude: • Lensaberrations. • Sphericalaberrationsoccurwithlenses,just likewithmirrors. • Anothertypeofaberrationiscalledchromatic aberration,whichissimilartowhathappensin aprism.
