M23_KNIG7655_00_ITG_CH23

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1、CHAPTER23RayOpticsRecommendedclassdays:4BackgroundInformationStudentsapproachopticswithanaiveconceptionoflightanditsproperties.Asthelastchapternoted,allstudentsknowthat“lighttravelsinastraightline,”butfewhaveaclearunderstandingofwhatthisactuallymeans.Manystudentsdon’trecognizethatlightisaphysicale

2、ntity,withanexistenceapartfromitssourcesandeffects.Theydon’tmakeacleardistinctionbetweenlightandvision.Asanexampleofstudents’naiveviews,interviewsfindthatmanystudentsfrommiddleschoolthroughcollegethinkthatlighthasafiniterangebefore“runningdown.”Afterall,theydon’tseetheirheadlightbeamsduringtheday,

3、soapparentlythelightfromtheheadlightsonlytravelsafewfeetduringdaylighthours!Andwhilemoststudentsknowthat“whitelightconsistsofallcolors,”fewcaninterpretthisstatement.Manystudentsthinkthatacolorfilteraddscolortowhitelight.Inshort,studentshavescatteredideasaboutlight,butnotacoherentmodelthatcanbeused

4、forreasoning.Wewantthemtolearntwomodels,thewavemodelandtheraymodel,inquicksuccession.Studentswhogoontomodernphysicswillbepresentedwithathirdmodel,thephotonmodel.Theresearchevidenceisthatstudentsemergefromopticswithaconfused,hybridmixoftherayandwavemodels.Afterinstruction,themajorityofstudentsgivei

5、ncorrectanswerstosimplequalitativequestionsaboutimageformationandaboutinterference.StudentunderstandingofrayopticswasstudiedbyGoldbergandMcDermott(1987)andbyWosilaitetal.(1999a).Muchofthisworkwasbasedonpost-instructioninterviewswithstudentsinthecalculus-basedphysicscourse.Theyfoundthatstudentshadd

6、ifficultyapplyingthemostbasicideasoftheraymodeloflight.Whenpresentedwitha1-cm-widerectangularapertureandapointsourceoflight,themajorityofstudentsthoughtthatthepatternoflightonascreenwouldbedominatedbydiffractioneffects.Evenafterbeingledtorecognizethattheimagewouldbeabrightrectangle,noneoftheinterv

7、iewedstudentscoulddetermineitssize.Theyalsohadgreatdifficulty23-1predictingthepatternoflightonthescreenifanaperturewereilluminatedbytwopointsourcesoflight.Fewstudentshaddevelopedaworkingmodeloflightpropagationtha