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GeometricOptics
LearningGoals
Lookingforwardat…• howaplanemirrorformsanimage,andwhyconcaveandconvexmirrorsformimagesofdifferentkinds.• howimagescanbeformedbyacurvedinterfacebetweentwotransparentmaterials.• whataspectsofalensdeterminethetypeofimagethatitproduces.• whatcausesvariousdefectsinhumanvision,andhowtheycanbecorrected.• howmicroscopesandtelescopeswork.
Introduction
• Thissurgeonperformingmicrosurgeryneedsasharp,magnifiedviewofthesurgicalsite.• Toobtainthis,she’swearingglasseswithmagnifyinglenses.• Howdomagnifyinglenseswork?• Howdolensesandmirrorsformimages?• Weshalluselightraystounderstandtheprinciplesbehindopticaldevicessuchascameralenses,theeye,microscopes,andtelescopes.
Reflectionataplanesurface
• LightraysfromtheobjectatpointP arereflectedfromaplanemirror.• ThereflectedraysenteringtheeyelookasthoughtheyhadcomefromimagepointP’
Imageformationbyaplanemirror
Imageformationbyaplanemirror:Signrules
Characteristicsoftheimagefromaplanemirror
• Inaplanemirror,theimageisvirtual,erect,reversed,andthesamesizeastheobject.
Theimageisreversed• Theimageformedbyaplanemirrorisreversed;theimageofarighthandisalefthand,andsoon.
Sphericalmirrorwithapointobject
• AsphericalmirrorwithradiusofcurvatureR formsarealimageP’ ofthepointobjectP.
GeneralSignconventions(mirror&lens)
• IftheobjectpointP isonthesamesideastheincidentlight,thens ispositive.• IftheimagepointP’ isonthesamesideasthereflectedlight,thens’ ispositive.• IfthecenterofcurvatureCisonthesamesideastheoutgoing light,thenR ispositive.
Focalpointandfocallength• Whentheobjectisveryfarfromthesphericalmirror,theincomingraysareparallel.• Thebeamofincidentparallelraysconverges,afterreflectionfromthemirror,toafocal point,pointF.• Thedistancefromthevertextothefocalpoint,denotedbyf,iscalledthefocal length.
Focalpointandfocallength
• Withtheobjectatthefocalpoint,thereflectedraysareparalleltotheopticaxis.• Thereflectedraysmeetonlyatapointinfinitelyfarfromthemirror,sotheimageisatinfinity.
Imageofanextendedobject:Sphericalmirror
• Shownishowtodeterminetheposition,orientation,andheightofanimageformedbyaconcavesphericalmirror.
Imageformationbyaconvexmirror• Ifthemirrorisconvex,sothatR isnegative,theresultingimageisvirtual(thatis,theimagepointisontheoppositesideofthemirrorfromtheobject),erect,andsmallerthantheobject.
Focalpointandfocallengthofaconvexmirror
• Whenincomingraysthatareparalleltotheopticaxisarereflectedfromaconvexmirror,theydivergeasthoughtheyhadcomefromthevirtual focalpoint F atadistancefbehindthemirror.• Thecorrespondingimagedistances isnegative.
Focalpointandfocallengthofaconvexmirror
• WhentheincomingraysareconvergingasthoughtheywouldmeetatthevirtualfocalpointF,thentheyarereflectedparalleltotheopticaxis.
Graphicalmethodoflocatingimages
Graphicalmethodoflocatingimages
Imageofapointobjectatasphericalsurface
Apparentdepthofaswimmingpool
• Whenlighttravelsthroughaplanesurfacebetweentwoopticalmaterials,theimagehasthesamelateralsize(m =1)andisalwayserect.• s’=-s/1.33• Theapparentdepthofapoolislessthanitsactualdepth.
R→∞
Lensmaker’s equation
22
Magnification
m = y 'y= −s 's
Typesoflenses• Shownbelowarevarioustypesoflenses,bothconverginganddiverging.
• Anylensthatisthickeratitscenterthanatitsedgesisaconverging lenswithpositivef ;andanylensthatisthickeratitsedgesthanatitscenterisadiverging lenswithnegativef.
1f= (n−1) 1
R1−1R2
"
#$$
%
&''
R1 > 0 > R2R1 =∞0 > R2
0 > R2 > R1 R2 > 0 > R1
f > 0 f < 0
R1 > R2 > 0R1 =∞R2 > 0
Thinconverginglens
• Alens isanopticalsystemwithtworefractingsurfaces.• Thesimplestlenshastwosphericalsurfacescloseenoughtogetherthatwecanignorethedistancebetweenthem;wecallthisathin lens.
Thinconverginglens• RayspassingthroughthefirstfocalpointF1 emergefromaconverginglensasabeamofparallelrays.
• fiscalledthefocal length.
Imageformedbyathinconverginglens
Thindiverginglens• Whenabeamofparallelraysisincidentonadiverging lens,theraysdivergeafterrefraction.• Thefocallengthofadiverginglensisanegativequantity,andthelensisalsocalledanegative lens.
Thindiverginglens
• Incident raysconvergingtowardthefirstfocalpointF1ofadiverginglensemergefromthelensparallel toitsaxis.
Graphicalmethodsforlenses
• Shownbelowisthemethodfordrawingthethreeprincipal rays forarealimageformedbyaconverginglens.
Graphicalmethodsforadiverginglens
31
Example
Cameras
• Whenacamera isinproperfocus,thepositionoftheelectronicsensorcoincideswiththepositionoftherealimageformedbythelens.
Cameralensbasics
• Thefocallengthf ofacameralensisthedistancefromthelenstotheimagewhentheobjectisinfinitelyfaraway.
• Theeffectiveareaofthelensiscontrolledbymeansofanadjustablelensaperture,anearlycircularholewithdiameterD.
• Photographerscommonlyexpressthelight-gatheringcapabilityofalensintermsoftheratiof/D,calledthef-numberofthelens:
Theeye• Theopticalbehavioroftheeyeissimilartothatofacamera.
Defectsofvision
• Anormaleyeformsanimageontheretinaofanobjectatinfinitywhentheeyeisrelaxed.• Inthemyopic (nearsighted)eye,theeyeballistoolongfromfronttobackincomparisonwiththeradiusofcurvatureofthecornea,andraysfromanobjectatinfinityarefocusedinfrontoftheretina.
Nearsightedcorrection
• Thefarpointofacertainmyopiceyeis50cminfrontoftheeye.• Whenadiverginglensoffocallengthf =−48cmisworn2cminfrontoftheeye,itcreatesavirtualimageat50cmthatpermitsthewearertoseeclearly.
Farsightedcorrection• Aconverginglenscanbeusedtocreateanimagefarenoughawayfromthehyperopiceyeatapointwherethewearercanseeitclearly.
Angularsize
• Themaximumangular sizeofanobjectviewedatacomfortabledistanceistheangleitsubtendsatadistanceof25cm.
𝜃 ≈ tan 𝜃 =𝑦
25𝑐𝑚
Themagnifier• Theangular magnification ofasimplemagnifieris:
Thecompoundmicroscope
Theastronomicaltelescope
• Thefigurebelowshowstheopticalsystemofanastronomicalrefracting telescope.
Thereflectingtelescope
• TheGeminiNorthtelescopeusesanobjectivemirror8metersindiameter.
43
Example:Inatelescope,theobjectivelensformsareal,reducedimageIoftheobject.Thisimageistheobjectfortheeyepiece,whichformsanenlarged,virtualimageofIwhichisatinfinity(formastcomfortableviewingbyaeye).
ϑ =−y 'f1
and ϑ ' = y 'f2
angular magnification M =ϑ 'ϑ= −
f1f2
(-ve for the inverted final image)