chapter 10: light reflection and refraction · chapter 10 light-reflection and refraction 1. which...

$: Chapter 10: LIGHT REFLECTION AND REFRACTION · Chapter 10 Light-Reflection and Refraction 1. Which of the following can make a parallel beam of light when light from a point source$
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NCERTExemplarSolutions

Chapter10

Light-ReflectionandRefraction

1.Whichofthefollowingcanmakeaparallelbeamoflightwhenlightfromapoint

sourceisincidentonit?

(a) Concavemirroraswellasconvexlens

(b) Convexmirroraswellasconcavelens

(c) Twoplanemirrorsplacedat90°toeachother

(d) Concavemirroraswellasconcavelens

Ans.(a)Concavemirroraswellasconvexlens

Explanation:Whenapointsourceoflightisatthefocusofaconcavemirrororaconvex

lens,thenemergentraysoflightmakeaparallelbeamoflight.Thisisthereason;concave

mirrorisusedasreflectorofheadlights.

2. A10mmlongawlpinisplacedverticallyinfrontofaconcavemirror.A5mmlong

imageoftheawlpinisformedat30cminfrontofthemirror.Thefocallengthofthis

mirroris

(a) –30cm

(b) –20cm

(c) –40cm

(d) –60cm

Ans.(b)–20cm

Exercise

Multiple Choice Questions (MCQs)

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Explanation:objectsizeh=10mm,imagesizeh’=5mm,imagedistancev=-30cm,object

distance=?,f=?

magnification=

or or

oru=-60

Now,itcanbecalculatedasfollows:

or or

(thenegativesignconfirmsthatthecalculatedfocallengthis

ofaconcavemirror

)

3. Underwhichofthefollowingconditionsaconcavemirrorcanformanimagelarger

thantheactualobject?

(a) Whentheobjectiskeptatadistanceequaltoitsradiusofcurvature

(b) Whenobjectiskeptatadistancelessthanitsfocallength

(c) Whenobjectisplacedbetweenthefocusandcentreofcurvature

(d) Whenobjectiskeptatadistancegreaterthanitsradiusofcurvature

Ans.(c)Whenobjectisplacedbetweenthefocusandcentreofcurvature

Explanation:WhenobjectisplacedbetweenFandC,anenlargedimageisformedbeyondC.


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4. Figure10.1showsarayoflightasittravelsfrommediumAtomediumB.Refractive

indexofthemediumBrelativetomediumAis

(a)


(b)

(c)

(d)

Ans.(a)

Explanation:RefractiveindexofmediumBrelativetomediumAcanbecalculatedas

follows:

bysnell'slaw

nB/nA=sin60/sin45

5. AlightrayentersfrommediumAtomediumBasshowninFigure10.2.The

refractiveindexofmediumBrelativetoAwillbe

(a) greaterthanunity



(b) lessthanunity

(c) equaltounity

(d) zero

Ans.(b)lessthanunity

Explanation:Inthiscase,therayoflightbendsawayfromnormalwhenitentersfrom

mediumAintomediumB.ThisshowsthatmediumBisopticallyrarerthanmediumA.

Hence,speedoflightinmediumBismorethanthatinmediumA.So,ratioofspeedoflight

inmediumAtospeedoflightinmediumBwillbelessthanone.

6. BeamsoflightareincidentthroughtheholesAandBandemergeoutofboxthrough

theholesCandDrespectivelyasshownintheFigure10.3.Whichofthefollowingcould

beinsidethebox?

(a) Arectangularglassslab

(b) Aconvexlens

(c) Aconcavelens

(d) Aprism

Ans.(a)Arectangularglassslab

Explanation:Inthiscase,incidentraysfallperpendicularlyonthepointofincidence.A

rectangularglassslabwouldrefractandthenre-refractit..


7. AbeamoflightisincidentthroughtheholesonsideAandemergesoutoftheholes

ontheotherfaceoftheboxasshownintheFigure10.4.Whichofthefollowingcouldbe

insidethebox?

(a) Concavelens

(b) Rectangularglassslab

(c) Prism

(d) Convexlens

Ans.(a)Concavelens

Explanation:Theincidentraysareparallelandemergentraysarediverging.Hence,itisa

concavelens(diverginglens).

8. Whichofthefollowingstatementsistrue?

(a) Aconvexlenshas4dioptrepowerhavingafocallength0.25m

(b) Aconvexlenshas–4dioptrepowerhavingafocallength0.25m

(c) Aconcavelenshas4dioptrepowerhavingafocallength0.25m

(d) Aconcavelenshas–4dioptrepowerhavingafocallength0.25m

Ans.(a)Aconvexlenshas4dioptrepowerhavingafocallength0.25m

Explanation:Thepositivesignwiththefocallengthindicatesthatitisaconvexlens.


9. Magnificationproducedbyarearviewmirrorfittedinvehicles

(a) islessthanone

(b) ismorethanone

(c) isequaltoone

(d) canbemorethanorlessthanonedependinguponthepositionoftheobjectinfrontofit

Ans.(a)islessthanone

Explanation:Convexmirrorisusedinrearviewmirrors.Convexmirroralwaysmakes

smallerimages.Hence,magnificationproducedbyrearviewmirrorislessthanone.

10. RaysfromSunconvergeatapoint15cminfrontofaconcavemirror.Whereshould

anobjectbeplacedsothatsizeofitsimageisequaltothesizeoftheobject?

(a) 15cminfrontofthemirror

(b) 30cminfrontofthemirror

(c) between15cmand30cminfrontofthemirror

(d) morethan30cminfrontofthemirror

Ans.(b)30cminfrontofthemirror

Explanation:Inthiscase,f=-15cmandhencec=-30cm;becauseradiusofcurvatureis

doublethefocallength.WhenandobjectisplacedonC,itsimageisofthesamesize,

invertedandisformedonC.

11. Afulllengthimageofadistanttallbuildingcandefinitelybeseenbyusing

(a) aconcavemirror

(b) aconvexmirror

(c) aplanemirror


(d) bothconcaveaswellasplanemirror

Ans.(b)aconvexmirror

Explanation:Fieldofviewofaconvexmirrorismorethananyothertypeofmirror.Hence,

afulllengthimageofadistanttallbuildingcandefinitelybeseenbyusingthis.

12. Intorches,searchlightsandheadlightsofvehiclesthebulbisplaced

(a) betweenthepoleandthefocusofthereflector

(b) veryneartothefocusofthereflector

(c) betweenthefocusandcentreofcurvatureofthereflector

(d) atthecentreofcurvatureofthereflector

Ans.(b)veryneartothefocus

Explanation:Reflectorsofheadlightsandsearchlightsareintheshapeofconcavemirror.

Whensourceoflightisplacedatthefocus,thereflectedlightappearslikeabeamoflight.

13. Thelawsofreflectionholdgoodfor

(a) planemirroronly

(b) concavemirroronly

(c) convexmirroronly

(d) allmirrorsirrespectiveoftheirshape

Ans.(d)allmirrorsirrespectiveoftheirshape

14. Thepathofarayoflightcomingfromairpassingthrougharectangularglassslab

tracedbyfourstudentsareshownasA,B,CandDinFigure10.5.Whichoneofthemis

correct?


(a) A

(b) B

(c) C

(d) D

Ans.(b)B

Explanation:Whenrayoflightentersintoglass(opticallydenser)fromair,itbendstowards

thenormal.Whenrayoflightfromglassemergesintoair,itbendsawayfromnormal.This

iscorrectlyshownbyfigureB.

15. Youaregivenwater,mustardoil,glycerineandkerosene.Inwhichofthesemediaa

rayoflightincidentobliquelyatsameanglewouldbendthemost?

(a) Kerosene

(b) Water

(c) Mustardoil

(d) Glycerine


Ans.(d)Glycerine

Explanation:Refractiveindexofwateris1.33,thatofkeroseneis1.44,thatofmustardoilis

1.46andthatofglycerineis1.47.

16. Whichofthefollowingraydiagramsiscorrectfortherayoflightincidentona

concavemirrorasshowninFigure10.6?

(a) Fig.A

(b) Fig.B

(c) Fig.C

(d) Fig.D

Ans.(d)FigD

Explanation:Incaseofconcavemirror,anincidentrayparalleltotheprincipalaxispasses

throughFafterreflection.

17. Whichofthefollowingraydiagramsiscorrectfortherayoflightincidentonalens

showninFig.10.7?


(a) Fig.A.

(b) Fig.B.

(c) Fig.C.

(d) Fig.D.

Ans.(a)Fig.A

Explanation:Inconvexlens,theincidentraypassingthroughFgoesparalleltotheprincipal

axisafterrefraction.

18. Achildisstandinginfrontofamagicmirror.Shefindstheimageofherhead

bigger,themiddleportionofherbodyofthesamesizeandthatofthelegssmaller.The

followingistheorderofcombinationsforthemagicmirrorfromthetop.

(a) Plane,convexandconcave

(b) Convex,concaveandplane

(c) Concave,planeandconvex

(d) Convex,planeandconcave

Ans.(c)Concave,Planeandconvex


Explanation:WhenobjectisbetweenFandPofconcavemirror,anenlargedanderect

imageisformedbehindthemirror.So,thechildisseeingherheadinaconcavemirror.

Planemirrormakessimilarsizedimageasobject.So,themiddleportionofherbodyappears

tobeofsamesizeinmirror.Aconvexmirrormakessmallerimageandhencelegsare

appearingsmallerinmirror.

19. Inwhichofthefollowing,theimageofanobjectplacedatinfinitywillbehighly

diminishedandpointsized?

(a) Concavemirroronly

(b) Convexmirroronly

(c) Convexlensonly

(d) Concavemirror,convexmirror,concavelensandconvexlens

Ans.(d)Concavemirror,convexmirror,concavelensandconvexlens

Explanation:Incaseofalloftheabove,whenanobjectisatinfinity,imageishighly

diminishedandpointsized.


Short Answer Questions

20.Identifythedeviceusedasasphericalmirrororlensinfollowingcases,whenthe

imageformedisvirtualanderectineachcase.

(a)Objectisplacedbetweendeviceanditsfocus,imageformedisenlargedandbehindit.

Ans.Concavemirror

(b)Objectisplacedbetweenthefocusanddevice,imageformedisenlargedandonthesame

sideasthatoftheobject.

Ans.Concavelens

(c)Objectisplacedbetweeninfinityanddevice,imageformedisdiminishedandbetween

focusandopticalcentreonthesamesideasthatoftheobject.

Ans.Concavelens

(d)Objectisplacedbetweeninfinityanddevice,imageformedisdiminishedandbetween

poleandfocus,behindit.

Ans.Convexmirror

21. Whydoesalightrayincidentonarectangularglassslabimmersedinanymedium

emergesparalleltoitself?Explainusingadiagram.

Ans.Whenarayoflightentersfromonerarermedium(sayair)toanotherdensermedium

(sayglass),itbendstowardsthenormal.Whenthesamerayoflightexitsfromsecond

mediumintofirstmedium,itbendsawayfromthenormal.Inthiscase,theextentofbending

ofrayatoppositeparallels(air-glassinterfaceandglass-airinterface)issame.Duetothis,the

emergentrayisparalleltoincidentray.


22. Apencilwhendippedinwaterinaglasstumblerappearstobebentattheinterface

ofairandwater.Willthepencilappeartobebenttothesameextent,ifinsteadof

waterweuseliquidslike,keroseneorturpentine?Supportyouranswerwithreason.

Ans.Weknowthatpencilappearstobebentattheinterfaceofairandwaterbecauseof

refractionoflight.Thedegreeofrefractiondependsonrefractiveindexofagivenliquid.

Refractiveindicesofkerosene,waterandotherliquidswouldbedifferent.Hence,degreeof

bendwouldbedifferentincaseofdifferentliquids.

23. Howistherefractiveindexofamediumrelatedtothespeedoflight?Obtainan

expressionforrefractiveindexofamediumwithrespecttoanotherintermsofspeed

oflightinthesetwomedia?

Ans.Therefractiveindexcanbeseenasthfactorbywhichthespeedandthewavelengthof

theradiationarereducedwithrespecttotheirvacuumvalues.

n=c/v(wheren:refractiveindex,c=speedoflight,v:velocityoflightinthatmedium)

Refractiveindexofonemediuminrelationtoasecondmediumisgivenbyratioofspeedof

lightinsecondmediumtospeedoflightinfirstmedium.


Ifv1isthespeedoflightinfirstmediumandv2isthespeedoflightinsecondmedium,then

refractiveindexoffirstmediuminrelationtosecondmediumcanbegivenasfollows:

24. Refractiveindexofdiamondwithrespecttoglassis1.6andabsoluterefractive

indexofglassis1.5.Findouttheabsoluterefractiveindexofdiamond.

Ans.Absoluterefractiveindexofamediumshowstheratioofspeedoflightinairtospeed

oflightinthatmedium.Letusassumecisthespeedoflightinair,v1isspeedoflightinglass

andv2isspeedoflightindiamond.

Rdg=1.6----(1)

Absoluterefractiveindexofglass,Rga=1.5-----(2)

multiplyingequation(1)and(2),RdaXRga

So,absoluterefractiveindexofdiamond=2.4

25. Aconvexlensoffocallength20cmcanproduceamagnifiedvirtualaswellasreal

image.Isthisacorrectstatement?Ifyes,whereshalltheobjectbeplacedineachcase

forobtainingtheseimages?

Ans.Thisiscorrectstatement.

WhenanobjectisplacedbetweenFandF2ofaconvexlens,itsenlarged,invertedandreal

imageisformedbeyond2F2,i.e.ontheothersideoflens.So,forthisweneedtoplacethe

objectbetween20cmand40cmofthelens.

WhenanobjectisplacedbetweenFandOofaconvexlens,itsenlarged,erectandvirtual

imageisformedbeyondF2,i.e.onthesamesideoflens.So,forthisweneedtoplacethe

objectatadistancelessthan20cmfromthelens.



26. Sudhafindsoutthatthesharpimageofthewindowpaneofhersciencelaboratory

isformedatadistanceof15cmfromthelens.Shenowtriestofocusthebuilding

visibletoheroutsidethewindowinsteadofthewindowpanewithoutdisturbingthe

lens.Inwhichdirection,willshemovethescreentoobtainasharpimageofthe

building?Whatistheapproximatefocallengthofthislens?

Ans.LetusassumethatthewindowpaneisbetweenF2andinfinityfromthislensandthisis

aconvexlens.WeknowthatwhentheobjectisbetweeninfinityandF2,itsinvertedandreal

imageisformedbetween2Fand2F2.

Now,thedistantbuildingisatinfinityfromthelens.Itsimagewouldbeformedat2F.So,the

screenneedstobemovedtowardsthelensinordertogetasharpimage.Itsapproximate

focallengthis10cm(lessthanimagedistanceinearliercase).

27. Howarepowerandfocallengthofalensrelated?Youareprovidedwithtwolenses

offocallength20cmand40cmrespectively.Whichlenswillyouusetoobtainmore

convergentlight?

Ans.Powerofalensisreciprocaltoitsfocallength.So,smallerfocallengthmeansmore

power.Outofthegivenlenses,thelenswith20cmasfocallengthhasmorepowerthanthe

lenswith40cmasfocallength.Thelenswithhigherpowershouldbeusedtoobtainmore

convergentlight.

28. Underwhatconditioninanarrangementoftwoplanemirrors,incidentrayand

reflectedraywillalwaysbeparalleltoeachother,whatevermaybeangleofincidence.

Showthesamewiththehelpofdiagram.

Ans.Whentwoplanemirrorsareatrightanglestoeachother,incidentrayandreflectedray

willalwaysbeparalleltoeachother,whatevermaybetheangleofincidence.Letususe

followingfiguretounderstandthis.


EDandDCaretwoplanemirrorsplacedatrightanglestoeachother.Anisincidentrayon

CDandMNisnormalatpointofincidence.NQisreflectedrayfromNandisincidenton

pointQ.

QBisreflectedrayandPQisnormaltoED.

(anglei=angler)

(sameasabove)

(MNisnormalonCD)...(1)

(mirrorsareatrightangle)

Hence,in ;

(acuteanglesofarighttrianglearecomplementary)…(2)

Fromequations(1)and(2);


Hence,

(because and arecomplementary)

Now, and aresupplementary(PQisnormalonDE)

So,

Or,

Sincecorrespondinganglesareequal

Hence, proved

Thismeansthatincidentrayandreflectedraywillalwaysbeparallel;irrespectiveofvalue

ofangleofincidence.

29. Drawaraydiagramshowingthepathofraysoflightwhenitenterswithoblique

incidence(i)fromairintowater;(ii)fromwaterintoair.

Ans.

1. thelightslowsdownwhenitpassesfromararermediumintoadensermedium.Also,the

lightraybendsmoretowardsthenormal.(leftimage)

2.fromadensermediumtoararermediumthelightraybendsawayfromthenormal.(right

image)


Long Answer Questions

30. Drawraydiagramsshowingtheimageformationbyaconcavemirrorwhenan

objectisplaced

(a) betweenpoleandfocusofthemirror

Ans.

(b) betweenfocusandcentreofcurvatureofthemirror

Ans.

(c) atcentreofcurvatureofthemirror

Ans.


(d) alittlebeyondcentreofcurvatureofthemirror

Ans.

(e) atinfinity

Ans.

31. Drawraydiagramsshowingtheimageformationbyaconvexlenswhenanobjectis

placed

(a) betweenopticalcentreandfocusofthelens


Ans.

(b) betweenfocusandtwicethefocallengthofthelens

Ans.

(c) attwicethefocallengthofthelens

Ans.

(d) atinfinity


Ans.

(e) atthefocusofthelens

Ans.

32. Writelawsofrefraction.Explainthesamewiththehelpofraydiagram,whenaray

oflightpassesthrougharectangularglassslab.

Ans.Lawsofrefractionareasfollows:

(a) Theincidentray,therefractedrayandnormalatthepointofincidence;alllieinthesame

plane.

(b) Theratioofsineofincidencetosineofrefractionisaconstant,forthelightofagiven

colourandforagivenpairofmedia.ThislawisalsocalledSnell’sLaw.


ABCDisaglassslab.EFisincidentraywhichisincidentonpointOonair-glass

interface.

NOisnormaland whichisangleofincidence.

N’O’isnormalextendedtowardstheglassslaband whichisangle

ofrefraction.

OO’isrefractedrayfromsurfaceAB.ItbehaveslikeincidentrayonsurfaceCD.

therayEFbendswhenitenterstheslabtobecomeOO'.

MO’andO’M’arenormalonsurfaceCD.

GHistheemergentray.

whichisangleofincidenceatsurfaceCD.

whichisangleofrefractionatsurfaceCD.

ItisobservedthatEF,NOandOO’lieinthesameplane;whichisinaccordancetothe

firstlawofrefraction.

Itisalsoobservedthat whichmeansemergentrayisparalleltoincident

ray.Thishappensbecausethedegreeofbendatoppositesurfacesofglassslabis

same.

33. Drawraydiagramsshowingtheimageformationbyaconcavelenswhenanobject

isplaced

(a) atthefocusofthelens

Ans.

(b) betweenfocusandtwicethefocallengthofthelens

Ans.


(c) beyondtwicethefocallengthofthelens

Ans.

34. Drawraydiagramsshowingtheimageformationbyaconvexmirrorwhenanobject

isplaced

(a) atinfinity

Ans.

(b) atfinitedistancefromthemirror

Ans.


35. Theimageofacandleflameformedbyalensisobtainedonascreenplacedonthe

othersideofthelens.Iftheimageisthreetimesthesizeoftheflameandthedistance

betweenlensandimageis80cm,atwhatdistanceshouldthecandlebeplacedfromthe

lens?Whatisthenatureoftheimageatadistanceof80cmandthelens?

Ans.Magnificationis3timesandimagedistancev=80cm

Magnification=

Or,

Or,

Since,objectdistanceistakenasnegative;aspersignconventionso,

or,

Natureofimage:Sinceimageisformedontheothersideoflenshenceitisarealimageand

invertedimage.

NatureofLens:Enlargedimageisformedbyconvexlensandnotbyconcavelens,soitisa

convexlens.

36. Sizeofimageofanobjectbyamirrorhavingafocallengthof20cmisobservedtobe

reducedto1/3rdofitssize.Atwhatdistancetheobjecthasbeenplacedfromthe

mirror?Whatisthenatureoftheimageandthemirror?

Ans.Sinceimagesizeis1/3ofobjectsize,soimagedistanceis1/3ofobjectdistancebecause

h’/h=v/u

Usingthemirrorformula,wecancalculateobjectdistanceandimagedistance


or,

or,

or,u=-40cm

Usingthis,wecanfindimagedistance:

or,

or,

or,

Butabovevalueofimagedistancedoesnotmatchwithourinitialassumption.Thismeans

thatthemirrorisnotaconcavemirrorbutaconvexmirror.Letuscalculatewiththe

assumptionthatitisaconvexmirror.

or,

or,


or,

Natureofmirror:convexmirror

Natureofimage:Smallerthanobject,erectandvirtual

37. Definepowerofalens.Whatisitsunit?Onestudentusesalensoffocallength50

cmandanotherof–50cm.Whatisthenatureofthelensanditspowerusedbyeachof

them?

Ans.Thedegreeofdivergenceorconvergenceachievedbyagivenlensiscalledpowerofthe

lens.TheunitofpoweroflensisdiopterandisexpressedbyD.

Focallengthoflensusedbyfirststudentisinpositivehenceitisaconvexlens.Thelensof

secondstudentisaconcavelens.

Poweroflens(firststudent)=+2

Poweroflens(secondstudent)=-2

38. Astudentfocussedtheimageofacandleflameonawhitescreenusingaconvex

lens.Henoteddownthepositionofthecandle,screenandthelensasunder

Positionofcandle=12.0cm

Positionofconvexlens=50.0cm

Positionofthescreen=88.0cm

(i) Whatisthefocallengthoftheconvexlens?

Ans.

Imagedistancev=88-50=38cm


Focallengthcanbecalculatedusingthelensformula;

or,

or,

(ii) Wherewilltheimagebeformedifheshiftsthecandletowardsthelensatapositionof

31.0cm?

Ans.objectdistanceu=50–31=19cm

Inthiscase,objectdistance=focallength

Thismeansthatimagesisformedatinfinity.

(iii) Whatwillbethenatureoftheimageformedifhefurthershiftsthecandletowardsthe

lens?

Ans.Ifhefurthershiftsthecandletowardsthelens,theobjectcomesbetweenFandO.In

thiscase,imageisvirtual,enlargedanderectandisformedonthesamesideoflens.

(iv) Drawaraydiagramtoshowtheformationoftheimageincase(iii)assaidabove.

Ans.



chapter 10: light reflection and refraction · chapter 10 light-reflection and refraction 1. which...

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