Post on 28-Jul-2021
CBSEClass10Science
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)
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
4. Figure10.1showsarayoflightasittravelsfrommediumAtomediumB.Refractive
indexofthemediumBrelativetomediumAis
(a)
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(b)
(c)
(d)
Ans.(a)
Explanation:RefractiveindexofmediumBrelativetomediumAcanbecalculatedas
follows:
bysnell'slaw
nB/nA=sin60/sin45
5. AlightrayentersfrommediumAtomediumBasshowninFigure10.2.The
refractiveindexofmediumBrelativetoAwillbe
(a) greaterthanunity
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(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..
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(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?
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(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
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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?
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(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
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
EDandDCaretwoplanemirrorsplacedatrightanglestoeachother.Anisincidentrayon
CDandMNisnormalatpointofincidence.NQisreflectedrayfromNandisincidenton
pointQ.
QBisreflectedrayandPQisnormaltoED.
(anglei=angler)
(sameasabove)
(MNisnormalonCD)...(1)
(mirrorsareatrightangle)
Hence,in ;
(acuteanglesofarighttrianglearecomplementary)…(2)
Fromequations(1)and(2);
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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)
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
Long Answer Questions
30. Drawraydiagramsshowingtheimageformationbyaconcavemirrorwhenan
objectisplaced
(a) betweenpoleandfocusofthemirror
Ans.
(b) betweenfocusandcentreofcurvatureofthemirror
Ans.
(c) atcentreofcurvatureofthemirror
Ans.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(d) alittlebeyondcentreofcurvatureofthemirror
Ans.
(e) atinfinity
Ans.
31. Drawraydiagramsshowingtheimageformationbyaconvexlenswhenanobjectis
placed
(a) betweenopticalcentreandfocusofthelens
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
Ans.
(b) betweenfocusandtwicethefocallengthofthelens
Ans.
(c) attwicethefocallengthofthelens
Ans.
(d) atinfinity
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
Ans.
(e) atthefocusofthelens
Ans.
32. Writelawsofrefraction.Explainthesamewiththehelpofraydiagram,whenaray
oflightpassesthrougharectangularglassslab.
Ans.Lawsofrefractionareasfollows:
(a) Theincidentray,therefractedrayandnormalatthepointofincidence;alllieinthesame
plane.
(b) Theratioofsineofincidencetosineofrefractionisaconstant,forthelightofagiven
colourandforagivenpairofmedia.ThislawisalsocalledSnell’sLaw.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
(c) beyondtwicethefocallengthofthelens
Ans.
34. Drawraydiagramsshowingtheimageformationbyaconvexmirrorwhenanobject
isplaced
(a) atinfinity
Ans.
(b) atfinitedistancefromthemirror
Ans.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
or,
or,
or,u=-40cm
Usingthis,wecanfindimagedistance:
or,
or,
or,
Butabovevalueofimagedistancedoesnotmatchwithourinitialassumption.Thismeans
thatthemirrorisnotaconcavemirrorbutaconvexmirror.Letuscalculatewiththe
assumptionthatitisaconvexmirror.
or,
or,
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform
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.
www.physicsgurukul.com | www.successrouter.comOnline Support & Assessment Platform