Post on 18-Mar-2021
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CBSEClass10Science
NCERTExemplarSolutions
Chapter11
HumanEyeandColourfulWorld
1.Apersoncannotseedistinctlyobjectskeptbeyond2m.Thisdefectcanbecorrected
byusingalensofpower
(a) +0.5D
(b) –0.5D
(c) +0.2D
(d) –0.2D
Ans.(b)-0.5D
Explanation:Thispersonissufferingfrommyopia.Heneedsaconcavelensandhence
powerwouldbeinnegative.
2. Astudentsittingonthelastbenchcanreadtheletterswrittenontheblackboardbut
isnotabletoreadtheletterswritteninhistextbook.Whichofthefollowing
statementsiscorrect?
(a) Thenearpointofhiseyeshasrecededaway
(b) Thenearpointofhiseyeshascomeclosertohim
(c) Thefarpointofhiseyeshascomeclosertohim
(d) Thefarpointofhiseyeshasrecededaway
Exercise
Multiple Choice Questions (MCQs)
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Ans.(a)Thenearpointofhiseyeshasrecededaway
Explanation:Inhypermetropiathenearpointofeyemovesawayfrom25cm.Duetothis,
thepersonneedstokeepabookatmorethan25cmtoreaditproperly.
3. AprismABC(withBCasbase)isplacedindifferentorientations.Anarrowbeamof
whitelightisincidentontheprismasshowninFigure11.1.Inwhichofthefollowing
cases,afterdispersion,thethirdcolourfromthetopcorrespondstothecolourofthe
sky?
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)
Ans.(b)(ii)
Explanation:IfprismiskeptwithbaseBCatbottom,thentheemergentbandofcolour
wouldshowvioletatthebottom.IfprismiskeptwithbaseBCattop,thenvioletwouldbeat
top;followedbyindigoandblue.
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4. Atnoonthesunappearswhiteas
(a) lightisleastscattered
(b) allthecoloursofthewhitelightarescatteredaway
(c) bluecolourisscatteredthemost
(d) redcolourisscatteredthemost
Ans.(b)allthecoloursofthewhitelightarescatteredaway
Explanation:Skywillappeardarkincaseofoption‘a’.Itwillappearblueincaseofoption
‘c’andwillappearredincaseofoption‘d’.
5. Whichofthefollowingphenomenaoflightareinvolvedintheformationofa
rainbow?
(a) Reflection,refractionanddispersion
(b) Refraction,dispersionandtotalinternalreflection
(c) Refraction,dispersionandinternalreflection
(d) Dispersion,scatteringandtotalinternalreflection
Ans.(c)Refraction,dispersionandinternalreflection
Explanation:Dispersionresultsinwhitelightgettingsegregatedintoitscomponentcolours.
Refractionbendstheincidentlighttoananglethatiscausesinternalreflection;andfinally
rainbowisformed.
6. Twinklingofstarsisduetoatmospheric
(a) dispersionoflightbywaterdroplets
(b) refractionoflightbydifferentlayersofvaryingrefractiveindices
(c) scatteringoflightbydustparticles
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(d) internalreflectionoflightbyclouds
Ans.(b)refractionoflightbydifferentlayersofvaryingrefractiveindices
Explanation:Duetorefractionoflightbydifferentlayersofvaryingrefractiveindices,the
apparentpositionofsourceoflightkeepsonchanging.Duetothis,starsappeartotwinkle.
7. Theclearskyappearsbluebecause
(a) bluelightgetsabsorbedintheatmosphere
(b) ultravioletradiationsareabsorbedintheatmosphere
(c) violetandbluelightsgetscatteredmorethanlightsofallothercoloursbytheatmosphere
(d) lightofallothercoloursisscatteredmorethanthevioletanbluecolourlightsbythe
atmosphere
Ans.(c)violetandbluelightgetscatteredmorethanlightsofallothercoloursbythe
atmosphere
8. Whichofthefollowingstatementsiscorrectregardingthepropagationoflightof
differentcoloursofwhitelightinair?
(a) Redlightmovesfastest
(b) Bluelightmovesfasterthangreenlight
(c) Allthecoloursofthewhitelightmovewiththesamespeed
(d) Yellowlightmoveswiththemeanspeedasthatoftheredandthevioletlight
Ans.(c)Allthecoloursofthewhitelightmovewiththesamespeed
9. Thedangersignalsinstalledatthetopoftallbuildingsareredincolour.Thesecanbe
easilyseenfromadistancebecauseamongallothercolours,theredlight
(a) isscatteredthemostbysmokeorfog
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(b) isscatteredtheleastbysmokeorfog
(c) isabsorbedthemostbysmokeorfog
(d) movesfastestinair
Ans.(b)isscatteredtheleastbysmokeorfog
10. Whichofthefollowingphenomenacontributessignificantlytothereddish
appearanceofthesunatsunriseorsunset?
(a) Dispersionoflight
(b) Scatteringoflight
(c) Totalinternalreflectionoflight
(d) Reflectionoflightfromtheearth
Ans.(b)Scatteringoflight
Explanation:Redcolourscatterstheleastandhencetravelsthefarthest.Duringsunsetor
sunrise,lighthastotravelalongerdistancetoreachus.Hence,onlyredlightreachestous
andtheskyappearsreddish.
11. Thebluishcolourofwaterindeepseaisdueto
(a) thepresenceofalgaeandotherplantsfoundinwater
(b) reflectionofskyinwater
(c) scatteringoflight
(d) absorptionoflightbythesea
Ans.(b)Reflectionofskyinwater
Explanation:Wateriscolourless.Itscolourappearstobesameastheobjectreflectedbyit.
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12. Whenlightraysentertheeye,mostoftherefractionoccursatthe
(a) crystallinelens
(b) outersurfaceofthecornea
(c) iris
(d) pupil
Ans.(b)Outersurfaceofthecornea
13. Thefocallengthoftheeyelensincreaseswheneyemuscles
(a) arerelaxedandlensbecomesthinner
(b) contractandlensbecomesthicker
(c) arerelaxedandlensbecomesthicker
(d) contractandlensbecomesthinner
Ans.(a)arerelaxesandlensbecomesthinner
14. Whichofthefollowingstatementiscorrect?
(a) Apersonwithmyopiacanseedistantobjectsclearly
(b) Apersonwithhypermetropiacanseenearbyobjectsclearly
(c) Apersonwithmyopiacanseenearbyobjectsclearly
(d) Apersonwithhypermetropiacannotseedistantobjectsclearly
Ans.(c)Apersonwithmyopiacanseenearbyobjectsclearly
Explanation:Thisisthereason;myopiaisalsoknownasnearsightedness.
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Short Answer Questions
15. Drawraydiagramseachshowing
(i)myopiceyeand
Ans.
(ii) hypermetropiceye
Ans.
16. Astudentsittingatthebackoftheclassroomcannotreadclearlytheletterswritten
ontheblackboard.Whatadvicewilladoctorgivetoher?Drawraydiagramforthe
correctionofthisdefect.
Ans.Thisstudentisunabletoseefaroffobjects.Thismeansthatthestudentissuffering
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frommyopia.Doctorwillprescribeaconcavelensofsuitablefocallength.
17. Howareweabletoseenearbyandalsothedistantobjectsclearly?
Ans.Humaneyeshavepowerofaccommodation.Whenwehavetoseedistantobjects,the
eyemusclesrelaxandlensbecomesthin.Duetothis,thefocallengthofthelensincreases
andtheeyeisabletoseedistantobjects.Whenwehavetoseenearbyobjects,theeye
musclescontractandlensbecomesthick.Duetothis,thefocallengthofthelensdecreases
andtheeyeisabletoseenearbyobjects.
18. Apersonneedsalensofpower–4.5Dforcorrectionofhervision.
(a)Whatkindofdefectinvisionisshesufferingfrom?
Ans.Myopia
(b)Whatisthefocallengthofthecorrectivelens?
Ans. or,
(c)Whatisthenatureofthecorrectivelens?
Ans.Thenegativesignshowsthatitisaconcavelens.
19. Howwillyouusetwoidenticalprismssothatanarrowbeamofwhitelightincident
ononeprismemergesoutofthesecondprismaswhitelight?Drawthediagram.
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Ans.Forthis,oneprismisplacednearanotherprismsothatoneprismisinerectposition
andanotherprismisininvertedposition.Whenrayofwhitelightentersthefirstprism,
dispersionoflighttakesplace.Whenlightsofdifferentcolourspassthroughthesecond
prism,theyrecombinetomakearayofwhitelight.
20. Drawaraydiagramshowingthedispersionthroughaprismwhenanarrowbeam
ofwhitelightisincidentononeofitsrefractingsurfaces.Also,indicatetheorderofthe
coloursofthespectrumobtained.
Ans.
Orderofcoloursfrombottomtotop:Violet,Indigo,Blue,Green,Yellow,OrangeandRed.
21. Isthepositionofastarasseenbyusitstrueposition?Justifyyouranswer.
Ans.Thedensityofatmosphericlayersincreasesaswemovefromtoptobottom.Duetothis,
starlightbendstowardsthenormalasitpassesthroughdifferentlayersofatmosphere.Due
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tothis,theapparentpositionofstarisalittleaboveitsactualpositioninsky.
22. Whydoweseearainbowintheskyonlyafterrainfall?
Ans.Formationofrainbowisonlypossiblewhendispersionoflighttakesplacethrougha
suitablesurface.Afterrainfall,someraindropsremainintheclouds.Moreover,theopposite
sideofskyworkslikeascreenonwhichrainbowisformed.Hence,rainbowisseeninthe
skyonlyafterrainfall.
23. Whyisthecolouroftheclearskyblue?
Ans.Outofallthecoloursinthevisiblespectrum,bluecolourscattersthemost.Duetothis,
itisthebluecolourwhichreachesoureyes.Asaresult,thecolourofskyappearsblue.
24. WhatisthedifferenceincoloursoftheSunobservedduringsunrise/sunsetand
noon?Giveexplanationforeach.
Ans.Theskyappearsreddishduringsunrise/sunsetbutitappearswhiteatnoon.During
noon,thesunlighthastotravellessdistancetoreachus.Mostofthecoloursreachingusget
scattered.Duetothis,Skyappearswhiteatnoon.
Coloursneartheredendofthespectrumscattertheleast.Duringsunset,andsunrise,
sunlightneedstotravelmoredistancetoreachus.Redcolourisabletoreachusbecauseitis
scatteredtheleast.Hence,skyappearsreddishduringsunrise/sunset.
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Long Answer Questions
25.ExplainthestructureandfunctioningofHumaneye.Howareweabletoseenearby
aswellasdistantobjects?
Ans.Thehumaneyehasfollowingmainparts:
Cornea:Humaneyeissphericalinshape.Ithastoughwhitecoatwhichprotectstheinterior
oftheeye.Thefrontportionofthiscoatistransparentandiscalledcornea.
Iris:Thisisadarkmuscularstructurebehindthecornea.Uniquecolourofaperson’seyeis
becauseofcolourofiris.
Pupil:Thesmallopeningintheirisiscalledpupil.Iriscontrolsthesizeofthepupilandthus
controlstheamountoflightenteringtheeye.Lightenterstheeyethroughpupil.
Lens:Lensisthickeratthemiddleandismadeoftransparentmaterial.Lensfocusesthe
lightonthebackoftheeye;calledretina.
Retina:Thebackoftheeyeiscalledretina.Itworkslikeascreen;onwhichimageisformed.
.Rodsandconesfunctionasphotoreceptorsintheretinaoftheeye,turningvisiblelightinto
neuronalsignals,whicharesenttothebrain.Thisprocessiscalledtransduction.
Thesenervecellsareconnectedtoopticnerve.
FormationofImageinEye:Lightraysentertheeyethroughpupilandpassthroughlens.
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Lensfocuseslightraysonretina.Real,invertedandsmallerimageisformedonretina.Optic
nervecarriesthemessagetothebrain.Thebraininterpretsthemessageandwegetthe
senseofvision.
26. Whendoweconsiderapersontobemyopicorhypermetropic?Explainusing
diagramshowthedefectsassociatedwithmyopicandhypermetropiceyecanbe
corrected?
Ans.Whenapersonisunabletoclearlyseedistantobjects,heisconsideredamyopic
person.Suchapersonissufferingfrommyopia.Thishappenswhenimageisformedinfront
oftheretina.
Whenapersonisunabletoclearlyseeanearbyobject,heisconsideredahypermetropic
person.Suchapersonissufferingfromhypermetropia.Thishappenwhenimageisformed
behindtheretina
CorrectionofMyopia:Apersonsufferingfrommyopianeedstouseaconcavelensof
suitablefocallength.Theconcavelensdivergestherayscomingfrominfinity.After
refractionfromtheconcavelens,theraysappeartobecomingfromthefarpointofthis
person’seye.Duetothis,aclearimageofdistantobjectismadeontheretinaofthatperson.
Thatishowamyopicpersonisabletoclearlyseedistantobjects;withthehelpofsuitable
concavelens.
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CorrectionofHypermetropia:Ahypermetropicpersonneedstouseaconvexlensof
suitablefocallength.Theconvexlensconvergesthelightrayscomingfromanearbyobject.
Asaresult,theselightraysappeartobecomingfromthenearpointofthisperson’seyes.
Duetothis,aclearimageofnearbyobjectismadeontheretinaofthatperson.Thatishowa
hypermetropicpersonisabletoclearlyseenearbyobjects;withthehelpofsuitableconvex
lens.
27. Explaintherefractionoflightthroughatriangularglassprismusingalabelledray
diagram.Hencedefinetheangleofdeviation.
Ans.
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ABCisprismwithbaseBC
PEisincidentrayonsurfaceAB.Itmakes withthenormalNE.Thisangleis
angleofincidence.
Afterenteringtheprism,thelightraybendstowardsnormal.Inthiscase,EFis
refractedray. isangleofrefraction.
Oncetherefractedrayemergesfromprismintoair,itbendsawayfromnormal.In
thisfirgure,FSisemergentray. isangleofemergence.
AngleofDeviation:Theanglebetweenincidentrayandemergentrayiscalledangleof
deviation.Here, isangleofdeviationor
28. Howcanweexplainthereddishappearanceofsunatsunriseorsunset?Whydoesit
notappearredatnoon?
Ans.Coloursneartheredendofthespectrumscattertheleast.Thishappensbecauseof
shortwavelengthofreddishcolours.Duringsunsetandsunrise,sunlightneedstotravel
moredistancetoreachus.Redcolourisabletoreachusbecauseitisscatteredtheleast.
Hence,skyappearsreddishduringsunrise/sunset.
Theskyappearsreddishduringsunrise/sunsetbutitappearswhiteatnoon.Duringnoon,
thesunlighthastotravellessdistancetoreachus.Mostofthecoloursreachingusget
scattered.Duetothis,skyappearswhiteatnoon.
29. Explainthephenomenonofdispersionofwhitelightthroughaglassprism,using
suitableraydiagram.
Ans.
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Whenrayoflightentersaprism,itbendsbecauseofrefractionoflight.Whentherayoflight
finallyemergesoutoftheprism,itdeviatesdrasticallyfromitsoriginalpath.Thishappens
becauseofuniqueshapeofprism.
Differentcoloursinthevisiblespectrumhavedifferentspeeds.Duetothis,differentcolours
bendatdifferentanglesofdeviation.Asaresult,theemergentlightappearsasabandof
sevencolours;thecolourswhicharethecomponentsofwhitelight.ThesecoloursareViolet,
Indigo,Blue,Green,Yellow,OrangeandRed.
Segregationofwhitelightintoitsdifferentcomponentsiscalleddispersionoflight.
30. Howdoesrefractiontakeplaceintheatmosphere?Whydostarstwinklebutnotthe
planets?
Ans.Atmosphereismadeupofseverallayers.Thelayeratthetopisopticallyrare,whilethe
layeratthebottomisopticallydenser.Duetothis,whenlighttravelsthroughdifferent
layersoftheatmosphere,refractiontakesplace.Sincelightpassesthroughdenserand
denserlayerasitmovesthroughatmosphere,ittendstobendtowardsthenormal.
Starsareveryfarfromus;comparedtoplanet.Duetothis,starsserveaspointsourceof
light.Asaresult,evenaslightestchangeintheirapparentpositionintheskyisclearly
perceivedbyus.Hence,starsappeartotwinkle.
Planetsontheotherhand,areneartous.Hence,theydonotserveaspointsourceoflight.
Hence,minorchangesintheirapparentpositionarenotperceivedbyus.Hence,planetsdo
notappeartotwinkle.