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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)


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.

https://www.ncertbooks.guru/ncert-exemplar-problems-solutions-pdf/


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


(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


(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.


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.


Short Answer Questions

15. Drawraydiagramseachshowing

(i)myopiceyeand

Ans.

(ii) hypermetropiceye

Ans.

16. Astudentsittingatthebackoftheclassroomcannotreadclearlytheletterswritten

ontheblackboard.Whatadvicewilladoctorgivetoher?Drawraydiagramforthe

correctionofthisdefect.

Ans.Thisstudentisunabletoseefaroffobjects.Thismeansthatthestudentissuffering


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.


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


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.


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.


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.


CorrectionofHypermetropia:Ahypermetropicpersonneedstouseaconvexlensof

suitablefocallength.Theconvexlensconvergesthelightrayscomingfromanearbyobject.

Asaresult,theselightraysappeartobecomingfromthenearpointofthisperson’seyes.

Duetothis,aclearimageofnearbyobjectismadeontheretinaofthatperson.Thatishowa

hypermetropicpersonisabletoclearlyseenearbyobjects;withthehelpofsuitableconvex

lens.

27. Explaintherefractionoflightthroughatriangularglassprismusingalabelledray

diagram.Hencedefinetheangleofdeviation.

Ans.


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.


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.

https://www.ncertbooks.guru/ncert-exemplar-problems-solutions-pdf/

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