CBSEClass10Science

ImportantQuestions

Chapter11

HumanEyeandColourfulWorld

1MarksQuestions

1. Whenapersonismyopic,he/shecanclearlysee

(a) bothnearbyandfaroffobjects

(b) Onlynearbyobjects

(c) onlyfaroffobjects

(d) Neithernearbynorfaroffobjects

Ans.(b)Onlynearbyobjects

2. Thedefectofmyopiacanbecorrectedbyusing

(a) Concavelens

(b) Convexlens

(c) Eitherconcaveorconvex

(d) Acomplicatedcombinationoflenses.

Ans.(a)Concavelens

3. Thecolourthatisscatteredtheleastbythetinyparticlesandtheatoms/moleculesof

theatmosphereis

(a) Violet

(b) Green

1

(c) yellow

(d) Red

Ans.(d)Red

4. Whichofthefollowingphenomenoncontributessignificantlytothereddish

appearanceofthesunatsunriseorsunset?

(a) Dispersionoflight

(b) Scatteringoflight

(c) TotalinternalReflection

(d) Reflectionoflightfromtheearth

Ans.(b)Scatteringoflight

5. Thefocallengthoftheeyelensincreaseswheneyemuscles.

(a) arerelaxedandlensbecomesthinner

(b) contractandlensbecomesthicker

(c) arerelaxedandlensbecomesthicker

(d) Contractandlensbecomesthinner.

Ans.(a)arerelaxedandlensbecomesthinner

6. Definepowerofaccommodation?

Ans.Theabilityoftheeyelenstoadjustitsfocallengthiscalledpowerofaccommodation.

7. Whichpartofthehumaneyeprovidesmostoftherefractionforthelightrays

enteringtheeye?

2

Ans.CorneaandAqueoushumour

8. Whathappenstotheimagedistanceintheeyewhenweincreasethedistanceofan

objectfromtheeye?

Ans.Itremainsthesame

9. Whathappenstothepupiloftheeyewhenthelightisverybright?

Ans.Sizeofthepupilbecomessmall.

10. Whichpartofthehumaneyeconveystheelectricalsignalsgeneratedbythelight

sensitivecellsoftheretina,onthebrain?

Ans.Opticnerves

11. Thehumaneyecanfocusobjectsatdifferentdistancesbyadjustingthefocallength

oftheeyelens.Thisisdueto

(a) Presbyopia

(b) Accommodation

(c) Near-sightedness

(d) Far-sightedness

Ans.(b)Accommodation

12. Thehumaneyeformstheimageofanobjectatits

(a) Cornea

(b) Iris

(c) Pupil

3

(d) Retina

Ans.(d)Retina

13. Theleastdistanceofdistinctvisionforaneyelensiscausedbytheactionofthe

(a) 25m

(b) 2.5cm

(c) 25cm

(d) 2.5m

Ans.(c)25cm

14. Thechangeinfocallengthofaneyelensiscausedbytheactionofthe

(a) Pupil

(b) Retina

(c) Cilliarymuscles

(d) Iris

Ans.(c)Cilliarymuscles

15. Whatwouldhavebeenthecolouroftheskyiftherehadnotbeenanyatmosphere

aroundtheearth?

Ans.black.

16. Fordispersionoflightthroughaprismwhichcolourhasmaximumdeviation?

Ans.violet

17. Whatistheleastdistanceofdistinctvisionofanormalhumaneye?

4

Ans.25cm.

18. Namethemuscleresponsibleforbringingchangeinthefocallengthoftheeyelens?

Ans.Ciliarymuscle.

19. Nameonedefectofvisionwhichcannotbecorrectedbyanytypeofspectacle

lenses?

Ans.Cataract.

20. Stateoneeffectproducedbythescatteringoflightbytheatmosphere?

Ans.Tyndalleffect.

21. Whatisthenatureofimageformedontheretinaoftheeye?

Ans.Realandinverted.

22. Whattypeoflensisusedforcorrectinghypermetropia?

Ans.Convexlens.

23. Whowasthefirstpersontoobtainthespectrumofsunlight?

Ans.SirIsaacNewton.

24. Aslightrayspassfromairintoglassprism,aretheyrefractedtowardsoraway

fromthenormal?

Ans.Towardsthenormal.

25. Whichcolorhaslargestwavelength?

Ans.Redcolor.Ant

5

26. Whichdefectofvisioncanberectifiedusingaconcavelens?

Ans.Myopia.

27. Whatphenomenoncausestwinklingofstaronaclearnight?

Ans.Atmosphericrefraction.

28. Whatismeantbyscatteringoflight?

Ans.Changeofdirectionoflightonstrikingascattered.

2MarksQuestions

1. Thefarpointofamyopicpersonis80cminfrontoftheeye.Whatisthenatureand

powerofthelensrequiredtocorrecttheproblem.

Ans.

2. Drawadiagramtoshowthedispersionofwhitelightbyaglassprism.

Ans.

6

3. Namethephenomenonresponsiblefortheobservedtwinklingofstars.Willthis

twinklingbeobservedbyanobserveronthemoon.

Ans.Atmosphericrefractionisresponsiblefortwinklingofstars.

Theobserveronthemoonwillnotobservetwinklingofstarsasmoonhasnoatmosphere.

4. Namethepartoftheeyethat

(a) determinesthecolourofaperson’seye

(b) Controlstheamountoflightenteringtheeye

Ans.(a)Conescellpresentinretina

(b) Iris

5. Whatistheroleoftheciliarymuscles?

Ans.Theciliarymusclesholdstheeyelensinitspositionandwhenthemusclescontractand

relaxitchangestheshapeoftheeyelenswhichisturnincreasesordecreasesthefocal

lengthoftheeyelens

6. Whichconvexlensiscalledasconverginglens?

Ans.Aconvexlensiscalledasaconverginglensbecauseitfocusesalltheparallelbeamof

lightatitsfocus.

7. Statetheroleofeyelensinthehumaneye?

Ans.Iffocusesthelightsaysenteringtheeyetoformarealandinvertedimageoftheobject

ontheretina.

8. Whatismeantbypowerofaccommodationofeye?

Ans.Theabilityoftheeyelenstoadjustitsfocallength,soastoclearlyfocusrayscoming

fromdistantaswellasnearobjectsontheretina,iscalledthepowerofaccommodationof

theeye.

7

9. Apersonwithamyopiceyecannotseeobjectsbeyond1.2mdistinctly.Whatshould

bethecorrectivelensusedtorestorepropervision?

Ans.Apersonwithamyopiceyeshoulduseaconcavelensoffocallength1.2msoasto

restorepropervision.

10. Whatisthefarpointandnearpointofthehumaneyewithnormalvision?

Ans.Forahumaneyewithnormalvisionthefarpointisatinfinityandthenearpointisat

25cmfromtheeye.

11. Astudenthasdifficultyreadingtheblackboardwhilesittinginthelastrow.What

couldbethedefectthechildissufferingfrom?Howcanitbecorrected?

Ans.Thestudentissufferingfrommyopiaorshortsightedness.Thedefectcanbecorrected

bytheuseofconcavelensofsuitablepower.

12. Thefarpointofamyopicpersonis80cminfrontoftheeye.Whatisthenatureand

powerofthelensrequiredtocorrecttheproblem?

Ans.Tocorrectthemyopiathepersonconcernedshoulduseconcavelensoffocallength(f)=

-80cm=-0.80mPoweroflens(P)=1/f(m)=1/-0.80=100/-80=-1.25D.

13. Whyisanormaleyenotabletoseeclearlytheobjectsplacedcloserthan25cm?

Ans.Duetolimitofpowerofaccommodation,thefocallengthoftheeyelenscannotbe

decreasedbelowcertainminimumlimit.So,anormaleyecannotseeclearlytheobjects

placedcloserthanminimumdistance,callednearpointoftheeye.

14. Whathappentotheimagedistanceintheeyewhenweincreasethedistanceofan

objectfromtheeye?

Ans.Theimageisformedontheretinaevenonincreasingthedistanceofanobjectfromthe

eye.Infact,theeyelensbecomesthinneranditsfocallengthincreasesastheobjectismoved

awayfromtheeyeandconsequentlyimageisformedontheretina.

8

15. WhydoestheSunappearreddishearlyinthemorning?

Ans.Inearlymorning,theSunissituatednearhorizon.LightfromtheSunpassesthrough

thickerlayersofairandcoversalargerdistanceinearth’satmospherebeforereachingour

eyes.WhilepassingthroughatmospherebluelightismostlyscatteredawayandtheSun

appearreddish.

16. Apersonwearseyeglassoffocallength70cmwhatisthefarpointoftheperson?

Ans.l/f=l/v-l/u

V=?

F=-70cm

1/v=1/-70v=-70cm

17. Ifyoureyeglasseshavefocallength60cmwhatisyournorpoint?

Ans.l/f=l/v-l/u

1/60=l/v+1/25

v=-43cm.

18. Whydoweobserverandomwaveringorflickingoftheobjectsnearafireorona

veryhotday?

Ans.Areaabovethefireishot,anditsdensityandhencerefractiveindexchanges

frequently.Therefore,apparentimageoftheobjectalsochanges.

9

19. Whyarewenotabletoseethethingsclearlywhenwecomeoutofadarkroom?

Ans.Whenweareindark,pupilsizeisbigger.Aswecomeoutofdarkroom,itssizeneedsto

Becomesmaller.Forthattime-intervalpersonisunabletosee.

20. Whatisthefunctionofopticnerveinhumaneye?

Ans.Opticnervecarriestheimageformedontheretinatothebrainintheformofelectrical

signals.

21. Whydodifferentcoloursdeviatethoughdifferentanglesonpassingthrougha

prism?

Ans.Thisisbecausedifferentcolourstravelthroughglasswithdifferentspeedsandglass

hasdifferentrefractiveindexfordifferentcolours.

22. Namethedefectofvisioninperson

a. Whosenearpointismorethan25cmaway’?

b. Whosefarpointislessthaninfinity.

Ans.a.Hypermetropia

b.Myopia

23. Whatisaspectrum?

Ans.Thebandofsevencoloursobtainedbydispersionofwhitelightiscalledspectrum.

24. Whydoesclearskylookblue?

Ans.Theskyappearsbluebecausethetinyparticlesheldintheatmospherescatterlightsof

shorterwavelengthlikebluelight....thislightisreceivedbyoureyeandtheskyappears

blu...intheabsenceofatmosphere,scatteringdoesnottakeplaceandtheskyappears

dark....

10

25. Canvisiblelightbescatteredbyatoms/moleculesinearth’satmosphere?

Ans.Yes,assizeofmolecules/atomsimuchlessthenwavelengthoflight

26. Whydoesthesyappearacmouetoanastronaut?

Ans.Theupperatmospheredoesnothaveparticleordustetc.asaresultthereisno

scatteringoflightandhencetheskyappeardarkinsteadofbluetoanastronaut.

27. Whatisthebasiccauseofatmosphericrefraction?

Ans.Thebasiccauseofrefractionisvariationinopticaldensityofdifferentlayersofearth’s

atmosphere.Thesunpassesthroughearth’satmosphere.

28. Whatisrangeofvision?

Ans.Rangeofvisionofanormalhumaneyeisbetweenit’snearpointandfarpointis,from

25cmtoinfinity.

3MarksQuestions

1. (a)Whatishypermetropia?

(b) Whatarethetwocausesofthisdefectofvision?

Ans. (a)Hypermetropia is thedefect inwhichapersoncan seedistantobjects clearlybut

cannotseethenearbyobjects.

(b) Thetwopossiblecausesofthisdefectare

(i) Increaseisthefocallengthoftheeyelens

(ii) Eyeballgetsshortened

11

2. (a)Whatisscatteringoflight?

(b) Astronautsobservetheskyasdarkinsteadofbluewhy?

Ans. (a)The re-scattering of light by the scatterers present is the atmosphere in different

directionsafterbeingabsorbedbythemiscalledscattering.

(b) The blue of sky is due to scattering of blue colour by the earth’s atmosphere. For the

astronaut there would be no such scattering effect. Hence the sky appears dark to the

astronaut.

3. Apersonisknowntousealensofpower

(i) -5.5Dforhisdistantvision

(ii) +1.5Dforhisnearvision

Calculatethefocallengthofthelensusedforcorrectinghis

(a) Distantvisionand(b)Nearvisionproblems.

Ans.(i)Fordistantvision

(ii) Fornearvision

12

4. What ispresbyopia?State thecausesof thisdefect?Howispresbyopiaofaperson

corrected?

Ans.Presbyopiaisthedefectofhumaneyeinwhichapersonisunabletoseethenearbyas

wellasfaroffobjectsclearly.

Causes:

(1) Decreaseisthepowerofaccommodationoftheeyeduetoageing

(2) WeakeningoftheciliarymusclesItcanbecorrectedbyusingabifocallens(upperhalf

concaveandthelowerhalfconvex)

5. Therainbowisanaturalspectrumappearingintheskyafterarainshower

(a) Isitcorrecttosaythatarainbowisalwaysformedinadirectionoppositetosun?

(b) Canitbeseenonasunnyday?

(c) Arrange the sequence in correct sequential order Refraction, Internal Reflection,

Refraction&Dispersion

Ans.(a)Yes

(b) No,wecannotobservearainbowona sunnydaywhenwe lookat thesky througha

waterfall

orfountainwiththesunbehindus.

(c) Refractionanddispersion,internalreflection,Refraction.

6. (a)Writetwocausesofhypermetropia?

(b) Showdiagramtoshowthecorrectnessofhypermetropia?

Ans.(a)Causesofhypermetropia

(1) Increaseinthefocallengthoftheeyelens

(2) Decreaseinthesizeoftheeyeball.

13

(b)

7. Areporterrecordsthefollowingobservationsofanastronautfromhisspaceship.

(a) Thelengthofthedayissameasobservedontheearth.

(b) Skyappearsblackincolour.

(c) Thestarappearstotwinklewhiletheplanetsdonotdosoastheydoontheearth.

Justifyeachstatement

Ans. (a) It is incorrect as length of theday in spacewouldbe about fourminutes shorter

thanthatontheearth.

(b) Itiscorrectbecauseinspacewecannotobservescatteringoflightbecauseofabsenceof

atmosphere.

(c) Itisincorrectbecausetwinklingofstarsisduetoatmosphericrefractionbutinspacewe

donothaveatmosphereandatmosphericparticles.

14

8. A person needs a lens of power -5.5 dioptre for correcting his distinct vision. For

correctinghisnearvisionheneedsalens+1.5dioptre.Whatisthefocallengthofthe

lensrequiredforcorrecting(i)distinctvision,and(ii)nearvision?

Ans.(i)Poweroflensneededforcorrectiondistantvisionoftheperson(P)=-5.5D

Focallengthoflensrequiredforcorrectingdistantvision(f)

=1/P=1/-5.5m=0.18m=18cm.

(ii) Forcorrectingnearvisionthepoweroflensrequired(P)=+1.5D

Focallengthoflensrequiredforcorrectingnearvision(f)

=1/P=1/1.5m=0.67m=66.7cm.

9. Whydostarstwinkle?

Ans.Starstwinkleduetoatmosphericrefractionofstarlight.Asthestarsareveryawaythey

behaveasalmostpointsourcesoflight.Asonaccountofatmosphericrefraction,thepathof

raysoflightcomingfromthestargoesonvaryingslightly,theapparentpositionofthestar

fluctuates and the amount of light entering the eye flickers, so sometimes the star appear

brighterandatsomeothertime,fainter.Thusthestarstwinkle.

10. Explainwhytheplanetsdonottwinkle.

Ans.Planetsaremuchclosertotheearthandareseenasextendedsource.So,aplanetmay

beconsideredasacollectionofa largenumberofpoint-sized light sources.Although light

comingfromindividualpoint-sizedsourcesflickersbutthetotalamountoflightenteringour

eyefromalltheindividualpoint-sizedsourcesaverageouttobeconstant.Thereby,planets

appearequallybrighterandthereisnotwinklingofplanets.

15

11. Whydoestheskyappeardarkinsteadofbluetoastronaut?

Ans.Blue colour of the sky is on account of scattering of light of shorter wavelength by

particlesintheatmosphereofearth.Iftheearthhadnoatmosphere,therewouldnothave

beenanyscatteringandskywouldhavelookeddark.Whenastronautinhisspacecraftgoes

above the atmosphere of earth, sky appears dark to himbecause there is no scattering of

light.

12. Acertainpersonhasminimumdistanceofdistinctvisionof150cm .Hewishes to

readatadistanceof25cm.Whatfocallengthglassshouldheuse?Whatisthenatureof

eyedefect?

Ans.-U=-25cmV=-l5Ocm.

1/f=1

1/f=1/-1/(-25)

f=3Ocm.

fbeing+ve,lenseusedisconvexlens.

Hypermetropic

5MarksQuestions

1. A 14 year old student is not able to see clearly the questions written of the black

boardplacedatadistanceof5mfromhim.

(a) Namethedefectofvisionheissufferingfrom?

(b) Drawthediagramtoshowthisdefect?

(c) Namethetypeoflensusedtocorrectthisdefect?

(d) Nametwopossiblecauseofthisdefect.

(e) Drawthediagramtoshowhowthisdefectcanbecorrected.

16

Ans.(a)Thestudentissufferingfrommyopia.

(b)

(c) Concavelens

(d) (i)Decreaseinthefocallengthoftheeyelens

(ii) Eyeballgetselongated.

(e)

2. Make a diagram to show how hypermetropia is corrected. The near point of a

hypermetropiceyeis1m.Whatisthepowerofthelensrequiredtocorrectthisdefect?

Assumethatnearpointofthenormaleyeis25cm.

Ans.Diagramrepresentingthecorrectionofhypermetropiaisafollows:

17

Nearpointofdefectiveeyeis1mandthatofnormaleyeis25cm.

Here,u=-25cm,v=-1m=100cm.

Usinglensformula

1/f=1/v–1/u

1/f=1/-100+1/25=3/100

f=100/3cm=1/3m.

P=1/f(m)=1/0.33=+3.0D.

18