1 3.Blacksurfacesarepoorabsorbersofheat. TRUEORFALSE · 2020. 4. 17. · ASP-X-PHY-RL 1 Thevarioussourcesofenergyarethesun,thewind,water,fossilfuelsetc. Agoodsourceofenergyisonewhichsupplieslargeamountofusefulenergy,easilyavailable

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Thevarioussourcesofenergyarethesun,thewind,water,fossilfuelsetc.

Agoodsourceofenergyisonewhichsupplieslargeamountofusefulenergy,easilyavailable, economicaland

causeminimum environmentalpollution.

Electricityproducedbyflowingwaterisknownashydro-electricpower.

Biomassisamaterialwhichcontainscarbonandothercombustiblematerial.

Plants,woodanimalsandplantswastearetheexamplesofbiomass.

Gobargasorbio-gasistheexampleofabio-massenergysource.

Mainconstituentofabiogasorgobargasismethanegas.

Biogasplantisanarrangementofproducingbiogasfrom animaldung,humanexcreta,industrialand

domesticwastes.

Biogasplantisoftwotypes:a)Fixed-dometype,b)Floatinggasholdertype

Constantandrapiduseofconventionalsourcesofenergywouldultimatelyexhaustthesesourcesand hencea

needfortappingenergyfrom alternateornon-conventionalsourcesofenergyisseriously felt.

Solarconstantisdefinedastheenergyreceivedfrom thesuninonesecondbyaunitsquaremeter areaof

theouteredgeofearth’satmosphereexposedperpendiculartotheradiationofthesunatan averagedistance

betweenthesunandtheearth.

Valueofsolarconstant

Waterduetoitshighspecificheatcapacity isastorehouseofheatenergy.

Energyfrom seaoroceanwaterisaavailable intheform of

Energyofseawaves

Tidalenergyand

Oceanthermalenergy(OTE)

Theheatenergystoredinthehotspotsofearth’scrustiscalledgeo-thermalenergy.

Theenergyobtainedfrom theconversionofnuclearmassisknownasnuclearenergy.

Nuclearenergyisobtainedbytwoprocessesknownasnuclearfissionandnuclearfusion.

Nuclearfusionistheprocessoffusingorcombiningtogethertwosmallnucleitoform acomparatively big

nucleuswiththereleaseoflargeenergy.

Nuclearfusionreactionsoccuratveryhightemperature

Sourceofenergyareclassifiedintotwocategoriesi)conventionalornon–renewablesourcesof energy

andii)Non-conventionalorrenewablesourcesofenergy.

TRUEORFALSE

1. Solarenergyhasthegreatestpotentialofalltherenewablesourcesofenergies.

2. Reflectorisusedinsolarheatingdevicestoconcentratethesunheat.

3. Blacksurfacesarepoorabsorbersofheat.

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4. Glasscoverinsolarheatingdevicesallowstheshortwavelengthofinfraredradiationstoenterthe solar

cooker.

5. Sphericalreflectorsareusedtoobtainhightemperaturesinsolarheatingdevises.

6. Solarcellsaremadeofsemiconductormaterials.

7. Solarselenium solarcellcanconvert25%ofsunlightintoelectricalenergy.

8. Solarcellsandsolarpanelscannotbeusedinremoteareas.

FILLINTHEBLANKS

9. Solarcellconverts_________ energyinto____________

10. __________typeofreflectorisusedinaboxtypesolarcooker.

11. Inordertotrapthesun’sheatinsideasolarheatingdevice_________isused.

12. Solarcellissmallwaferof_________devices.

13. Theothernameofsolarcellis_________

14. Agroupofsolarcells,arrangedinadefinitepatterniscalleda_______

15. Theefficiencyofasolarcellisaround________

16. Flowingwatercontains___________form ofenergy.

17. Inhydroelectricity________form ofenergyconvertsto______form.

18. Periodicriseandfallofoceanwatertwiceinadayiscalled__________

19. Oceanthermalenergyisduetodifferencein_________

MULTIPLECHOICEQUESTIONS

20. Whichofthefollowingisnotanon-renewablesourceofenergy?

1)Coal 2)Petroleum 3)Wood 4)Naturalgas

21. Thefractionofsun’senergyreceivedontheearthisabout

1)47% 2)12% 3)38% 4)57%

22. Whichofthefollowingsourcesofenergyisdifferentfrom theother?

1)coal 2)plants 3)petroleum 4)lignite

23. Whichofthefollowingsourcesofenergyisdifferentfrom theother?

1)coke 2)gobargas 3)bitumen 4)anthracite

24. Choosetheonlyrenewablesourceofenergyfrom thefollowing

1)Coal 2)Geothermalpower 3)Uranium 4)Naturalgas

25. Thelongestandtheshortestwavelengths(inmicro)forredandvioletlightarerespectively

1)0.3,0.6 2)0.4,0.7 3)0.7,0.9 4)0.7,0.4

26. Thenuclearfuelinthesunis

1)Helium 2)Uranium 3)Hydrogen 4)Radium

27. Minimum velocityofwindrequiredforafunctionalwindmillisabout

1) 2) 3) 4)anyvelocitywilldo

28. Thetemperaturedifferencerequireduptoawaterleveldifferenceof1000m foroperatinganOTEC system

is

1) 2) 3) 4)

29. Theenergyliberatedbywoodonburningis

1)17kJ 2)15kJ 3)20kJ 4)33kJ

SYNOPSIS

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CHARACTERISTICSOFLIGHT:

Lightpossessesthefollowingcharacteristics:

Thespeedoflightinvacuum is3×108ms–1.

Lightcanpassthroughtransparentmedium likeglassorair,butcannotpassthroughopaquematerialsuchas

woodormetals.

Lighttravelsinastraightline.

Lightgetsreflectedwhenitfallsonpolishedsurfaces;likemirrors.

Lightsuffersrefractionwhenittravelsfrom onemedium toanother.

Thereisachangeinthewavelengthoflightwhenitmovesfrom onemedium intoanother.

Thespeedoflightchangeswhenitmovesfrom onemedium intoanother.Itsspeedisamaximum invacuum.

Thespeedoflightinglassandwaterislesserthanthatinvacuum.

SOMETERMS:

Source:Abodywhichemitslightinalldirectionsissaidtobethesourceoflight.Thesourcecanbeapointoneor

anextendedone.Asourceisoftwokinds.

● Self-luminous:Self-luminoussourceisthesourcewhichpossesseslightofitsown,e.g.,sun, electric

arccandle,etc.

● Non-luminous:Itisasourceoflightwhichdoesnotpossesslightofitsownbutreceiveslight

from anexternalsourceandscattersittothesurrounding,e.g.,moon,pageofyourbook, table,

etc.

Medium:Asubstancethroughwhichlightpropagatesortendstopropagateiscalledamedium.Itisoffollowing

threekinds:

● Transparent:Itisamedium throughwhichlightcanbepropagatedeasily,e.g.,glass,water,

etc.

● Translucent:Itisamedium throughwhichlightispropagatedpartially,e.g.,paper,ground,

glass,etc.

● Opaque:Itisamedium throughwhichlightcannotbepropagated,e.g.,wood,iron,etc.

Ray:Thestraightlinepathalongwhichthelighttravels,inahomogeneousmedium,iscalledaray.

Itisrepresentedbyanarrowheadonastraightline;thearrow

headgivesthedirectionofpropagationoflight.(Figurebelow)

Figure

Beam:Anumberofrayscombinedtogetheriscalledabeam.

Pencil:Anarrowbeam ofraysiscalledapencil.Apencilisofthreekinds:

● Convergent:Itisapencilinwhichwidthofthepencilgoesondecreasingastherays

proceedforward.(Figure)

Converging

● Divergent:Itisapencilinwhichalltheraysmeetatapointwhenproducedbackwardand

thewidthofpencilgoesonincreasingastheraysproceedforward.(Figure)

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Diverging

● Parallel:Itisapencilinwhichalltheraysmoveparalleltoeachotherandthewidthofthe

pencilremainsconstantthroughout.(Figure)

Parallel

Images:Animageisapointorplanetowhichraysareconvergedorfrom whichtheyappeartodivergeafter

reflectionorrefractionoflight.Imagesareformedatthepointwhereraysoflightactuallyintersectoratthepoint

from wheretheyappeartooriginate.Imagesareclassifiedas

RealandVirtual.

● Realimage:Whentheraysoflight,afterreflectionfrom amirror,actuallymeetatapoint, then

theimageformedbytheseraysissaidtobereal.Realimagescanbeobtainedona screen.

● Virtualimage:Whentheraysoflight,afterreflectionfrom amirror,appeartomeetata

point,thentheimageformedbytheseraysissaidtobevirtual.Virtualimagescan'tbe obtainedona

screen.

REFLECTIONOFLIGHT:

Whenalightraytravellinginamedium encountersaboundaryleadingintoasecondmedium,partoftheincident

rayisthrownbackintotheoriginalmedium.

Thisphenomenoniscalledreflectionoflight.Inotherwordsthebouncingbackoflightwhenitstrikesasmooth

orpolishedsurfaceiscalledreflectionoflight.

Reflectionisoftwotypes:

● Diffusereflectionorirregularreflectionand

● Specularreflectionorregularreflection.

LAWSOFREFLECTION:

Allsurfaceswhich,canreflectlight,obeythefollowingtwolaws,calledthelawsofreflection.Allreflectionstake

placeinaccordancewiththeselaws.

Firstlawofreflection:Theangleofincidence(i)isequaltotheangleofreflection(r).

Secondlawofreflection:Theincidentray,thereflectedrayandthenormalatthepointofincidencealllieinthe

sameplane.

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Theangleofincidenceisequaltotheangleofreflection.Mathematically,wehave .

SPHERICALMIRRORS:Amirrorisareflectingsurface.Wehavealreadyfamiliarizedourselveswithatypeof

mirrorcalledplanemirror.Anothervarietyofmirrorsarethecurvedmirrors.Acurvedmirrorisamirrorwitha

curvedreflectingsurface,whichmaybeeitherconvex(bulgingoutward)orconcave(bulginginward).Thecurved

mirrorsconverge(bringtogether)ordiverge(spreadout)theraysoflightincidentonthem.Themostcommon

typeofcurvedmirrorsisthesphericalmirror.

Asphericalmirrorisapartofasphericalreflectingsurfaceasshowninfigure.Sphericalmirrorsare oftwo

types:

(i)Concavemirrorand

(ii)Convexmirror.

Concavemirror:Aconcavemirror,orconvergingmirror,hasareflectivesurfacethatbulgesinward(awayfrom

theincidentlight).

Convexmirror:Aconvexmirror,fisheyemirrorordivergingmirrorisacurvedmirrorinwhichthereflective

surfacebulgestowardthelightsource.Figureshowsbothtypesofmirrors;withalightrayreflectingfrom the

polishedsurface.Thelawofreflectionapplies,justasitdoesforaplanemirror.For

eithertypeofsphericalmirror,thenormalisdrawnperpendiculartothemirroratthepointof

incidence.

Fig.ThecentreofcurvatureispointCandtheradiusofcurvatureisRforaconcavemirror,thereflectivesurfaceisthe

innerone,whileforaconvexmirroritistheouterone.

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TECHNICALTERMSCONNECTEDWITHSPHERICALMIRRORS:

PoleorVertex(P):Thecentralpointofamirroriscalleditspoleorvertex.

Centreofcurvature(C):Thecentreofthesphereofwhichthemirrorisapartiscalledthecentreofcurvature.It

isdenotedbyCasshowninfigure.

Fig.Thetechnicaltermsconnectedwithsphericalmirrors.

Radiusofcurvature(R):Theradiusofthesphereofwhichthemirrorisapartiscalledtheradiusofcurvature.It

isdenotedbyRasshowninfigure.

Principalaxis:Thestraightlinepassingthroughthepoleandthecentreofcurvatureofthemirroriscalledthe

principalaxis.InfigurePCistheprincipalaxis.

Principalfocus(F):Itisapointontheprincipalaxisatwhichtheraysparalleltotheprincipalaxismeetafter

reflectionorseem tocomefrom.Infigure,Fistheprincipalfocus.Inotherwordsaconcavemirrorhasareal

focuswhileaconvexmirrorhasavirtualfocus.

Focalplane:Aplanedrawnperpendiculartotheprincipalaxisandpassingthroughtheprincipalfocus.

Focallength:Thedistancebetweenthepoleandthefocusiscalledthefocallength.Itisrepresentedbyf.The

focallengthishalftheradiusofcurvaturei.e.,f= Infigure,PF=fisthefocallength.

Aperture:Thesizeofthemirroriscalleditsaperture.Itisalsodefinedastheeffectivediameter,ofthelight

reflectingareaofthemirror.(Figure)

RELATIONBETWEENRADIUSOFCURVATURE(R)ANDFOCALLENGTH(f)OFASPHERICALMIRROR

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Experimentsshowthatforasphericalmirror(concaveandconvex)thefocallengthofthemirrorishalfitsradius

ofcurvature.Mathematically,wehave .

Consideraconcavemirrorofsmallapertureasshowninfigure.

LetarayoflightAB,paralleltotheprincipalaxis,strikethemirroratpointB.

ThisincidentrayABisreflectedbythemirroralongthepathBF,whereFisthefocusofthemirror.

Thisrayfollowsthelawsofreflection.LetusjoinCtoBwiththehelpofadottedline.

SinceCBistheradiusofthesphereofwhichthismirrorismade,thereforeitisperpendiculartothemirrorat

pointB.

ThusthelineCBisanormalatpointB.From BdropaperpendicularBM ontheprincipalaxis.

Then,inaccordancewiththelawsofreflection

Fig.Raydiagram toshowthatthefocallengthofamirrorishalfitsradiusofcurvature.

ABC= FBC, i=r (Angleofincidence=Angleofreflection.)

But ABC= BCF (Alternateangles)

Therefore, BCF= FBC

Alsoin CBF, BFM =2 BCF=2i (exteriorangle=sum ofinterioroppositeangles)

Nowin CMB,tani ……(1)

Sinceapertureissmall,thereforeMC=PC

Alsoin FMB,tan2i ……(2)

Sinceapertureissmall,thereforeMF=PF.Alsoapertureissmallthereforetheanglesiand2iarealsosmall,

thereforetani iandtan2i 2i.Dividingequation(2)byequation(1),wehave

……(3)

SolvingwehavePC=2PF ……(4)

SincePC=RandPF=f,thereforeR=2for

……(5)

Forsphericalmirrors

Thusthefocallengthofasphericalmirrorishalfitsradiusofcurvature.

RAYSFORLOCATINGIMAGEINACONCAVEMIRROR

Fordrawingtheraydiagramstolocatetheimageformedbyasphericalmirror,thefollowingpointsshouldbe

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keptinmind.

Allraysparalleltotheprincipalaxis,afterreflection,passthroughtheprincipalfocusorseem tocomefrom it.

Fig:(i)

Arayoflightparalleltotheprincipalaxis,afterreflection,passesthroughtheprincipalfocusorseemstocome

from it.(Fig(i)aandb)

Fig.Arayoflightparalleltotheprincipalaxispassesorappearstopassthroughthefocusafterreflection.

Fig:(ii)

Arayoflightpassingthroughthefocusordirectedtowardsthe(Figure(ii)aandb)

Principalfocus,afterreflection,becomesparalleltotheprincipal.

Fig:(iii)

Arayoflightpassingthroughthecentreofcurvatureafterreflectionretracesitsoriginalpath.

(Fig(iii)aandb)

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Arayoflightincidentonthepoleofthemirrorgetsreflectedinaccordancewiththelawsofreflection.

(Figure(iv)aandb)

Althoughfourdifferentraysareshowntolocateanimage,onlytwoarereallyneeded;thethirdorthefourthrayis

usuallydrawntoserveasacheck.Inasimilarfashion,raysfrom allpointsoftheobjectlocatecorresponding

pointsontheimage,andthemirrorformsacompleteimageoftheobject.Theimageisrealifthelightrays

actuallypassthroughapointandtheimageisvirtualifthelightraysseem tocomefrom apoint.

Inalltheabovereflectionstakingplacethelawsofreflectionarestrictlyobeyed.

Fig.Imageformedwhenobjectisatinfinity. Fig.Imageformationwhentheobjectliesbeyondthe

centreofcurvature.

Fig.Imageformationwhentheobjectliesatthecentreof

curvature

Fig.ImageformationwhenobjectisplacedbetweenFand

C.

Fig.Imageformationwhenobjectliesatthefocus. Fig.ImageformationwhentheobjectliesbetweenFand

P.

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RAYDIAGRAMSFORFORMATIONOFIMAGEBYACONCAVEMIRROR

Anobjectcanbeplacedanywhereinfrontofaconcavemirrortoobtainitsimage.

Keepinginview,theabovepoints,theimageformedbyaconcavemirrorforvariouspositionsoftheobjectis

discussedasunder.

Whentheobjectisatinfinity:Considertwodifferentrayscomingfrom infinity,wheretheobjectisplaced.These

rays,afterreflectionfrom themirror,convergeatthefocustoform theimageofthedistantobject(Figure).The

imageformedisreal,invertedandextremelydiminishedinsize.Iftheraysarenotparalleltotheprincipalaxis

thentheimageisformedatthefocalplaneandisreal,invertedandextremelydiminished.

Whentheobjectliesbeyondthecentreofcurvature(C):Consideraraystartingfrom thetopoftheobjectand

paralleltotheprincipalaxis.Thisrayafterreflectionfrom themirrorpassesthroughthefocus.Anotherrayof

lightstartingfrom thetopoftheobjectandpassingthroughthefocusbecomesparalleltotheprincipalaxisafter

reflectionfrom themirror.Thesetworeflectedraysmeetbetweenthecentreofcurvatureandthefocustoform

therealimageoftheobject(Figure).Thisimageisreal,invertedanddiminished.

Whentheobjectliesatthecentreofcurvature(C):Consideraraystartingfrom thetopoftheobjectandparallel

totheprincipalaxis.Thisrayafterreflectionfrom themirrorpassesthroughthefocus.Anotherrayoflight

startingfrom thetopoftheobjectandpassingthroughthefocusbecomesparalleltotheprincipalaxisafter

reflectionfrom themirror.Thesetworeflectedraysmeetatapointlyingintheplaneofthecentreofcurvatureto

form therealimageoftheobject(Fig).Thisimageisreal,invertedandequaltothesizeoftheobject.

Whentheobjectliesbetweenthecentreofcurvature(C)andthefocus(F):Consideraraystartingfrom thetop

oftheobjectandparalleltotheprincipalaxis.Thisrayafterreflectionfrom themirrorpassesthroughthefocus.

Anotherrayoflightstartingfrom thetopoftheobjectandpassingthroughthefocusbecomesparalleltothe

principalaxisafterreflectionfrom themirror.Thesetworeflectedraysmeetatapointlyingbeyondthecentreof

curvaturetoform therealimageoftheobject(Figure).Thisimageisreal,invertedandbiggerthantheobject.

Whentheobjectliesatthefocus(F):Considertworaysstartingfrom theobjectplacedatthefocus.After

reflectionfrom themirrortheseraysbecomeparalleltotheprincipalaxis.Thesetworeflectedraysbeingparallel

toeachothermeetatinfinitytoform theimageoftheobject(Figure).Thisimageisreal,invertedandextremely

magnified.

Whentheobjectliesbetweenthefocus(F)andthepole(P)orvertex(V):Consideraraystartingfrom thetopof

theobjectandparalleltotheprincipalaxis.Thisrayafterreflectionfrom themirrorpassesthroughthefocus.

Anotherrayoflightstartingfrom thetopoftheobjectandpassingthroughthecentreofcurvatureretracesits

pathafterreflectionfrom themirror.Thesetworeflectedraysseem todivergefrom apointlyingbehindthe

mirror.Theimageisformedatthispoint(Fig).Thisimageisvirtual,erectandmagnified.

Theabovesixcaseshavebeensummarizedinthetablebelowforquickreference:

S.No. Positionofobject Positionofimage Natureofimage Use

1.

Atinfinity Attheprincipalfocusor

inthefocalplane

Real,invertedandextremely

diminishedinsize

Tocollectheat

radiationssolar

devices

2. Beyondthecentreof Betweentheprincipal Real,invertedand ___________

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curvature focusandcentreof

curvature

diminished

3.Atthecentreofcurvature Atthecentreof

curvature

Real,invertedandequalto

thesizeoftheobject

Reflectingmirrorfor

projectorlamps

4.Betweenfocusandthe

centre ofcurvature

Beyondcentreof

curvature

Real,invertedandbigger

thanobject

Infloodlights

5.Attheprincipalfocus Atinfinity Real,invertedandextremely

magnified

Intorches,headlights

6.Betweenthepoleandthe

principlefocus

Behindthe mirror Virtual,erectandmagnified Shavingmirror,

dentistmirror

FORACONCAVEMIRROR

Whenobjectisatinfinity,theimageisrealinvertedandextremelydiminished.

Whentheobjectliesbeyondthecentreofcurvature,theimageisreal,invertedanddiminished.

Whentheobjectliesatthecentreofcurvature,theimageisreal,invertedandequaltothesizeoftheobject.

Whentheobjectliesbetweenthefocusandthecentreofcurvature,theimageisreal,invertedandbiggerthanthe

object.

Whentheobjectliesattheprincipalfocus,theimageisreal,invertedandextremelymagnified.

Whentheobjectliesbetweenthepoleandtheprincipalfocus,theimageisvirtual,erectandmagnified.

Foraconcavemirrorforanobjectsituatedbeyondthefocustheimageisalwaysrealandinvertedwhereasforan

objectsituatedbetweenthepoleandthefocustheimageisvirtualanderect.

Fig.Imageformationbyaconvexmirror.

RAYDIAGRAMSFORFORMATIONOFIMAGEBYACONVEXMIRROR

Keepinginview,theabovepoints,theimageformedbyaconvexmirrorforvariouspositionsoftheobjectis

discussedasunder:

Whentheobjectisatinfinity:Considertworayscomingfrom infinity,wheretheobjectisplaced.Theserays,after

reflectionfrom themirror,seem tocomefrom thefocus.Thisistheimageofthedistantobject(Figure).This

imageformedisvirtual,erectandextremelydiminished.

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Fig.Imageformationbyaconvexmirror.

Whentheobjectliesbeyondthecentreofcurvature(C):Consideraraystartingfrom thetopoftheobjectand

paralleltotheprincipalaxis.Thisrayafterreflectionfrom themirrorseemstocomefrom thefocus.Anotherray

oflightstartingfrom thetopoftheobjectandtravellingtowardsthecentreofcurvaturereflectsbackonitself.

Thetworayswhenproducedbackwardsseem tomeetbehindthemirror.Thisistheimageoftheobject.(Figure).

Thisimageformedisvirtual,erectanddiminished.

Aconvexmirroralwaysformsavirtualanderectimagenomatterwhereinfrontofthemirrortheobjectisplaced.

Theimageisalwayssituatedbetweenthepoleandthefocus,irrespectiveofthepositionoftheobjectinfrontof

themirror.

Theabovetwocaseshavebeensummarizedinthetablebelowforquickreference.

ForaConvexMirror:

S.No. Positionofobject Positionofimage Natureofimage Uses

1. Atinfinity Appearsattheprincipal Virtual,erectand

extremelydiminished

Usedasarearview

mirrorandreflectorin

streetlights

2. Betweeninfinityandthe

polefig.

Appearsbetweenthe

principalfousandthepole

Virtual,erectand

diminished

Usedasrearviewmirror

andreflectorsinstreet

light

USESOFSPHERICALMIRRORS:

ConcaveMirror:

Tocollectheatradiationsinsolardevices:Heatradiationfrom thesuncomingfrom infinityisbroughttofocus

bytheconcavemirrorinitsfocalplaneoratthefocus.(Figure)

Reflectingmirrorforprojectorlamps:Theobjectisplacedatthecentreofcurvaturetoobtainanimageofthe

samesize.

Infloodlights:Thesourceoflightisplacedjustbeyondthecentreofcurvature.Thisilluminatesacertainsection

oftheground.

Intorches,headlights:Thesourceoflightisplacedatthefocustoobtainaparallelbeam oflight.

Shavingmirror,dentistmirror:Itproducesanerect,virtualandhighlymagnifiedimageofanobjectplaced

betweenitspoleandfocus.

Itisusedintheophthalmoscopebydoctorstoconcentratelightonasmallregionwhichistobeexamined.

Fig.Concavemirrorsareusedtoconcentratesunlightatsolarfarms

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Theyareusedtoconcentratesunlightinsolarfarms.

ConvexMirrors:

Itisusedasarearviewmirrorinautomobiles.Thisisduetothereasonthataconvexmirrorprovidesawider

fieldofviewthanaplaneorconcavemirror.

Itproducesanerect,diminishedandvirtualimage.Usuallyontherearviewmirroritiswritten'Objectsinthis

mirrorarecloserthantheyappear'(Figure).

Itisbecauseaconvexmirrordoesnotgivetheexactdistanceofthevehiclecomingfrom behindasitproducesa

diminishedimage.

Fig.Arearviewmirror

Itisusedasareflectorinstreetlampssoastodivergelightoveralargearea.

Fig.Aconvexmirrorisusedtodivergelight.

Fig.Fieldviewofa(a)convexmirrorand(b)planemirror.

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Fig.Theimageofcandlewhenitliesinfrontofaconvexmirror.

Theimageisvirtualerectanddiminished.

Itisusedasasecuritymirrorinshopsandonroadsatsharpbendsandconcealedentrances.

Fig.Convexmirrorsareusedassecuritymirrorsinshopsandonroadsat

sharpbendsandconcealedentrances.

Fig.Curvedsurfacesoffunhousemirrorsproduceinterestingimages

THENEW CARTESIANSIGNCONVENTIONANDTHEMIRRORFORMULA

Now-a-daysthenewCartesiansignconventionsarefollowedforassigning

Signstovariousdistanceswhicharetobemeasuredinaraydiagram.Thesemirror.(Realobject)

signconventionsareasfollows:

● Theobjectisalwaysplacedtowardstheleftofthemirrorsuchthatlightpropagatesfrom the left

towardstherightside.

● Alldistancesaremeasuredparalleltotheprincipalaxisandfrom thepoleofthemirror.The

distancesmeasuredinthedirectionoftheincidentrayaretakenaspositive,whilethose measuredoppositeto

thedirectionortheincidentrayaretakenasnegative.

● Distancesmeasuredperpendiculartotheprincipalaxisandintheupwarddirectionaretaken as

positiveandthosemeasuredperpendicularandbelowtheprincipalaxisaretakenas negative.Thesesign

conventionsareshowndiagrammaticallyinfigure.

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Formirrorsthefollowingresultshold:

1. uis-veiftheobjectisinfrontofthemirror. (Realobject)

2. uis+veiftheobjectisbehindthemirror. (Virtualobject)

3. vis-veiftheimageisinfrontofthemirror. (Realimage)

4. vis+veiftheimageisbehindthemirror. (Virtualimage)

5. Focallengthofaconcavemirroristakenas-ve.

6. Focallengthofaconvexmirroristakenas+ve.

Themirrorformula .

Fig.ThenewCartesianSignconventionforsphericalmirror.

● Themirrorformulaisarelationbetweentheobjectdistance(u),theimagedistance(v)and the

focallength(f)ofamirror.Accordingtothisformulathethreearerelatedas

Whentheimageformedbyasphericalmirrorisreal,itisalsoinvertedandisonthesamesideofthemirroras

theobject.Sincebothvanduarenegativethemagnificationisnegative.

● Thisformulaisvalidforboththeconcaveandtheconvexmirrors.

● Thisformulaisvalidforallsituationsforallsphericalmirrorsforallpositionsoftheobjects,

● YoumustusetheNewCartesianSignConventionwhilesubstitutingnumericalvaluesforu,v,f,and

Rinthemirrorformulaforsolvingproblems.Remembernosignconventionisusedfortheunknown

quantityinthenumericalproblem.

Whentheimageformedbyasphericalmirrorisvirtual,itisalsoerectandisontheothersideofthemirroras

theobject.Inthiscaseuis-veandvis+ve,thereforem ispositive.

LINEARMAGNIFICATION:

Itisdefinedastheratiooftheheightoftheimage(h')totheheightoftheobject(h).Mathematicallywehave

Linearmagnification

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Linearmagnificationisgivenbytheexpression

Thenegativesignisusedtotellwhethertheimageiserectorinverted.

Intermsoff,wehavem =

Ifm ispositive,theimageiserectw.r.t.theobjectandifm isnegative,theimageisinvertedw.r.t.theobject.

Ifm ispositive,theimageiserectw.r.t.theobjectandifm isnegative,theimageisinvertedw.r.t.theobject.

Sinceconcavemirrorcanproducebothrealandvirtualimagesthereforemagnificationofaconcavemirrorcan

bepositiveornegative.

Ontheotherhandaconvexmirroralwaysformsavirtualimage;thereforemagnificationforaconvexmirroris

alwayspositive.

Magnificationforaconcavemirrorcanbepositiveornegative,whilethatofaconvexmirrorisalwayspositive.

Itisalsoworthnotingthatifmagnificationisgreaterthanonethen,theimageisbiggerthantheobjectandifitis

lessthanonethen,theimageissmallerthantheobject.

Amagnificationequaltooneindicatesthattheimageandtheobjectareofthesamesize.

1. Anobject4cm highisplacedatadistanceof6cm infrontofaconcavemirroroffocallength12cm.Findthe

position,natureandsizeoftheimageformed.

Sol. Given,sizeofobject,h=4cm,u=–6cm,f=12cm

Usingmirrorformula

Thereforev=12cm

Thus,theimageisformedatadistanceof12cm behindtheconcavemirrorasvispositive.Thereforeimageis

virtual.

Nowm = orI=8cm

So,imageis8cm tall.SinceIispositive,soimageiserect.

2. Theradiusofcurvatureofasphericalmirroris30cm.Findthefocallengthofmirror.

Sol. Here,f= wehave;f= =15cm.

Thus,focallengthofthesphericalmirror=15cm.

3. Anobject4.0cm insizeisplacedatadistanceof25.0cm infrontofaconcavemirrorof radiusof

curvature40cm.Find

(i) theposition,

(ii) thesizeand

(iii) natureoftheimage.

Sol. Here,h=+4.0cm; u=–25.0cm (signconventions)

R=+40cm; f= =20.0cm

Step1. Determinationofthepositionsoftheimage.

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Using, ,weget

Thus,theimageisat11.11cm behindtheconvexmirror.

Step2.Determinationofsizeandnatureofimage.

Usingm = or h’=

So,theimageisinverted.

4. Aconcavemirrorusedinabushasradiusofcurvature3.5m.Ifthedriverofthebuslocatesacarat1.0m

behindthebus,findtheposition,natureandsizeoftheimageofthecar.

Sol. Here,R=3.5m;f= ;u=10.0m

Step1:Determinationofthepositionofthecar.

Using ,wehave or

Thus,thecarappearstobeat1.5m from theconcavemirror.

Step2:Determinationofthesizeandnatureoftheimage.

Using, weget,m =

Thus,thesizeoftheimageofthecaris0.15timestheactualsizeofthecar.Sincem ispositive,so image

ofthecariserect(i.e.,upright)

5. Theradiusofcurvatureoftheconvexmirrorusedonamovingautomobileis2.0m.A truckiscomingbehind

itatadistanceof3.5m.Calculate

(i) theposition

(ii) thesizeoftheimagerelativetothesizeofthetruck.Whatwillbethenatureofthe image?

Sol. Here,R=2.0mf= ;u=–3.5m

Step1:Determinationofpositionoftruck

Using ,weget

Thus,thetruckappearstobeat0.78m from theconvexmirror.

Step2:Determinationofsizeandnatureoftheimage

Using,m =

Thus,sizeoftheimageofthetruckis timestheactualsizeofthetruck.

Sincem ispositive,soimageoftruckiserectandvirtual.

6. Anobjectisplacedatadistanceof20cm from theaconvexmirroroffocallength10cm. Findthe

positionandnatureoftheimage.

Sol. Here,f=10cm (signconvention)

u=–20cm (signconvention)

Step1:Using, ,weget

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or

Step2:m=–

Sincemagnificationispositive,soimageformedisvirtual.

VERYSHORTANSWERTYPEQUESTION

1. Radiusofcurvatureofaconcavemirroris25cm.Whatisitsfocallength?

2. Anobject1cm highisheldnearaconcavemirrorofmagnification10.Howtallwillbeimage?

3. Namethevarioustypesofbeamsoflight.

4. Inwhichmirrorwouldyoubeabletoseeafullsizeimageofafarofflargeobject.

5. Canaconcavemirrorform avirtualimageofsamesizeastheobject?

6. Statetheexpressionforlateralmagnificationofaconcavemirrorintermsofobjectdistance andimage

distance.

SHORTANSWERTYPEQUESTIONS

7. Anobjectisplaced90cm awayfrom aconcavemirroroffocallength30cm.Findtheposition andthenature

oftheimageformed.

8. Aconcavemirrorproducesthreetimesmagnifiedrealimagesofanobjectplacedat10cm in frontofit.

Whereistheimagelocated?

9. Usethemirrorformulatoshowthatforanobjectlyingbetweenthepoleandfocusofaconcave mirror,the

imageformedisalwaysvirtualinnature.

10. Findtheposition,natureandsizeoftheimageofanobject3cm highplacedatadistance6cm from aconcave

mirroroffocallength12cm

11. Showthatthemirrorformulaforsphericalmirrorsholdsgoodforaplanemirrortoo.

12. Describetheimageformationshowninthediagram.

LONGANSWERTYPEQUESTIONS

13. Findtheposition,natureandsizeoftheimageofanobject3cm highplacedatadistanceof9cm from a

concavemirroroffocallength18cm.

14. Anobject5cm inheightisplacedatadistanceof20cm infrontofaconvexmirrorofradiusof curvature30cm.

Findtheimage,itsnatureandsize.

15. Discussthenatureofimageformed,whentheobjectmovesfrom infinitytowardstheconcave mirror.

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16. Anobjectofsize7cm isplacedat27cm infrontofaconcavemirroroffocallength18cm.Atwhatdistancefrom

themirrorshouldascreenbeplaced,sothatasharpfocusedimagecanbeobtained?Findthesizefindnatureof

theimage.

17. Whenanobjectisplacedatadistanceof60cm from adivergingsphericalmirror,themagnificationproducedis

0.5.Wheretheobjectshouldbeplacedtogetamagnificationof .

18. 1) Statetherelationbetweenobjectdistance,imagedistanceanfocallengthofaspherical

mirror.

2) Aconcavemirroroffocallength15cm form animageofanobjectkeptatadistance

of10cm from theimageformedbyit.

3) Drawaraydiagram toshowtheimageformedbyaconcavemirrorwhenanobjectis

placedbetweenpoleandfocusofthemirror.

HIGHERORDERTHIKINGSKILLS

19. Amanstands10m infrontofalargeplanemirror.Howformusthewalkbeforehe5awayform hisimage?20. Anobjectisplaced20cm infrontofaplanemirror.Themirrorismoved2cm towardstheobject. Thedistancebetweenthepositionsoftheoriginalandfinalimagesseeninthemirroris:

1)2cm 2)4cm 3)10cm 4)22cm21. Amansitsinanoptician’schair,lookingintoaplanemirrorwhichis2m awayhim andviewstheimageofachart

whichfacesthemirrorandis50cm behindhishead.Howfarawayfrom hiseyesdoesthechartappeartobe?22. ArayoflightstrikesaplanemirrorPQatanangleofincidenceof30°,isreflectedfrom theplanemirrorandthen

strikesasecondplanemirrorQRplacedatrightanglestothefirstmirror.Theangleofreflectionatthesecondmirroris:1)30° 2)45° 3)60° 4)90°Drawaray–diagram toillustrateyouranswer.

23. Explainhowtoreadthefollowingmessagewhichwasfoundonsomeblottingpaper:

TRUE(or)FALSE

24. Lightisanelectromagneticradiation.

25. Lightalwaysbehaveslikeawave.

26. Theeffectivewidthofasphericalmirrorfrom whichreflectioncantakeplaceiscalledits aperture.

27. Aconvexmirrorproducesavirtual,erectandmagnifiedimage.

28. Lawsofreflectionareapplicabletoalltypesofreflectingsurfaces.

29. Accordingtosignconventions,thedistancemeasuredinthedirectionofincidentlightistakes asnegative.

30. Thepoleofasphericalmirroristhecentreofthemirror.

31. Whenanobjectisatthe centreof curvatureof concavemirror,theimageformedwillbe virtualanderect.

32. Ifanobjectofheight1cm isplacednearaconcavemirrorofmagnification10,thentheheight oftheimage

willbe10cm.

33. Aconvexmirrorisusedintheophthalmoscope.

FILLINTHEBLANKS

34. Aconcavemirrorgivesreal,invertedandsamesizeimageiftheobjectisplacedat______

35. Aconcavemirrorgivesvirtual,erectandenlargedimageiftheobjectisplaced______

36. Focallengthofcombinationoftwothinlensesofpower+6Dand-2Dis_________

37. Theradiusofcurvatureofamirroris20cm,itsfocallengthis___________

38. Anincidentraymakes anglewiththesurfaceoftheplanemirrortheangleofreflectionis _________

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39. Ifthelinearmagnificationincaseofsphericalmirrorisgreaterthanone,thentheimageformed is_________


40. Thenumberofimagesobservablebetweentwoparallelmirrorsis

1)6 2)infinite 3)2 4)4

41. Thenumberofimagesformedbytwoplanemirrorsinclinedatanangle ofanobjectplaced

symmetricallybetweenmirrorsis

1)5 2)infinite 3)6 4)7

42. Howmanyimagesofhimselfdoesanobserverseeiftwoadjacentwallsandtheceilingofa rectangular

room aremirrorsurfaced?

1)6 2)7 3)3 4)5

43. Athickplanemirrorshowsanumberofimagesofthefilamentofanelectricbulb.Ofthese,the brightestimage

isthe

1)last 2)fourth 3)first 4)second

44. Alightbulbisplacedmidwaybetweentwoplanemirrorsinclinedatanangleof .The numbersof

imagesformedare

1)5 2)4 3)6 4)8

45. Choosethewrongstatement.

1)Aconcavemirrorcanform amagnifiedrealimage.

2)Aconcavemirrorcanform amagnifiedvirtualimage

3)Aconvexmirrorcanform adiminishedvirtualimage

4)Aconvexmirrorcanform adiminishedrealimage

46. Aman180cm highstandsinfrontofaplanemirror.Hiseyesareataheightof172cm from thefloor.Thento

seehisfullimageforminimum lengthofmirror,thelowerendofthemirrorshouldbeplacedataheightof

1)86cm from thefloor 2)94cm from thefloor

3)4cm from thefloor 4)8cm from thefloor

47. Amanstandingontheroadinfrontofalargewindowglasspaneseeshisimagebiggerthan himself.The

glasspaneis

1)Convexoutside 2)cylindricaloutside 3)plane 4)concaveoutside

48. Itisdesiredtophotographtheimageofanobjectplacedatadistanceof3m from aplanemirror. The

camera,whichisatadistanceof4.5m from themirror,shouldbefocusedfora distanceof

1)6m 2)7.5m 3)3m 4)4.5m

49. Anobject5cm longandapencil10cm longareplacedinfrontofapinholecamerasuchthattheirimageshave

thesamelength.Theratioofthedistanceoftheobjectfrom thepinholetothatofthepencilis

1)5:2 2)1:4 3)3:2 4)1:2

VERYSHORTANSWERQUESTIONS

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1. Accordingtothesignconvention,whichmirrorhasnegativefocallength?

2. Whereshouldanobjectbeplacedinordertouseaconvexlensasamagnifying?

3. Whatisasphericalmirror?

4. M doesnotshowlateralinversionwhereasPdoes.Explain.

5. Givetwodifferencesbetweenrealimageandvirtualimage.

6. Whydoesaconvexmirrorhasavirtualprincipalfocus?

SHORTANSWERQUESTIOINS

7. Giveoneexampleeachof

1)parallel 2)converging 3)divergingbeamsoflight.

8. Aconcavemirrorproducesthreetimesenlargedimageofanobjectplacedat10cm infrontof it.Calculatethe

radiusofcurvatureofthemirror.

9. Anobject4cm highplacedatadistanceof6cm infrontofaconcavemirroroffocallength

12cm.Findtheposition,natureandsizeoftheimageformed.

10. A2cm highobjectisplacedatadistanceof32cm from aconcavemirror.Theimageisreal, invertedand3

cm insize.Findthefocallengthofthemirrorandthepositionoftheimage.

11. A2cm highobjectisplacedperpendiculartotheprincipalaxisofaconcavemirror.Thedistanceoftheobject

from themirroris30cm,anditsimageisformed60cm from themirror,onthesamesideofthemirrorasthe

object.Findtheheightoftheimageformed.

12. Aconcavemirrorformsanerectimageofanobjectplacedatadistanceof10cm from it.Thesizeoftheimageis

doublethatoftheobject.Whereistheimageformed?

LONGANSWERQUESTIONS

13. Aconvexmirrorusedinabushasradiusofcurvature3.5m.Ifthedriverofthebuslocatesacarat 10m

behindthebus,findtheposition,natureandsizeoftheimageofthecar.

14. Anobject3cm highisplacedatadistanceof10cm infrontofaconcavemirroroffocallength20cm. Findthe

position,natureandsizeoftheimageformed

15. Theradiusofcurvatureofaconvexmirrorusedonamovingautomobileis2.0m.Atruckiscoming behind

itatdistanceof3.5m.Calculate

(i) thepositionand

(ii) thesizeoftheimagerelativetothesizeofthetruck.Whatwillbethenatureoftheimage?

16. Asquarewireofside3.0isplaced25cm awayfrom aconcavemirroroffocallength10cm.Whatisthearea

enclosedbytheimageofthewire?Thecentreofthewireisontheaxisofthemirror,withitstwosidesnormalto

theaxis.

HIGHERORDERTHIKINGSKILLS

17. Acommunicationssatelliteinorbitsendsaparallelbeam ofsignalsdowntoearth.Ifthesesignalsobeythesamelawsofreflectionaslightandaretobefocusedontoasmallreceivingaerial,whatshouldbethebestshapeofthemetal‘dish’usedtocollectthem?

18. Whenasphericalmirrorisheldtowardsthesunanditssharpimageisformedonapieceofcarbonpaperforsometime,aholeisburntinthecarbonpaper.1)Whatisthenatureofsphericalmirror?2)Whyisaholeburntinthecarbonpaper?3)Atwhichpointofthesphericalmirrorthecarbonpaperisplaced?

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4)Whatnameisgiventothedistancebetweensphericalmirrorandcarbonpaper?5)Whatistheadvantageofusingacarbonpaperratherthenawhitepaper?

19. Anobjectis100mm infrontofaconcavemirrorwhichproducesanuprightimage(erectimage).Theradiusofcurvatureofthemirroris:1)lessthan100mm 2)between100mm and200mm3)exactly200mm 4)morethan200mm

20. Avirtual,erectandmagnifiedimageofanobjectistobeproducedwithaconcavemirroroffocallength12cm.Whichofthefollowingobjectdistanceshouldbechosenforthispurpose?(i)10cm (ii)15cm (iii)20cmGivereasonsforyourchoice.

21. Aconcavemirrorhasafocallengthof25cm.Atwhichofthefollowingdistanceshouldapersonholdhisfacefrom thisconcavemirrorsothatitmayactasashavingmirror?1)45cm 2)20cm 3)25cm 4)30cmGivereasonforyourchoice.

22. Anobjectisplacedatthefollowingdistancefrom aconcavemirroroffocallength15cm,turnbyturn:1)35cm 2)30cm 3)20cm 4)10cmWhichpositionoftheobjectwillproduce:(i)amagnifiedrealimage? (ii)amagnifiedvirtualimage?(iii)adiminishedrealimage? (iv)animageofsamesizeastheobject?

TRUE(or)FALSE

23. Irregularreflectionoflightgivesasharpimage.

24. Sphericalmirrorscanalsobeusedastrickmirrors.

25. Focallengthofaplanemirrorisinfinity.

26. Concavemirrorformsafullsizeimageofafarofflarge.

27. Avirtualimagecannotbephotographed.

28. Linearmagnificationofaconvexmirrorisalwaysnegative.

29. Arayincidentalongnormaltothemirrorretracesitspath.

FILLINTHEBLANKS

30. Themagnificationofimagesincaseifanobjectisplaced10cm infrontofaconcavemirrorofradiusof

curvature15cm is__________

31. Arayoflightfallingnormallyonamirrorreflectsby________

32. Ifanobjectisplacedbetweentwoplanemirrorsinclinedat toeachotherthenthenumberofimages

formedis_________

33. Ifdistanceofasurfacefrom asourceismadetwice,theilluminanceofthesurfacebecomes __________

34. Iftheobjectisplacedatfocusofaconcavemirror,theimageisformedat……..


35. Aplanemirrorreflectingarayofincidentlightisrotatedthroughanangle aboutanaxis throughthe

pointofincidenceintheplaneofthemirrorperpendiculartotheplaneofincidence. Then

a)Thereflectedraydoesnotrotate

b)thereflectedrayrotatesthroughanangle

c)thereflectedrayrotatesthroughanangle2

d) theincidentrayisfixed

Oftheabovestatements

1)canddarecorrect 2)banddarecorrect

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3)Onlyaiscorrect 4)aandcarecorrect

36. Givenapointsourceoflight,whichofthefollowingcanproduceaparallelbeam oflight?

1)Concavelens

2)Twoplanemirrorsinclinedat toeachother

3)Convexmirror

4)Concavemirror

37. Arayoflightisincidentonaplanemirroratanangleofincidenceof .Thedeviation producedbythemirror

is

1) 2) 3) 4)

38. Anobjectisphotographedwhenplacedatadistanceof2m from thecamera.Ifthesameobjectbeplacedata

distanceof4m from thecamera,howwillthetimeofexposurebeaffected?

1)Itwillbeincreasedfourtimes. 2)Itwillbedecreasedfourtimes

3)Itwillbedecreasedtwotimes 4)Itwillbeincreasedtwotimes

39. Aconvexmirrorisusedtoform animageofarealobject.Thentheincorrectstatementis

1)theimageiserect 2)theimageisreal

3)theimageliesbetweenthepoleandthefocus.

4)theimageisdiminishedinsize

40. Aplanemirrorisapproachingyouat .Youcanseeyourimageinit.Atwhatspeedwillyourimage

approachyou.

1) 2) 3) 4)

41. Avirtualimage,largerthantheobjectcanbeproducedby

1)planemirror2)concavelens3)convexmirror 4)concavemirror

42. Forarealobject,aconvexmirroralwaysformsanimagewhichis

1)Virtualanderect 2)realand magnified

3)realandinverted 4)concavemirror

43. Theimageformedbyapinholecameraissharpestwhenthe

1)Cameraisusedinadarkroom 2)holeisverysmall

3)Objectisbrightlyilluminated 4)distancefrom thepinholetotheobjectissmall

44. Theimageofanobjectplacedontheprincipalaxisofaconcavemirroroffocallength12cm isformedatapoint

whichis10cm moredistantfrom themirrorthantheobject.Themagnificationoftheimageis

1)2 2)1.5 3) 4)2.5

SOLUTIONS&KEY

WARM UP

1. True 2. True 3. False 4. True

5. True 6. True 7. True 8. False.

9. Solar,Electrical 10. Planemirror 11. Glasssheet

12. Semiconductor 13. Photovoltaiccell

ASP-X-PHY-RL

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14. Solarpanel 15. 10-25%16. mechanical 17. Mechanical,Electrical

18. Tide 19. Temperatureoflayers

20. 3 21. 1 22. 2 23. 2 24. 2

25. 4 26. 3 27. 1 28. 1 29. 1

CLASSROOM WORKSHEET–1

1. R=25cm,f=?

2. Here

3. Therecanbethreetypesofbeamsoflight:

i)parallelbeam ii)Convergingbeam iii)Diverging beam

4. Aconvexmirrorformsafullsizeimageofafar-offlargeobject.

5. No,virtualimageformedbyaconcavemirrorisalwaysenlarged.

6.

7. AccordingtoCartesiansignconvention,

Negativesignmeansthatarealimageifformed45cm from themirroronthesamesideasthe object.

8. Magnification,m =-3(sinceimageisreal)

Objectdistance,u=-10cm

Imagedistance, =?

Weknowthatmagnificationforthemirror

9. usingmirrorformula, ,where andfareobjectdistance,imagedistanceandfocal length

Focallengthandobjectdistanceareboth–veforconcavemirror.

Itisgiven , becomes andispositive. ispositivewhichshowsavirtualimageis

formed.

10. Here

Usingmirrorformula,

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Imageisformedbehindthemirroratadistanceof12cm

Natureofimageisvirtual,erectandenlarged.Sizeofimage:Twicethesizeofobject

11. Themirrorformulaforsphericalmirrorsis

Foraplanemirror,f= ;So,

i.e.,theimageformedinaplanemirrorisasfarbehindastheobjectinfrontofit.Thisshows thatthemirror

formulaforsphericalmirrorsistrueforplanemirrortoo.

12. Thediagram describesasfollows:

1)TheobjectisplacedbetweenFandCinfrontofaconcavemirror.

2)TheimageisformedbeyondCinfrontoftheconcavemirror.

3)Theimageformedisinverted,enlargedandreal.

13. Given

Accordingtomirrorformula,

Thepositivesignof showsthattheimageisformedbehindthemirrorandisvirtualand erect.

Accordingtomagnificationformula,

The heightofimageis6cm

14. Given,


Thepositionofimageisatadistanceof8.57cm from the poleofthemirrorbehindthemirror(as is

positive).Theimageisvirtualanderectandsmallerinsize.

Accordingtomagnificationformula,

15. Whentheobjectisatinfinity,theimageisformedatfocuswhichishighlydiminished,realandinverted.Aswe

bringtheobjectclosertothemirror(beyondCatC,betweenCandF,atF)theimagemovesaway,from themirror,

increasesinsizebutremainsrealandinverted.WhentheobjectisplacedbetweenPandF,theimageformedis

behindthemirror,enlarged,virtualanderect.Theaboveexplanationcanbeshowninagraphisasfollows:

16. Given

Screenisplacedatapositionwheretheimageisformed.


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Theimageifformedatadistanceof54cm from thepoleofthemirrorinfrontofthemirror

(as isnegative).Sincetheimageisinfrontofthemirror,theimageisrealandinverted.

Accordingtomagnificationformula

17. Given, Accordingtomagnificationformula,


Note:Thefocallengthispositivebecausethemirrorisdiverging(convex)

For

Magnification ,u=?

Again,usingmirrorformula,

18. 1) Therelationbetweenobjectdistance,imagedistanceandfocallengthofaspherical

mirrorisgivenby Wheref=focallengthofsphericalmirror, =image

distance,u=objectdistance.Thisrelationiscalledmirrorequation.

2) Foraconcavemirror,Given:f=-15cm,u=-10cm, =?,m=?

From themirrorequation

=+30cm

Also,Magnification,

Hence,theimageisformedatadistanceof30cm behindthemirror.

Natureofimage:Virtual,erect.

Sizeofimage:Magnified,i.e.,thricethesizeofobject.

3) Positionofobject:Betweenpoleandfocus

Mirror:Concave

Positionofimage:Behindthemirror

Natureofimage:Virtual,enlargedanderect

19. 7.5m 20. (2) 21. 4.5m 22. (3)

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23. Theimpressiononblottingpaperisthemirrorimageofthewrittenmessage;Holdthewritten

Messageinfrontofamirrortoreadit.

24. True 25. False 26. True 27. False 28. True

29. False 30. True 31. False. 32. True 33. False

34. Atthecentreofcurvature 35. betweenFandO 36. 25cm

37. 10cm 38. 39. Enlarged

40. 2 41. 1 42. 1 43. 4 44. 4

45. 4 46. 1 47. 4 48. 2 49. 4

HOMEWORKSHEET-1

1. Aspersignconvention,thefocallengthofaconcavemirrorisconsiderednegative.

2. Aconvexlensactsasamagnifyingglass,whenitformsavirtual,erectandmagnifiedimageof anobject.For

this,theobjectistobeplacedbetweenprincipalfocusandopticalcentreofthe lens.

3. Asphericalmirroristhatmirrorwhosereflectingsurfaceisapartofahollowsphereofglass. Onesideofthe

mirrorisreflectingandtheothersideisopaque.

4. ThisisbecauseM hasleft-rightsymmetrybutPisnotsymmetrical.

5. Realimage:

i) Arealimagecanbecaughtonascreensinceitisformedbyactualmeetingofrays.

ii) Arealimageisalwaysinverted.

Virtualimage:

i) Avirtualimagecannotbecaughtonascreensinceitisformedbymeetingof

imaginaryrays.

ii) Avirtualimageisalwayserect.

6. Inaconvexmirror,thereflectedraysdonotactuallypassthroughthefocus(F).So,aconvex mirrorhasa

virtualprincipalfocus,whichissituatedbehindthemirror.

7. 1) Abeam oflightcomingfrom afaroffobjectisaparallelbeam oflight

2) Aparallelbeam oflightaftersufferingreflectionfrom aconcavemirrorreturnsbackas a

convergingbeam oflight.

3) Abeam oflightcomingfrom aflashinglightorfrom theheadlightofacar/truckisa

divergingbeam oflight.

8. Here ;

9. Here,sizeofobject,h=4cm

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Thus,imageisformedatadistanceof12cm behindtheconcavemirroras ispositive

Therefore,imageisvirtual.

Using

Soimageis8cm tall.Since ispositive,soimageiserect

10. Wehave, from thequestion,

Wehave

11. Thesituationisshowninfigure

Wehave,

Thus

Theheightoftheimageis4.0cm.Theminussignshowsthatitisontheothersideoftheaxis,i.e.,itisinverted.

12. From thequestion,

Thus,theimageisformed20cm behindthemirror(from thepositivesignof ).

13. Here,R=3.5m,f= =17.5m;u=–10.0m

Using, ,wehave or v= =1.5m

Thus,thecarappearstobeat1.5m from theconvexmirror.

Using,m = wegetm = =0.15

Thus,thesizeoftheimageofthecaris0.15timestheactualsizeofthecar.

Sincem ispositive,soimageofthecariserect(i.e.,upright).

14. Heresizeofobject,h=3cm

u=–10cm; f=–20cm

Using ,wehave; v=20cm

Thus,imageisformedatadistanceof20cm from theconcavemirror.Sincevispositive,soimageisformed

behindtheconcavemirror.Itmeanstheimageisvirtual.

Usingm= weget or h’=2xh=2x3=6cm

Thus,sizeoftheimageis6cm

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15. Given,R=2.0m,f= ;u=–3.5m;Using ,weget

Thus,thetruckappearstobeat0.78m from theconvexmirror.

Using,m =–

Thus,sizeoftheimageofthetruckis timestheactualsizeofthetruck.

Sincem ispositive,soimageofthetruckiserectandvirtual.

16. (4):Here,h1=3cm,u=–25cm,

f=–10cm,v=?,m =?

As

v=–50/3cm

Asimageisreal

m =

orh2=–2cm [minussignforinvertedimage]

Areaofsquarewire=(–2)2=4sq.cm

17. Concavemetaldish:Itwillcollecttheparallelbeam ofsatellitesignals atitsfocuswhere receivingaerialisfixed.18. (1)Concavemirror

(2)Alotofsun’sheatraysareconcentratedatthepoint ofsun’simagewhichburntheholein carbonpaper3)Atthefocus 4)Focallength5)Ablackcarbonpaperabsorbsmoreheatraysandburnsaholemoreeasily(thanawhitepaper)

19. (4) 20. 10cm;becauseitislessthanfocallength 21. 20cm22. (i)20cm (ii)10cm (iii)35cm (iv)30cm.23. False 24. True 25. True 26. False

27. True 28. True 29. True

30. 3 31. Zerodegree 32. 11 33. Onefourth 34. Infinity.

35. 1 36. 4 37. 2 38. 1 39. 2

40. 1 41. 4 42. 1 43. 2 44. 2

Documents

1 3.Blacksurfacesarepoorabsorbersofheat. TRUEORFALSE · 2020. 4. 17. · ASP-X-PHY-RL 1 Thevarioussourcesofenergyarethesun,thewind,water,fossilfuelsetc. Agoodsourceofenergyisonewhichsupplieslargeamountofusefulenergy,easilyavailable