After completing this chapter, the students will be able to

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9. Dispersion of Light eLearn.Punjab

Students’ Learning OutcomesAfter completing this chapter, the students will be able to:• Explainrefractionoflightanditscauses.• Discusstheeffectsofrefractionwithexamples.• Listthecoloursoflightusingaprism.• Describethedispersionoflightbyaprism.• Identifydifferentusesoflightofdifferentcoloursathome,schoolandcountryandexplainthe

relationshipofchoiceofcolourstotheirpurpose.• Definespectrumoflight.• Identifyprimarycoloursandshowhowtheyarecombinedtoformsecondarycolours.• Identifyadeviceintheirsurroundingsthatusesdifferentcombinationofcolours.• Demonstratehowspinningofarainbowdiscresultsintheappearanceofwhitedisc.• Explainwhyanopaqueornon-luminousobjectappearstobeofcertaincolour.

Animation 9.2 : RefractionSource and Credit: District196

WehavelearntafewpropertiesoflightinclassVI.Inthischapterweshalldiscusssomemorepropertiesoflight.Youmayhaveseensomeofthesescenesaroundyou:

• Adeeptubfilledwithwaterappearslessdeep.• Apuddleofwaterontheroadonahot,sunnyday• AbeautifulrainbowintheskyafterrainAllthesephenomenaaretheresultofapropertyoflight,calledrefraction.

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9.1: Refraction

Weknowthatlightdoesnotneedamaterialmediumtotravel.Lighttravelsatdifferentspeedsindifferentmediums.Lighttravelsthefastestthroughthevacuum.Whenlightpassesfromonetransparentmediumtoanother,itchangesitsspeedanddirection(orbends).Thisbendingof light iscalledrefraction.But,whenlightfallsperpendiculartothesurfaceofthemedium,itdoesnotchangeitsdirection.

Fig. 9.1: The pencil in the glass of water looks as it has been broken at the water line. It is because of refraction of light.

Fig. 9.2: A light beam bends as it travels from air into glass and also from glass into air.

Important TermsWecanunderstandthetermrefractionwiththehelpofthefollowingterms:Incident Ray:Therayoflightthattravelsinonemediumandfallsonthesurfaceofthesecondmedium.Refracted Ray:Therayoflightthatchangesitsdirectioninthesecondmedium.Normal:Animaginaryline,drawnperpendicularlyonthesurfaceofthemediumatthepointwhereincidentrayfalls(pointofincidence).Angle of Incidence:Theanglebetweenthenormalandtheincidentray.Itisdenotedby‘i’.Angle of Refraction:Theanglebetweenthenormalandtherefractedray.Itisdenotedby‘r’.

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Activity 9.1 Effect of Refraction

You will need: • aplate• water• acoinProcedure1.Takeashort,opaqueplateandplaceacoinatitsbottom.2.Positionyourselfinsuchawaythatthecoinisjustnotvisibletoyou.3.Askyourpartnertopourwaterintotheplateuntilyoucanseethecoinclearly.What happens?Whenthecoinisnotvisibletoyou,theraysoflighttravellingfromthecoininastraightlinearenotabletoenteryoureyes.Theseraysareblockedbytheedgeoftheplate.Assoonaswaterispouredintheplate,thechangeofmediumoccurs.Nowlighthastotravelfromwaterintoair.Duetorefractionoflightitbendstoenteryoureyes.Thecoinisnowvisibletoyouduetorefraction.

Animation 9.3: Total Internal RefractionSource and Credit: The University of Sydney

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9.2: Refraction in Different Mediums (Glass and Water)

Whenlightpassesfromairtowaterorglass,itbendstowardsthenormal.Theangleofincidenceisgreaterthantheangleofrefraction(Fig:9.3a).

∠i>∠rWhenlightpassesfromwaterorglasstoair,itbendsawayfromthenormal.Theangleofrefractionisgreaterthantheangleofincidence(Fig:9.3b).

∠r>∠i

Fig. 9.3: (a) Light bends towards the normal when passes from air into water or glass. (b) It bends away from the normal when passes from water or glass into air.

9.3: Laws of Refraction

Therearetwolawsofrefraction.1. Theincidentray,therefractedrayandthenormalatthepointofincidence,allliein

thesameplane.2. Theratioofthespeedoflightinvacuumtoitsspeedinanothermediumisalways

constant.

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Activity 9.2

You will need• Aglassslab• Adrawingboard• Drawingpins• Whitesheetofpaper• Commonpins• GeometryboxProcedure1. Fixawhitesheetofpaperonadrawingboard.2. Putaglassslab in thecentreof thepaper.Draw

theboundaryoftheslabasABCD.3. FixtwopinsPandQonthepaper insuchaway

thattheymakeananglewiththeslab.4. Lookfortheimageofthepinsfromtheotherside

throughtheslab.FixtwomorepinsRandSinlinewiththeimagesofPandQ.

5. Removetheslabandthepins.Markthepositionofthepins.

6. JoinPandQastheincidentraytoABatpointO.7. JoinRandSastherefractedraytoCDatpointL.8. JoinOandL.9. DrawanormalNOMatpointO.10. FixPandQpinsatdifferentpositionsandplaceR

andSpinsontheothersideoftheslabaccordingly.You can see that the incident ray, the refracted ray and normal at the point of incidence, all lie in the plane of paper.

Using the knowledge from the above activitystudytherefractionoflightthroughaprism.

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9.3.1: Refractive Index

The speedof light varies indifferentmediums. Somemediumscause light tobendmore thanotherswhenitpassesthroughthem.Thedegreetowhichamediumcanbendlightisgivenbyitsrefractiveindex.Intermsofspeedof light,wecandefinerefractive indexas,“refractive index istheratioofthespeedoflightinvacuumtoitsspeedinthemedium”.

9.4: Real and Apparent Depth

Sometimesrefractionoflightgivesusafalseimpressionofthedepthandpositionofobjectsinwaterorglass.Forexample,wehavenoticedthatclearswimmingpoolslookshallowerthantheiractualdepth.Itisbecauseofrefractionoflight.Lighttravelsfasterinairthaninwater.Whenlightpassesfromadensermedium(water)toararemedium(air),itbendsawayfromthenormal.Whenthisrefractedlightentersoureyes,thebottomofthepoolandobjectslyingonthebottomappearclosetousthantheyreallyare(Fig.9.5).

Fig.9.4: Willebrord Snell (1580–1626) was a Dutch astronomer and mathematician. In 1621, he presented the law of refraction.

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9.5: Critical Angle

When lightrayspass fromadensermedium(waterorglass) toararemedium(air), theybendawayfromthenormal.Theangleofrefractionisgreaterthantheangleofincidence.Iftheangleof incidenceisgradually increased,astagewillcomewhenmaximumrefractionoccursandtheangleofrefractionbecomes90o(Fig.9.6).Heretherefractedraybecomesparalleltothesurfaceoftherefractingmedium.Theangleofincidenceforwhichtheangleofrefractionis90oiscalledthecritical angle.Itisdenotedby‘C’.Criticalangleforwaterisabout49owhileforglassis42o.

Fig.9.5. It is because of the refraction of light that the chest (box) appears higher in the water than actually is.

9.6: Total Internal Reflection

Whenlightpassesfromglassorwatertoair(densertorarermedium),itbendsawayfromthenormal.Butwhenangleofincidence(i)isgreaterthanthecriticalangle‘C’,thelightraysreflectinthesamedensermedium.Thisphenomenoniscalledtotal internal reflection(Fig.9.7).

Fig. 9.6: C is the critical angle of glass.

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Fig. 9.7: Total internal reflection

Totalinternalreflectiontakesplaceonlywhen:1. Lightpassesfromadensermedium(waterorglass)toararemedium(air).2. Theangleofincidenceofallraysmustbegreaterthanthecriticalangleofthatdensermedium.

Ci∠ > ∠

Fig. 9.8: The underwater reflection of the turtle is the result of total internal reflection.

Fig. 9.9: Once the angle of light beam becomes greater than the critical angle,

it is totally reflected at the surface of the water.

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9.7: Applications of Total Internal Reflection

Manyopticalinstrumentsusetheprincipleoftotalinternalreflectionfortheirworking.

PrismsAprism isablockofglasswiththreerectangularandtwotriangularsurfaces.Aright angled prismhasone90oandtwo45oangles.Thecriticalangleforglassisabout42o.Whenlightenterstheprism,itwillundergototalinternalreflection.

Binocular

Thecriticalangle forglass isaround42o.When lightentersaright-angledprism,itmakesananglegreaterthanthecriticalangle.Itcausestotalinternalreflectiontotakeplace.Abinocularusesreflectingprismstoseedistantobjects(Fig.9.10).

Animation 9.4: Total internal Refraction with critical view Source and Credit: Science Joy Wagon

Fig: 9.10: A binocular uses reflecting prisms to see distant objects.

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Periscope

Wecanseeobjectswhicharehigherthanoureyeswiththe help of a periscope. A simple periscope consists ofa tube,at theendsofwhicharefitted tworightangledprisms.Thefirstprismturnslightcomingfromtheobjecttowardsthesecond.Thesecondprismturnsittooureyes.The prisms use the principle of total internal reflection(Fig.9.11).Periscopesareusedinsubmarines,tanks,etc.

Fig:9.11: Prisms in a periscope help to see objects which are higher than eyes.

MiragesHave you ever seen water on the road ahead whiletravellingonahot,sunnyafternoon?Butwhenyougetthere the road isperfectlydry (Fig.9.12).Thewaterwasneverthere.Whatyousawwasamirage.AMirageisanimageofsomedistantobjectwhichappearstousduetotherefractionandtotalinternalreflectionoflight.Theairhigherupiscoolerthantheairneartheroad.Lighttravels fasterwhen it reaches thewarmerair. The lightraysbendastheytraveldownwardduetorefraction.Nearthegroundwhereair isevenmorewarm,the lightraystravelalmostparalleltothegroundbutcontinuetobendinotherdirection(totalinternalreflection).Whenweseethesebendinglightrays,ourbrainassumesthattherayshavetravelledinastraightline.Theseraysseemtousasreflectingfromwater.Asaresult,weseeamirage.Deserttravelersoftenobservemirages.

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Fish Eye View

Wehavestudiedthatwhenlighttravelsfromonemediumintoanother, its speed changes, which causes the light to refractat theboundary.As light travels fromwater toair, itwillbendawayfromtheperpendiculartothesurface.Whentheangleofincidenceisgreaterthan49degrees,allthelightisreflectedbackinto thewater (total internal reflection).Whenfish looksup, itwillseereflectedviewofthesidesandbottomofthepond,whiledirectlyabove,itseesacompressedviewofoutsideworldduetorefraction.

Fig. 9.13: A fish looks the water above as a mirror due to total internal reflection.

Animation 9.5: PrismSource and Credit: [email protected]

Animation 9.6: Total internal refflaction of light

Source and Credit: [email protected]

Animation 9.7: Total internal refflaction of light

Source and Credit: [email protected]

The critical angle of glass is 42o. Total internal reflectionmakes light transmission over longdistancespossibleinopticalfibres.Opticalfibresarethintransparentglassfibresinwhichlighttravelsduetototalinternalreflection.Thesefibresarecommonlyusedincommunication,e.g.intelephonetransmissions,TVprogramsandcomputers.Anopticalfibrecancarrythousandsofphonecallsatthesametime.Findotherusesofopticalfibresinmedicinesandindustries.

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9.8: Dispersion of Light

Sunlightisoftencalledwhitelight,althoughit isacombinationofdifferentcolours.Wecanseethesecoloursinarainbow.Thesecoloursarered,orange,yellow,green,blue,indigoandviolet.Wecanalsospiltwhitelight intoitscoloursbypassingitthroughtheprism.Thebandofsevencoloursobtainediscalledspectrumofwhitelight.Thesplittingofwhitelightintoitscomponentcoloursiscalleddispersionoflight(Fig.9.14).

Animation 9.8: Critical angleSource and Credit: Science Joy Wagon

Activity 9.3 Obtaining a Spectrum of Colours

You will need: • Aprism• AwhitecardboardProcedureShineanarrowbeamofsunlightononerectangularsurfaceofaprisminsuchawaythataspectrumofcoloursformsonawhitecardboardscreenontheothersideoftheprism.Identifythecoloursoflight,seenbyyou.Whichcolourisatthetopandwhichoneisatthebottom?

Fig. 9.14: Spectrum of light

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Activity 8.5 White light is a combination of seven colours.You will need: • acardboard• astring• glazedpapersofdifferentcoloursProcedure1. Takearoundcardboard.Pasteequal-sizedpiecesofglazedpapers

ofsevencoloursoflight.2. Maketwoholesnearthecentreofthecardboard.Makealoopofa

stringthroughtheseholes.Holdthetwoendsoftheloopintoyourhandsandalsotwistthestringstrongly.

3. Letloosethestringandstretchyourhandsinandoutalternately.Repeat thisagainandagain.Thecardboardwill start revolvingasshowninfigure.Watchcarefullytherevolvingcardboard.

Things to thinki.Canyoustillseethedifferentcoloursonthecardboard?ii.Whichcolourcanyouseeintherevolvingcardboard?

9.8.1: Why does White Light get Dispersed?

Whenabeamof lightentersaprism,all thecoloursofwhite lightrefractatdifferentangles– itcausesthewhitelighttosplitintoitscomponentcolours.Redlightbendstheleast.Violetlightbendsthemostandrefractsbythelargestangle. Inthisway,white lightdispersesintoitscomponentcolours{Fig.9.15(a)}.WhenthisspectrumisagainpassedthroughanotherprismasshownintheFig.9.15(b),abeamofwhitelightisobtained.

Fig. 9.15(a) Fig. 9.15(b)

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Rainbow Formation

Arainbowisanaturaldemonstrationofrefraction,dispersionandtotalinternalreflectionoflight.Whenwhite lightof theSunpassesthroughtinyraindropssuspendedafterrainfall,arainbowmayappear.Raindropsintheairactliketinyprisms.Theyrefractandreflectthesunlightandthenseparateitintodifferentcolours.Thecolourschemeofrainbowisthesameasinthespectrummadebytheprism.Sinceredcolourbendsthe leastandvioletcolourbendsthemostfromitsoriginalpath,sointherainbow,theredcolourappearsatthetopandthevioletcolourappearsatthebottom.Theothercoloursappearinbetweenthesetwocolours(Fig.9.16).

Fig. 9.16: A rainbow forms when sunlight is refracted and totally reflected by tiny water droplets.

Do You Know?• WecanonlyseearainbowinfrontofuswhentheSunisshiningbehindus.• Arainbowusuallyshowsallthesevencoloursofwhitelight.

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Activity 9.5 Seeing a RainbowYou will need: • asunnyday• arunninghosepipeProcedure1. StandwithyourbacktotheSun.2. Spraywaterfromthehosepipe.(Placeyourthumboverthehole

attheendofthehosetogetasprayingeffect.)3. Watchthesprayagainstadarkbackground(grassorwall).• Can you see a rainbow in the water droplets of the sprinkling

water?• Whatistheorderofthesecoloursfrombottomtothetop?

9.9: Colours of Light

An understanding of colours is very useful in photographyandtheaterlightings.Peoplewhoworkwithlightsofdifferentcoloursmustknowhowtoproducelightsofvariouscoloursfromafewbasiccolours.Thecoloursthatcanbeusedtomakeany other colour are called primary colours. These are red,blueandgreen.Wecanmixthelightofthreeprimarycolourstoproducewhitelight.

Red + Blue + Green = WhiteWhen two primary colours mix, they produce asecondary colour. Cyan, yellow and magenta aresecondary colours. A colour television uses differentcombinationsofcolours.

Red + Green = YellowRed + Blue = Cyan

Blue + Green = MagentaWecanobtainothercoloursoflightbymixinglightsofprimaryandsecondarycolours.

Fig. 9.17: Red, blue and green are primary colours of light.

Animation 9.9: Color mixingSource and Credit: [email protected]

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Activity 9.6 Mixing the Colours of Light

Colours of the light thrown on the screen

Colour which appears on the screen

Red + BlueBlue + GreenRed + Green

Red + Green + Blue

9.10: Colours of Objects

Whenwhitelightshinesonnon-luminousobjectstheyreflectsomecoloursandabsorballtheothers.Thecolourofanobjectisthecolouroflightitreflects.Aredobjectappearsredbecauseitreflectsredcolouroflightandabsorbsalltheothercolours(Fig.9.18).Thegrassofourlawnappearsgreenasitreflectsgreenlightintooureyes.Whydoesabluecarappearblue?Whenall the coloursof lightare reflected intooureyes, theobject appears white. And, when all the colours of light areabsorbedbytheobject,itappearsblack.Blackobjectsdonotreflectanylight.Objectsofcoloursotherthanprimarycoloursreflectmixtureofcolours.

Fig. 9.18: The petals of this rose appear red because they reflect red light. The leaves ap-pear green because they reflect green light.

You will need:• Threetorches• Red,blueandgreencellophanepapersProcedure:1. Takethreetorches.Pasteagreencellophanepaperon

theglassofoneofthetorch,redcellophanefilteronthesecondandabluecellophanepaperonthethirdtorch.

2. Throw lightsofdifferent coloursonawhite screen insuchawaythatlightofonecolourfallsonthelightoftheothercolour.

Observemixingofcoloursandfillthetable.

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Key Points• Whenlightpassesfromatransparentmediumtoanother,itchangesspeedandbends.This

bendingoflightiscalledrefraction.• Refractioncausesimagestoforminoureyes,arainbowtotakeplace,etc.• Whenlightpassesthroughaprism,itrefractsandbendsatanangle.Aprismcansplitwhite

lightintoitscomponentcolours.• Red,orange,yellow,green,blue,indigoandvioletarethecomponentcoloursofwhitelight.• Thebandofsevencoloursoflightiscalledthespectrumoflight.• Arainbowdischasallthesevencoloursoflight.Whenitisspinned,whitediscisseen.• Red,blueandgreenare threeprimarycoloursof light.Primarycolourscombine tomake

secondarycoloursoflight.• Thecolourofanobjectsisthecolouroflightitreflects.Aredflowerreflectsredcolourand

appearsred.Awhitesurfacereflectsallthecoloursoflightandappearswhite.Ablacksurfacereflectsnocolour.

Animation 9.10: RainbowSource and Credit: University of Utah

Questions1. Complete each of the following sentences by writing the correct term.i.Theycancarrythousandsofphoneconversationsatthesametime_______ii.Thebendingoflight,whenitentersfromonemediumtoanother_______iii.Theangleofincidenceatwhichmaximumrefractionoccurs _______iv.Theratioofthespeedoflightinvacuumtoitsspeedinanothermedium_______v .Thesplittingofwhitelightintoitscomponentcolours

After completing this chapter, the students will be able to

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Transcript of After completing this chapter, the students will be able to