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CBSEClass10Science

RevisionNotes

CHAPTER–10

LIGHTREFLECTION&REFRACTION

Lightisaformofenergy,whichgivesusthepowerofvision.

Inthischapterwewillstudythephenomenaofreflectionandrefractionusingtheproperty

oflighti.e.straightlinepropagation(Lightwavetravelfromonepointtoanother,alonga

straightline).

Rayoflight:Itisalineinthedirectionofmovementoflight.

Beamoflight:Itisbucnhofraysoflight.

Parallelbeam:Alltheraysareparallel.

ReflectionofLight

Whenthelightisallowedtofallonhighlypolishedsurface,suchasmirror,mostofthelight

getsreflected.

LawsofReflection

1.Theangleofincidenceisalwaysequaltoangleofreflection.

2.Theincidentray,reflectedrayandthenormaltothereflectingsurfaceatthepointof

incidencelieinthesameplane.

ImageformedbyPlaneMirror(Planereflectingsurface)

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1.Virtual(imaginary)&Erect:Theimagethatdonotformonscreen.

Realimagescanberecordedonthescreen.

2.Laterallyinverted(Theleftsideofobjectappearonrightsideofimage)

3.Thesizeofimageisequaltothatofobject.

4.Theimageformedisasfarbehindthemirrorastheobjectisinfrontofit.

ReflectionoflightbysphericalMirrors

Mirrors,whosereflectingsurfacearecurvedinwardoroutwardsphericallyarecalled

sphericalmirror.

Forexample-Spoon Thecurvedsurfaceofshinningspooncanbeconsideredascurved

mirror.

Ifitiscurvedinward Actasconcavemirror

Ifitiscurvedoutward Actasaconvexmirror.

FewBasictermsrelatedtoSpherialMirror

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1. Principalaxis:Linejoiningthepoleandcentreofcurvatureofthesphericalmirror.

2. Pole:Thegeometricalcentralpointofthereflectingsphericalsurface.(aperture),

denotedby(P).

3. Aperture:Thewidthofreflectingsphericalsurface.

4. Centreofcurvature:Thecentreofthehollowglasssphereofwhichthespherical

mirrorisapartiscalledascentreofcurvature.

5. Radiusofcurvature:Thedistancebetweenthepoleandthecentreofcurvature.i.e.

PC=RorTheradiusofthehollowsphereofwhichthemirrorisapart.

6. Focuspoint:Thepointontheprincipalaxis,whereallparallelraysmeetafterreflection

iscalledasPrincipalFocusorFocus.Itisdenotedbyletter‘F’.

7. Focallength:Thedistancebetweenthepoleandfocuspointi.e.PF=f

8. RelationshipbetweenfocallengthandRadiusofcurvature.

ImageFormationbySphericalMirror

Beforewelearntheformationofimageorraydiagram,letusgothroughfewtips

(a)Remember,arayoflightwhichisparalleltoprincipleaxisalwayspassthroughfocus

(meetatfocus)orvice-versa.

(b)Arayoflightwhichpassesthroughcentreofcurvature(itisalsoknownasnormalatthe

pointofincidenceonsphericalmirror)willretracetheirpathafterreflection.

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(c)Arayoflightfallingonpolegetreflectedatthesameangleontheothersideofprincipal

axis.

Note:Arayoflightpassesthroughcentreofcurvatureofreflectingsphericalsurfacealways

actasnormalatthepointofincidence.Ifweknowthenormalwecandrawangleof

incidenceandangleofreflection

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Note1:Theimagewillonlyformwhentwoormoreraysmeetsatapoint.Imageformation

byaconcavemirrorfordifferentpositionoftheobject.

TheraydiagramsgiveninNCERTBooksarealsogoodtobefollowed.

ImageFormationbyConcaveMirror

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ImageformationbyConvexMirror

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UsesofConcaveMirror

1Usedintorches,searchlightandheadlightofvehicle.

2.Usedtoseelargeimageoffaceasshavingmirror

3.Usedbydentisttoseelargeimagesoftheteeth

4.Largeconcavemirrorusedtofocussunlight(heat)insolarfurnaces.

UsesofConvexMirror

Usedasrear-viewmirrorinvehiclesbecauseitgiveserectimage.Italsohelpsthedriverto

viewlargearea.

SignConventionforReflectionbySphericalMirror

1. Theobjectisalwaysplacedtotheleftsideofmirror.

2. Alldistanceshouldbemeasuredfrompole(P);paralleltoprincipalaxis.

3. Take'P'asorigin.Distancesmeasured

Rightoftheorigin(+x-Axis)aretakenpositive

Leftoftheorigin(–x-Axis)aretakennegative

Perpendiculartoandaboveprincipalaxis(+y-Axis)aretakenpositive

Perpendiculartoandbelowprincipalaxis(–y-Axis)aretakennegative

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MirrorFormula

f distancebetweenFandPole

v distanceofimagefromPole

u distanceofobjectfromPole

R distancebetweencentreofcurvatureandpole.

Where

Magnification

Itisexpressedastheratiooftheheightoftheimagetoheightoftheobject.

From1and2equation

When imageheightfromprincipleaxis.

Objectheightfromprincipleaxis

FewtipstoremembersignconventionforSphericalmirror

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f u v

CONCAVE -ve(real) -ve(real)-ve(real)

+ve(virtual)

CONVEX +ve +ve +ve

h–isalways+ve

h´–is+veforvirtual,–veforReal.

RefractionofLight

RefractionofLight:HappensinTransparentmediumwhenalighttravelsfromone

mediumtoanother,refractiontakesplace.

ArayoflightbendsasitmovesfromonemediumtoanotherRefractionisduetochangein

thespeedoflightasitentersfromonetransparentmediumtoanother.

Speedoflightdecreasesasthebeamoflighttravelfromrarermediumtothedensermedium.

SomeCommonlyobservedphenomenonduetoRefraction

Youreyes.

Rainbows.

Lightbendinginaglassofwater.

Glasses.

Cameralenses.

Objectdislocationinwater.

Binoculars..

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RefractionthroughaRectangularGlassSlab

WhenaincidentrayoflightAOpassesfromararermedium(air)toadensermedium(glass)

atpoint.OoninterfaceKL,itwillbendstowardsthenormal.AtptO1,oninterfaceNMthe

lightrayenteredfromdensermedium(glass)torarermedium(air)herethelightraywill

bendawayfromnormalOO1isarefractedrayO Bisanemergentray.Iftheincidentrayis

extendedtoC,wewillobservethatemergentrayO1BIparalleltoincidentray.Theraywill

slightlydisplacedlaterallyafterrefraction.

Note:Whenarayoflightisincidentnormallytotheinterfaceoftwomediaitwillgo

straight,withoutanydeviation.

LawsofRefractionofLight

Theincidentray,therefractedrayandthenormaltotheinterfaceoftwotransparentmedia

atthepointofincidence,alllieinthesameplane.

Theratioofsineofangleofincidencetothesineofangleofrefractionisaconstant

i.e.

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Forgivencolourandpairofmedia,thislawisalsoknownasSnell’sLaw

Constantnistherefractiveindexforagivenpairofmedium.Itistherefractiveindexofthe

secondmediumwithrepecttofirstmedium.

RefractiveIndex

Therefractiveindexofglasswithrespecttoairisgivenbyratioofspeedoflightinairtothe

speedoflightinglass.

c Speedoflightinvacuum= speedoflightinairismarginallyless,

comparedtothatinvacuum.

Refractiveindexofairwithrespecttoglassisgivenby

Refractiveindexofwater

Refractiveindexofwater

SphericalLens

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Atransparentmaterialboundbytwosurfaces,ofwhichoneorbothsurfacesarespherical,

formsalens.

ConvexlensConcavelens

1.Bulgingoutwards1.Bulginginwards.

2.Converginglens.2.Diverginglens.

ConcaveLens

Alensboundedbytwosphericalsurfaces,curvedinwardsisknownasdoubleconcavelens

(orsimplyconcavelens)

Itisalsoknownasdiverginglensbecauseitdivergesthelight.

FewBasicTermsRelatedtoSphericalLens

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1.Centreofcurvature:Alens,eitheraconvexlensoraconcavelensiscombinationoftwo

sphericalsurfaces.Eachofthesesurfacesformapartofsphere.Thecentreofthesetwo

spheresarecalledcentreofcurvaturerepresentedbyC1andC2.

2.Principalaxis:Imaginarystraightlinepassingthroughthetwocentresofcurvature

3.OpticalCentre:Thecentralpointoflensisitsopticalcentre(O).Arayoflight,when

passesthrough'O'itremainsundeviatedi.e.itgoesstraight.

4.Aperture:Theeffectivediameterofthecircularoutlineofasphericallens.

5.Focusoflens:Beamoflightparalleltoprincipalaxis,afterrefractionfrom

1.Convexlens,convergetothepointonprincipalaxis,denotedbyF,knownasPrincipal

focus

2.Concavelens,appeartodivergefromapointontheprincipalaxisknownasprincipal

focus.

ThedistanceOF2andOF1iscalledasfocallength

TipsforDrawaomgRayDiagram

1. Afterrefraction,arayparalleltoprincipalaxiswillpassthroughF.

2. AraypassesthroughF,afterrefractionwillemergeparalleltoprincipalaxis

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Imageformationbyaconvexlensforvariouspositionofobject

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ImageFormationbyConcaveLens

SignConventionforRefractionbySphericalLens

Similartothatofsphericalmirror,onlythedifferenceisthatallthemeasurementaremade

fromopticalcentre'O'

Lensformula

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Magnification

Itisdefinedastheratiooftheheightofimagetotheheightofobject.

Itisalsorelatedto‘u’&‘v’

Fromequation(1)&(2)

Ifmagnification

m>1,thenimageismagnified

m=1,imageisofsamesize

m<1,imageisdiminished

FewTipstoRememberSignConventionforSphericalLens

f u v

CONCAVE -ve -ve -ve(virtualimagealways)

CONVEX +ve -ve+ve(real)

-ve(virtual)

hisalways+ve

h´–veforRealand+veforVirtual&Errect.

PowerofLens

Thedegreeofconvergenceordivergenceoflightrayachievedbyalensisknownaspowerof

alens.

ItisdefinedasthereciprocalofitsfocallengthRepresentedbyP.

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IfFisgiveninmeter,then

IfFisgivenincm,then

SIunitofpowerofalensis“diopter”denotedby‘D’

IdiopterorID ItisthepoweroflenswhosefocallengthisIm

Powerofconvavelensordiverginglensisalwaysnegative

Ifanyopticalinstrumenthasmanylens,thennetpowerwillbe

Whatyouhavelearnt

Lightseemstotravelinstraightlines.

Mirrorsandlensesformimagesofobjects.Imagescanbeeitherrealorvirtual,

dependingonthepositionoftheobject.

Thereflectingsurfaces,ofalltypes,obeythelawsofreflection.Therefracting

surfacesobeythelawsofrefraction.

NewCartesianSignConventionsarefollowedforsphericalmirrorsandlenses.

Mirrorformula, givestherelationshipbetweentheobject-distance

(u),image-distance(v),andfocallength(f)ofasphericalmirror.

Thefocallengthofasphericalmirrorisequaltohalfitsradiusofcurvature.

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Themagnificationproducedbyasphericalmirroristheratiooftheheightofthe

imagetotheheightoftheobject.

Alightraytravellingobliquelyfromadensermediumtoararermediumbendsaway

fromthenormal.Alightraybendstowardsthenormalwhenittravelsobliquelyfrom

ararertoadensermedium.

Lighttravelsinvacuumwithanenormousspeedof3×108ms-1.Thespeedoflightis

differentindifferentmedia.

Therefractiveindexofatransparentmediumistheratioofthespeedoflightin

vacuumtothatinthemedium.

Incaseofarectangularglassslab,therefractiontakesplaceatbothair-glassinterface

andglass-airinterface.Theemergentrayisparalleltothedirectionofincidentray.

Lensformula, givestherelationshipbetweentheobject-distance(u),

image-distance(v),andthefocallength(f)ofasphericallens.

Powerofalensisthereciprocalofitsfocallength.TheSIunitofpowerofalensis

dioptre.