cbse class 10 science revision notes chapter – 10
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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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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.