for students of class X

Score ‘n’ Learn

Physics

CONTENTS

Light 7Chapter 1

The Human Eye and the Colourful World

45Chapter 2

Electricity 67Chapter 3

Magnetic Effects of Electric CurrentChapter 4

Sources of Energy 119Chapter 5

97

The following 2 pages will give you a

glimpse of how effectively we have used

illustrations along with simple language

to explain a concept. The following pages

are from the part of the physics book

for X grade, these pages detail -

“Atmospheric Scattering of light”

62

PHYSICS Score ‘n’ Learn

5. Whichcoloursundergothemaximumandtheminimumdeviationduringdispersion?

6. Whatarethenecessaryconditionsforobservingarainbow?7. Whydostarstwinkle?8. Howmanyminutesbeforetheactualsunrisetimecanthesunbeseen?9. Whydostarsseemhigherthantheyactuallyare?10. Whatarethevariouscoloursofthespectrum?11. Whataretheadvantagesofhavingtwoeyes?

Atmospheric Scattering of lightSomenaturalphenomenaarisingbecauseofthescatteringoflightcanbeeasilyunderstoodbyunderstandingtheTyndalleffect.

Tyndall EffectTheTyndalleffectisnamedaftertheBritishphysicistJohnTyndall(1820-1893).Hediscoveredthatthepathofabeamoflightbecomesvisiblewhenitpassesthroughaheterogeneousmixture.Thishappensdue to thescatteringof lightbytheparticlessuspendedinthemedium.Thephenomenonofthescatteringof lightby theparticles inamediumiscalled theTyndalleffect.Thiseffectbecomesmorepronouncedasthenumberofsuspendedparticlesinthemediumincreases.Inourdailylives,weobservethiseffectinthefollowingsituations:Abeamofsunlightbecomesvisiblewhenitentersaclosed,dustyroomthrougha small crack.The tiny dust particles in the room scatter the beam of light.Hence,thebeamoflightisvisible.Wecanseethelightfromtheheadlightsofacaronfoggydays.Littlesunlightpassesthroughthethickcanopyinadenseforest.However,itisscatteredbythetinydropletsofwaterintheatmosphere.

Why is the colour of the clear sky blue?Youhavelearntthatred-colouredlighthasthelongestwavelengthandvioletorblue-colouredlighthastheshortestwavelength.Sincethecolourofscatteredlight also depends on the size of the suspended particles, blue light will bemorescatteredbyaminuteparticle,whileredlightwillbemorescatteredbyarelativelylarge-sizedparticle.Thesizesofthemoleculesofairandotherparticlespresentintheatmospherearesmallerthanthewavelengthsofvisiblelight.Hence,theseparticlesscatterlightsofshorterwavelength(i.e.,blue)moreeffectivelythanlightsoflongerwavelengths (i.e., red). When sunlight passes through the atmosphere, the fine particlespresentintheatmospherescatterbluelightmoreeffectivelythanredlight.Thescatteredbluelightmakestheskyappearbluetooureyes.IftheEarthlackedatmosphere,thenitwouldhavelookeddarkastherewouldhavebeennoscatteringoflight.Thiswasnoticedbytheastronautswhotravelledintothespace.Tothem,theskyappearedtobedarkbecauselightcannotbescatteredintheouterspaceasitlacksatmosphere.The scattering of red-coloured light finds its application in various fields. It is usedindangersignalsbecauseitisleastscatteredbyfog,smoke,orthedustparticlespresentintheair.

Tyndall effect

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THE HUMAN EYE AND THE COLOURFUL WORLD

Why does the Sun Appear to be Red at Sunrise and Sunset?Sunlighttravelsagreatdistancethroughtheatmospheretoreachusatsunriseandsunset.Bluecolour,whichhasashorterwavelength,getsscatteredawayintheatmosphere.Hence,thelightthatreachesusconsistsmainlyofredcolour,whichhasalongerwavelength.Thisiswhythesunappearstoberedincolouratsunriseandsunset.Atnoon,sunlighttravelsarelativelyshorterdistancethroughtheatmospheretoreachusasthesunisdirectlyoverhead.Inthiscase,onlyasmallportionofbluelightgetsscatteredintheatmosphere.Thisisthereasonwhythesunappearstobewhiteatnoon.

Figure 32: Colour of sun

Tounderstandthescatteringoflightfurther,letusundertakeanactivity.

Takeastrongsourceoflightandfocusitslightwiththehelpofaconverginglens.Allowthelightbeamfromthelenstopassthroughaglass tankcontainingclearwater.After this,allowthebeamtofallonacardboardhavingacircularhole.Obtainan imageof thecircularholeona screenwith thehelpof asecondconverginglens.Afterthis,add2mLofsulphuricacidandabout200gofsodiumthiosulphateinthewaterandlookattheimage.Whatdoyouobserveonthescreen?

Sectional Assessment 4 Time: 25 minutes

1. Whatisthefunctionoftheopticnerve?2. Namethetwotypesofcellsthatarepresentintheretinaoftheeye.3. Whatarethethreedefectsofvision?4. Whatdoyouunderstandbythescatteringoflight?5. Briefly explain why the colour of the sky is blue.6. DescribeTyndalleffect.7. Whyisredcolourcommonlyusedindangersignals?8. Whydoestheskyappeardarktoanastronaut?

The Idea box

Recall

Scattering is a very useful phenomenon. Rutherford used this in his famous gold foil experiment to determine the structure of an atom.

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sectional assessments and comprehensive exercises at

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