Chapter13– LightandReflection

Itislikelythatsomeofyouaregoingtofindthischapterfairlydifficult.Myadvice:• Donotfallbehind• CompletethepracticeproblemsinclasswhileIamcompletingthem.EventheonesIdoontheboard.• Ifyouneedhelp,stayinofficehours(TuesdayandThursdayafterschool)

Chapter13-1:CharacteristicsofLight

Pages442to446

Bytheendofthissectionyoushouldbeableto:q Identifythecomponentsoftheelectromagneticspectrum

q Calculatethefrequencyorwavelengthofelectromagneticradiation

q Recognizethatlighthasafinitespeed

q Describehowthebrightnessofalightsourceisaffectedbydistance.

Electromagnetic(EM)Waves- EMwavesarenon-mechanicalwaves,meaningtheydonotneedamediumtotravelthrough.

- Theycontainbothanelectricandamagneticfield.- FormalDefinition:Waves whicharesynchronizedoscillationsofelectricandmagneticfieldsthatpropagateatthespeedoflightthroughavacuum.

ElectromagneticSpectrum

Prismscanseparatewhitelight

• Nameoftheband?• Nameofthealbum?• Yearofthealbum?• HowmanyweeksdidthisalbumstayontheBillboardTop200?

• Nameoftheband?PinkFloyd• Nameofthealbum?DarkSideoftheMoon• Yearofthealbum?1973• HowmanyweeksdidthisalbumstayontheBillboardTop200?741weeks,(from1973– 1988)

Donotcalloutanswers!Wewilldiscussafter.

1. Howmanydifferenttypesofelectromagneticradiation(EMR)arethere?2. WhichtypeofEMRhasthe shortestwavelength?3. WhichtypeofEMRhasthehighest frequency?4. Whichcolorofvisiblelighthasthehighest frequency?5. Whichcolorofvisiblelighthasthelongest wavelength?

Donotcalloutanswers!Wewilldiscussafter.

1. Howmanydifferenttypesofelectromagneticradiation(EMR)arethere?7types2. WhichtypeofEMRhasthe shortestwavelength?Gammarays3. WhichtypeofEMRhasthehighest frequency?Gammarays4. Whichcolorofvisiblelighthasthehighest frequency?Violet5. Whichcolorofvisiblelighthasthelongest wavelength?Red

Lighttravelsslowerinamediumthaninavacuum

• Speedoflightinvacuum:2.9979x108 m/s.• Speedoflightinair:2.9970x108 m/s.• Speedoflightinice:2.2900x108 m/s• Speedoflightinwater:2.2500x108 m/s

• Eachtimelighthitsanatom,itisabsorbedandthenre-emitted.Sincethisprocesstakestime,itslowsdowntheprocess.Themoredensethemedium,theslowerlighttravels

**Recallthaticeisactuallylessdensethanliquidwater,hencewhyicefloatsinaglassofwater.

“WhenamIevergoingtoneedscience?Iwanttobeanewsreporter…”

Thesunis149,600,000kmfromtheEarth!That’sthelengthof1,496,000soccerfields!

Question:AssumingthecircumferenceoftheEarthis40075km,howmanylapsaroundtheEarthcouldlightmakein1.0second.Recall,thespeedoflightinairis2.9970x108m/s.Note:Youwillhavetodoaunitconversion.

Solution• Question:AssumingthecircumferenceoftheEarthis40075km,howmanylapsaroundtheEarthcouldlightmakein1.0second.Recall,thespeedoflightinairis2.9970x108 m/s.Note:Youwillhavetodoaunitconversion.• Convertthecircumferencetometers:40075kmx(1000m/1km)

=40,075,000m• Findouthowfarlighttravelsin1.0second(v=d/t)

d=vt =(2.9970x108 m/s)(1.0s)=2.9970x108 m/s• DivideyouranswerbythecircumferenceoftheEarthtofindouthowmanylapsitwoulddo:

Laps=(2.9970x108m/s)/(4.0075x107m) =7.478laps=7.5laps

Thespeedoflight… whofiguredthisout?• GalileoGalilei(1564– 1642)firsttriedtodeterminethespeedoflight,butwasunsuccessful.Whenwatchingacanonbefiredoffinthedistance,itwasobservedthattheflashoflightcouldbeheardbeforethesound.Heconcludedthatlighttravelsfasterthansound.• OleRömer (Danishastronomer)in1676madethefirstreal(althoughslightlyincorrect)measurementofthespeedoflightatanobservatoryinParis.UsingeclipsesonJupitercreatedbyitsmoonIo,hewasabletodeterminethespeedoflighttothree-quartersoftherealvalue.

Ifyouwanttoreadmore,checkoutthefollowinglink:http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html

Allelectromagneticwavesmoveatthespeedoflight

Note:Forallcalculationsinthischapterregardingthespeedoflight,wewillbeusing3.00x108 m/s.

Wavespeedequation: 𝒄 = 𝒇𝝀

c=speedoflight(m/s)=3.00x108 m/sf=frequency(Hz)𝝀 =wavelength(m)

ExampleProblemThemostcommonFMradiobandsusedforbroadcastingradiois87.5to108MHz. Whatarethelongestandshortestwavelengthsinthisfrequencyrange?

SolutionConvertMHztoHztobeabletousethewavespeedequation:• 87.5MHz*(1,000,000Hz/MHz) =87,500,000Hz• 108MHz*(1,000,000Hz/MHz)=108,000,000Hz

Calculateeachindividualwavelength• For87.5MHz: 𝒄 = 𝒇𝝀 so3.00x108m/s=(87,500,000Hz) 𝝀

𝝀 =3.42857… m=3.43m• For108MHz: 𝒄 = 𝒇𝝀 so3.00x108m/s=(108,000,000Hz) 𝝀

𝝀 =2.7777… m=2.78 m

Therangeofwavelengthsisfrom2.78mto3.43m.

LuminousFlux• Luminous:Fulloforsheddinglight;brightorshining• Flux:Continuouschange

• LuminousFlux:Therateofwhichlightisemittedfromasource.Luminousfluxismeasuredinlumens(lm).

Theluminousfluxdividedbytheareaofthesurfaceiscalledtheilluminance(measuredinlm/m2),decreasesastheradiussquaredwhenyoumoveawayfromalightsource.

Chapter13-2:FlatMirrors

Chapter13-2:FlatMirrors

Pages447to450

Bytheendofthissectionyoushouldbeableto:q Distinguishbetweenspecularanddiffusereflection

q Applythelawofreflectionforflatmirrors

q Describethenatureofimagesformedbyflatmirrors.

Howlightisreflecteddependsonthesurface…

IncomingandReflectingAnglesareEqual

Reflectionofflatmirrors• Thesimplestmirrortoanalyzeistheflatmirror(versusconcave/convex)• Flatmirrorscreatevirtualimages:Animagefromwhichlightraysappeartodiverge,eventhoughtheyarenotactuallyfocusedthere.Avirtualimagecannotbeprojectedonascreen.

Appearstobebehindthemirror- virtual

PracticalUsesofFlatMirrors– InteriorDecoratingTheuseofmirrorscanmakehallwayslooklongerorroomslooklarger.Smallclothingstoresoftenusethistechniquetolooklarger.

PracticalUsesofFlatMirrors– InteriorDecorating

Doubletheamountofsunlightinyourroombyallowingittobounceoffreflectivesurfaces.Hangalargemirrordirectlyacrossfromthelargestwindowinyourroom,orarrangeanarrayofsmallermirrorstohelpbrightenadarkstaircase.

InfinityMirrors– Artwork– YayoiKusama“YayoiKusama (Japaneseartist)hadabreakthroughin1965whensheproducedInfinityMirrorRoom—Phalli’sField.Usingmirrors,shetransformedtheintenserepetitionofherearlierpaintingsandworksonpaperintoaperceptualexperience.”

Wanttoseemore? • Checkoutthewebsite:http://hirshhorn.si.edu/kusama/the-exhibition/

Standingbetweentwomirrors(French:Mise enabyme)

Mostmirrorsreflect~90%ofthelightthattheyreceive,soastheimagescontinueon,theyslowlybecomedarker.

Examplefrompage447inyourtextbookofanobjectplacedbetweentwoflatmirrors.Again,noticehoweachvirtualimagebecomesabitdarkerthanthelast.

ImagelocationcanbepredictedwithraydiagramsTherelationshipbetweentheobjectdistance fromthemirror,whichisrepresentedasp,andtheimagedistance,whichisrepresentedasq,issuchthattheobjectandimagedistancesareequal.Similarly,theimageoftheobjectisthesamesizeastheobject.

Practice– HowtoDrawaRayDiagram(youwillhavetocopythisdowninyournotes)

Let'simaginethatagirlislookingatamirrorwithherlefteyeclosedandthereisagreenarrowinfrontofthemirrorasshowntotheleft.Howisthelightreflectedintohereye?

Writteninstructionscanbefound herehttp://www.physicsclassroom.com/class/refln/Lesson-2/Ray-Diagrams-for-Plane-Mirrors