Light&theElectromagneticSpectrum

ElectromagneticWaves

• ElectromagneticRadiationisatransverse wavecomposedofanelectricwaveandamagneticwave.•Unlikemechanicalwaves,EMwavescantravelthroughavacuum.• TheEMspectrumcontainswaveswithaarangeofwavelengthsandfrequencies.•Weonlyseelightintherangeof400-700nm

speedoflight=c =3.0*108m/sc =λ*f

Ex1:HowlongdoesittakeforlightformthesuntoreachEarthifthesunis1.5x1011 maway?

Ex2:Microwaveovensemitwavesofabout2450MHz.Whatisthewavelengthofthislight?

Transparency

•Lightisenergycarriedinanelectromagneticwavethatisgeneratedbyvibratingelectriccharges.•Whenlighthitselectronsinamaterial,theyvibrate.• Iffrequencyoflighthittingamaterial≠naturalfrequencyofthematerial,theelectronsareforcedintovibrationwithsmallamplitudes.• Onlyasmallamountofenergyislosttoheat.• Energyofthevibratingelectronsisreemittedastransmittedlight.

•Materialsthattransmitlightaretransparent.

Opacity

• Iffrequencyoflighthittingamaterial=naturalfrequencyofthematerial,theelectronsareforcedintovibrationwithlargeamplitudes.• Therefore,moreenergyislosttoheat.

•Materialsthatabsorblightwithoutreemissionandthusallownolightthroughthemareopaque.

Glass:Anexample

•Glassis_______________ tovisiblelight.•However,electronsinglasshaveanaturalvibrationfrequencyintheultravioletrange.•UVlightshinesonglassandalotofenergy________ toheat•Therefore,UVlight___________passthroughglass.•Glassis_______________ toUVlight.

Shadows

• Athinbeamoflightiscalledaray.•Whenlightshinesonanobject,someoftheraysmaybestoppedwhileotherspassoninastraightlinepath• Ashadow isformedwherelightrayscannotreach• Fuzzypartaroundtheedgesoftheshadowhappenswhen:• Lightfromonesourceisblockedbutwhereotherlightfillsin•Wherelightfromasourceisonlypartiallyblocked

Polarization

•Whenlightfromalamporthesunshinesonapolarizingfilter,thelightthatistransmittedispolarized.• Lightwillpassthroughapairofpolarizingfilterswhentheirpolarizationaxesarealigned,butnotwhentheyarecrossedatrightangles.

Holdanuprightfingeratarm’slengthandseehowitswitchespositionrelativetothebackgroundasyoualternatelycloseeacheye.

3D

• Visioninthreedimensionsdependsonthefactthatbotheyesgiveimpressionssimultaneously,eacheyeviewingascenefromaslightlydifferentangle.• Theviewseenbyeacheyeisdifferent.• Thecombinationofviewsintheeye-brainsystemgivesdepth.• Apairofphotographsormovieframes,takenashortdistanceapart(aboutaverageeyespacing),canbeseenin3-Dwhenthelefteyeseesonlytheleftviewandtherighteyeseesonlytherightview.

3DMovies!

•Moviesprojectthepairofviewsthroughpolarizationfiltersontoascreen.• Theirpolarizationaxesareatrightanglestoeachother,sotherighteyeseesonlytherightviewandthelefteyeseesonlytheleftview.• Overlappingpictureslookblurrytothenakedeye.• Toseein3-D,theviewerwearspolarizingeyeglasseswiththelensaxesalsoatrightangles.• Eacheyeseesaseparatepicture,justasinreallife.Thebraininterpretsthetwopicturesasasinglepicturewithafeelingofdepth.