33. Electromagnetic Waves

Transcription

1 33. letroageti Waves 33-. Maxwell s Raibow - Maxwell showed that a bea of light is a eletroageti wave a travelig wave of eletri ad ageti fields. The Spetru of letroageti Wave

2 fλ f : frquey 8 3 MHz : λ 3 6 f ye sesitivity (Visible light)

3 33-3. The Travelig letroageti Wave, Qualitatively LC Osillator ω / LC Curret ad Charge i the dipole atea varies siusoidally. letri ad ageti fields also varies siusoidally. Travelig letroageti wave (Tasvere wave). r ad r ; perpediular to the travelig diretio. r r. 3. The travelig diretio; the diretio of r r 4. r ad r ; varies siusoidally with the sae frequey ad i phase.

4 letroageti Wave Assue r yˆ si si r zˆ ( kx ωt) ( kx ωt) at positio x ad tie t ( 99,79,456 /s ) Aplitude ad agitude ratio of the eletri ad ageti fields: Speed of wave (i vauu): 8 ω / k 3. / s μ ε Wave legth: λ π/k

5 33-4. The Travelig letroageti Wave, Quatitatively Idued letri Field si si ( kx ωt) ( kx ωt) seget h dx Faraday s law r r dφ ds dt Mageti flux Φ x ( h dx) k os r r ds dφ dt d hdx dt h d h dx t ( + d) h h hd d dt or d dx ( kx ωt) ω os( kx ωt) ω k d dt x t

6 Idued Mageti Field si si ( kx ωt) ( kx ωt) seget h dx Maxwell s law r r ds μ ε Mageti flux Φ dφ dt ( h dx) r r ds dφ dt ( + d) h + h hd d hdx dt h d μ ε h dx d dt or d dx μ ε d dt x μ ε t x k os t μ ε k / ω μ ε ( kx ωt) μ ε μ ε ω os( kx ωt) ( ) μ ε

7 33-5. ergy Trasport ad the Poytig Vetor Poytig Vetor : Itesity of a eletroageti wave r S I μ r r S avg rs μ (Watt/ ) S μ I S ave eergy / tie area [ ] avg [ si ( kx ωt) ] avg μ ergy desity assoiated with a letri field : ergy desity assoiated with a Mageti field : I avg μ S S eergy / tie area μ rs power area u u avg μ ε ist μ power area ist u u μ ε

8 Total istataeous eergy desity : u u u ave + u ( ) ε μ ε ave ε μ I S ave u ave μ ε Variatio of Itesity with distae Power of the soure : P s Itesity at a distae r I Ps 4πr I S ave μ μ At a distae r μ I μp πr s For P s 8 W ad r V/.9-7 T

9 33-6. Radiatio Pressure iidet refleted Total absorptio ase Moetu hage: Δp ΔU ΔU: iidet eergy Total refletio ase Moetu hage: Δp Fore F Δt Δp ΔU I power area eergy / tie area ΔU IAΔt Total absorptio ase Δp IAΔt / Pressure F F p r A I IA Total refletio ase Δp IAΔt Pressure / F I p r A F IA

10 33-7. Polarizatio Polarized Light Polarized radoly or upolarized Polarizatio diretio Partially Polarized: ot opletely rado

11 Itesity of Trasitted Polarized Light y osθ Iodee of upolarized light I I os θ I I[ os θ ] avg I I I I I I o I os 6 o 3 3 I3 I os 3 I I I

12 33-8. Refletio ad Refratio The Nature of Light letroageti Wave Wave Property Light Photo Partile Property Photo hf (Uit : ergy --- Quatized ) h J s f : frequey (s - ) Refleted ray θ θ, : Idex of refratio Refrated ray siθ siθ siθ siθ siθ si θ v v

13 Law of Refratio Idex of refratio θ Wavefrots No hage i the frequey fλ v fλ v f λ θ Wavefrots θ θ λ i λ i l siθ l siθ f λ λ v v l l siθ fλ l siθ f λ siθ si θ siθ siθ v v

14 Idex of Refratio (λ 589) Media Idex Mediu Idex Vauu Air (STP) Water ( C) Aetoe thyl alhol Sugar solutio (3%) Fused quartz Sugar solutio (9%) xatly Typial row glass Sodiu hloride Polystyree Carbo disulfide Heavy flit glass Sapphire Heaviest flit glass Diaod θ θ Udfleted oditio. > θ < θ 3. < θ > θ

15 Chroati Dispersio Idex of refratio varies with the wave legth λ. hroati dispersio λ 47 λ 7 λ 7 λ 47 pris (quartz)

16 Raibow : a exaple of hroati dispersio Priary raibow Seodary raibow

17 33-9. Total Iteral Refletio si θ If θ si si siθ siθ si θ θ Total refletio θ : Critial agle Quartz :.45 ~.47 for visible light Total refletio si45º.44 <

18 33-. Polarizatio by Refletio rewster agle: If θ si + θ r o 9 fully polarized refleted light siθ siθ θ siθ si osθ 9 r ( θ ) ta θ rewster s agle (air), θ ta rewster s law