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Lecture3– Chapter2
• MoreonMaxwell’sEquations• QuantumPropertiesof Radiation• FluxandIntensity• SolidAngle
ATMOS5140
Maxwell’sEquations
• Changingelectricfieldinducesamagneticfield• Changingmagneticfieldinducesanelectricfield• Doesnotneedmaterialmedium(vacuumworks)• QuantitativeInterplaydescribedbyMaxwell’sEquations
Direction of propagation
Electric field vector
Magnetic field vector
Maxwell’sEquations
Gauss’sLawofElectricField
Gauss’sLawofMagneticField
Faraday’sLaw
Ampere’sLawandMaxwell’sadditionofdisplacementcurrentdensity
D=ElectricdisplacementE=ElectricfieldB=MagneticinductionH=MagneticfieldJ=ElectricCurrent
=densityofelectriccharge
Maxwell’sEquationsD=ElectricdisplacementE=ElectricfieldB=MagneticinductionH=MagneticfieldJ=ElectricCurrent
𝜀 = 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦𝑜𝑓𝑠𝑝𝑎𝑐𝑒𝜇 = 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐𝑝𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦𝑜𝑓𝑡ℎ𝑒𝑚𝑒𝑑𝑖𝑢𝑚
𝜎 = 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦𝑜𝑓𝑚𝑒𝑑𝑖𝑢𝑚
Maxwell’sEquationsD=ElectricdisplacementE=ElectricfieldB=MagneticinductionH=MagneticfieldJ=ElectricCurrent
=densityofelectriccharge
• Sinusoidalplane-wavesolutionsareparticularsolutionstotheelectromagneticwaveequation.• Thegeneralsolutionoftheelectromagneticwaveequationinhomogeneous,linear,time-independentmediacanbewrittenasalinearsuperpositionofplane-wavesofdifferentfrequenciesandpolarizations.
Maxwell’sEquationsforplanewaves
FigurefromTimGarrett
Maxwell’sEquationsforplanewaves
Maxwell’sEquationsforplanewaves
ComplexVectors(realandimaginaryparts)Ifyouneedarefresher:watchKhanvideo
https://www.youtube.com/watch?v=kpywdu1afas
Maxwell’sEquationsforplanewaves
ComplexVectors(realandimaginaryparts)
Constantcomplexvectors
Constantcomplexwave vectors(hereonlyconsiderrealpart)
WaveVector
• Thewavevector,k,countsthewavenumberinaparticulardirection.
• Spatialfrequencyofawave• Forexample,thewavenumberofthebluelineisthenumberofnodes(reddots)youseeperunitdistancetimes π.
• Count2nodesin4unitsofdistance,or3nodesin6unitsofdistance½nodesperunitdistance.
• Wavenumberisthenumberofradiansperunitofdistance
k=1/2π
Maxwell’sEquationsforplanewaves
Ifk’’=0,thentheamplitudeofthewaveisconstantThenthemediumisnonabsorbing!
• Indexofrefraction(alsocalledrefractiveindex)- N
Maxwell’sEquationsforplanewaves
Maxwell’sEquationsforplanewaves
Inavacuum:
k’’=0
PHASESPEED
Maxwell’sEquationsforplanewaves
RefractiveIndex
• Therefractiveindexofamaterialiscriticalindeterminingthescatteringandabsorptionoflight,withtheimaginarypartoftherefractiveindexhavingthegreatesteffectonabsorption.
RealworlduseofIndexofRefractions
Fig.3
CitationFredericE.Volz,"InfraredRefractiveIndexofAtmosphericAerosolSubstances,"Appl.Opt. 11, 755-759(1972);https://www.osapublishing.org/ao/abstract.cfm?uri=ao-11-4-755
Image©1972OpticalSocietyofAmericaandmaybeusedfornoncommercialpurposesonly.Reportacopyrightconcernregardingthisimage.
EasiertomeasureRefractiveIndexintheIRthaninthevisible
Fig.2
CitationFredericE.Volz,"InfraredRefractiveIndexofAtmosphericAerosolSubstances,"Appl.Opt. 11, 755-759(1972);https://www.osapublishing.org/ao/abstract.cfm?uri=ao-11-4-755
Image©1972OpticalSocietyofAmericaandmaybeusedfornoncommercialpurposesonly.Reportacopyrightconcernregardingthisimage.
A variety of values for the refractive index of BC has been used in global climate models including the OpticalPropertiesofAerosolsandClouds(OPAC)value of 1.74 − 0.44i [Hess et al., 1998]. As reviewed by Bond and Bergstrom [2006], reported values of the refractive index of light absorbing carbon vary widely; the real part, n, appears to vary from that of water to that of diamond, and the imaginary part, k, varies from that of negligibly absorbing material to that of graphite. Bond and Bergstrom [2006] hypothesize that strongly absorbing carbon with a single refractive index exists and that some of the variation in reported values results from void fractions in the material. Based on agreement between measured real and imaginary parts of the refractive index of light absorbing carbon, Bond and Bergstrom [2006] recommended a value of 1.95 − 0.79i at 550 nm. They caution, however, that this value may not represent void-free carbon. Stier et al. [2007] found that this value led to better agreement with observed atmospheric absorption, compared with the OPAC value. Motekiet al. [2010] found that the refractive index of ambient BC in the Tokyo urban area was about 2.26 − 1.26i at 1064 nm. The OPAC assumption for imaginary refractive index is not taken from combustion-generated particles, is lower than either of the latter two recommendations, and would lead to a MAC prediction about 30% lower if all other factors were equal.
• Indexofrefraction- N• Flux– F(W/m2)
Maxwell’sEquationsforplanewaves
Flux- Poynting vector
• Theinstantaneousfluxdensityoftheelectricfieldinthedirectionofthewaveis
• Foraharmonicwave(averageovercompletecycle)
𝐹 = 12 𝑐𝜀=𝐸
?
𝑆 = 𝐸×𝐻
• Indexofrefraction- N• Flux– F(W/m2)• Absorptioncoefficient– 𝛽E
Maxwell’sEquationsforplanewaves
AbsorptionConsiderscalaramplitudeofthewave
Nowgobacktoourdefinitionofflux
Recallthatourimaginarypartofwavevectorisresponsibleforabsorption
Absorption
𝛽E = 4π𝑛H/λ
KLM=distancerequiredforthewave’senergytobeattenuatedtoe-1 (about37%)
Absorption
Polarizationparameterapplyingtotransversewavesthatspecifiesthegeometricalorientationoftheoscillations
the"polarization"ofelectromagneticwavesreferstothedirectionoftheelectricfield.
Polarization
(a) (b) (c)
(d) (e) (f )
Linearpolarization
Circularpolarization
Ellipticalpolarization(HybridofLinearandCircular)
QuantumPropertiesofRadiation
Wavevs.Particle
• WaveNatureofRadiationmatters• ComputingScatteringandreflectionpropertiesofatmosphericparticles
• Airmolecules,aerosol,clouddroplets,raindrops• AndSurfaces
• QuantizedNatureofradiation• AbsorptionandEmissionofradiationbyINDIVIDUALAtomsandmolecules
• Photochemicalreactions
• BroadbandRadiation• Widerangeoffrequencies
• MonochromaticRadiation Radiation• Singlefrequency(onecolor)• Morecommonlyuse:quasimonochromatic(rangeoffrequencies)
• Coherent• Perfectsynchronization• ArtificialSource– radar,lidar,microwave
• IncoherentRadiation• Notphaselocked• Nosynchronization• Naturalradiationintheloweratmosphere
Monochromatic Flux
Broadband Flux
Wm-2 um-1
Wm-2
SphericalCoordinates
ElevationorZenith
SphericalCoordinates
ΩΩz = cos θ
Ω x = sin θ cos φ
Ωy = sin θ sin φ
x
y
z
θ
φ
Steradian
dθ
θ
φ
dφ
z
x y
θ
dω = sinθ dθ dφ
Intensity
dA ∝ cos 𝜃 = 𝑛U V ΩX
RelationshipbetweenFluxandIntensityUpwardFlux– Integrateoverupperhemisphere
DownwardFlux– Integrateoverlowerhemisphere
RelationshipbetweenFluxandIntensity
RelationshipbetweenFluxandIntensity