Transcript Document
Light Scattering Vijay Natraj Ge152 February 9, 2007 What is Scattering? • Process by which particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions • No energy transformation results, only a change in the spatial distribution of the radiation • Function of ratio of particle diameter to wavelength of incident radiation α = 2π r / λ Scattering Regimes Scattering Directions total forward scattering dominant forward scattering radiation direction even forward and backward scattering Petty 2004 Rayleigh Scattering • a << 1 [2p r << l] – Scattered radiation is evenly divided between the forward and backward hemispheres an example Rayleigh Scattering • Rayleigh scattering by air molecules: ~ λ-4 • Blue scattered more than red Why are Sunsets Red? • Long path for light to travel • Rayleigh scattering of all but red light Chesapeake Light sunset Mie Scattering • 0.1 < a < 50 – Scattering of sunlight by particles of haze, smoke, smog, and dust usually falls within this regime an example Geometric Optics • a > 50 – Angular distribution of scattered radiation can be described by geometric optics (ray tracing) – Scattering of visible radiation by cloud droplets, raindrops, and ice particles falls within this regime an example Geometric Optics Geometric Optics – small scattering angles: diffracted light predominant – other than diffraction, most of the light scattered into forward hemisphere due to rays passing through particle with two refractions – ~ 80-120° scattering angle: reflection from outside of particles – maximum in backscattering direction (glory): incident edge rays Geometric Optics – Gradual change in scattering angle at ~ 137o (for water) results in a “bunching up” of rays • “focusing” of energy on a narrow range of scattering angle gives rise to the bright ring that we call a rainbow – Secondary rainbow arises from two internal reflections (scattering angle of ~ 130o) and resides 7o outside of the primary rainbow when viewed with the sun at your back Aerosol Effects on Climate Aerosol Effects on Climate • What if aerosols reflect incoming sunlight back to space? – Net cooling below the aerosols – Called the Direct Effect • What if dissolved aerosols in cloud droplets changes their size or optical properties? – Could enhance or decrease scattering leading to cooling or warming. – Called the Indirect Effect Global Climatology of Aerosol Types – Seven basic aerosol types: sulfate(land/water), seasalt, carbonaceous, black carbon, mineral dust (accumulated/coarse) – Each mixing group is a combination of 4 aerosol components – Lognormal distribution Global Climatology of Aerosol Types Scattering Matrix • Describes transformation from incident to viewing direction • In many cases, function only of scattering angle • (1,1) and (1,2) elements describe intensity and linear polarization of outgoing radiation; former called phase function Phase Function Rayleigh Phase Function Linear Polarization Rayleigh Linear Polarization Radiative Effect 8 Normalized Jacobian 6 4 2 0 -2 Type 1a 1b 1c 2a 2b 3a 3b 4a 4b 4c 5a 5b 5c -4 -6 -8 -10 0.758 0.760 0.762 0.764 0.766 Wavelength [m] 0.768 0.770 0.772