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Vijay Natraj

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Light Scattering Vijay Natraj Ge152 February 9, 2007 – PowerPoint PPT presentation

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Title: Vijay Natraj


1
Light Scattering
  • Vijay Natraj
  • Ge152
  • February 9, 2007

2
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

a 2p r / ?
3
Scattering Regimes
4
Scattering Directions
total forward scattering
dominant forward scattering
radiation direction
even forward and backward scattering
Petty 2004
5
Rayleigh Scattering
  • a ltlt 1 2p r ltlt l
  • Scattered radiation is evenly divided between the
    forward and backward hemispheres

an example
6
Rayleigh Scattering
  • Rayleigh scattering by air molecules ?-4
  • Blue scattered more than red

7
Why are Sunsets Red?
  • Long path for light to travel
  • Rayleigh scattering of all but red light

8
Mie Scattering
  • 0.1 lt a lt 50
  • Scattering of sunlight by particles of haze,
    smoke, smog, and dust usually falls within this
    regime

an example
9
Geometric Optics
  • a gt 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
10
Geometric Optics
11
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

12
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

13
Aerosol Effects on Climate
14
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

15
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

16
Global Climatology of Aerosol Types
17
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

18
Phase Function
19
Rayleigh Phase Function
20
Linear Polarization
21
Rayleigh Linear Polarization
22
Radiative Effect
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