1. Vijay Natraj Ge152 February 9, 2007 Light Scattering

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 α = 2π r / λ

3. Scattering Regimes

4. Scattering Directions radiation direction total forward scattering dominant forward scattering even forward and backward scattering Petty 2004

5. Rayleigh Scattering  << 1 [2  r << ] –Scattered radiation is evenly divided between the forward and backward hemispheres an example

6. Rayleigh Scattering Rayleigh scattering by air molecules: ~ λ -4 Blue scattered more than red

7. Why are Sunsets Red? Chesapeake Light sunset Long path for light to travel Rayleigh scattering of all but red light

8. Mie Scattering 0.1 <  < 50 –Scattering of sunlight by particles of haze, smoke, smog, and dust usually falls within this regime an example

9. Geometric Optics  > 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. –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 ~ 137 o (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 ~ 130 o ) and resides 7 o outside of the primary rainbow when viewed with the sun at your back

13. Aerosol Effects on Climate

14. 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