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 α = 2π r /λ

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 << 1 [2p r << l] • 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. Long path for light to travel Rayleigh scattering of all but red light Chesapeake Light sunset Why are Sunsets Red?

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

9. 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

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