Homework 2

Light scattering

Due on Thursday, April 23th, 2009.

Problem set

/files/homework_scattering.pdf

Answers

/files/once_in_a_blue_moon.pdf
/files/ice_vs_water.pdf

1. use x= 2 pi / lambda

2. Once in a blue moon. Most of you correctly indicated that blueing is due to a particular region of the Mie resonance (between x=6-11) and that large particles such as dust are responsible. One needs to assume lambda=0.4-0.7 to calulate possible range of particles. But the question still remains why we "never" see blue moon even though there are particles in the atmosphere of that size. I liked one answer that the possible solution may be due to the polydisperse size distribution of serosols in the atmosphere. Another hypothesis given was that only particles high in the atmosphere contribute (but that doesn't explain why we rarely see blue moon). I am attaching Chapter from the book by Craig Bohren - "Clouds in a glass of beer". Unfortunatelly, he doesn't seem to give full answer either, but for monodisperse case he is right. See also
http://www.philiplaven.com/mieplot.htm.
How can you get color by knowing the intensity?

3. Forward scatter is just that. But sometimes it is a bit broader and means several degrees from 0.
Backscatter is well defined for lidar (180). But in satellite application backscatter may indicate range of angles. Sidescatter is around 90. But may mean 65-135, say. Sometimes (certain nephelometers) we define forward scattering hemisphere (5-90) and back scattering hemisphere (90-180)

4. Many optical instruments use forward scatter to measure sizes. For example FSSP uses 4-12 scattering angles to measure sizes. One plots intensity from such directions as a function of size and finds semi-empirical relationship between size and intensity.

See for example
http://www.skytechresearch.com/Publications/TrudiCAO1985_FSSPcalibration.pdf

5. Refractive index differences - see several pages from Stevens's book.
/files/ice_vs_water.pdf

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