Subject: Re: Colors
Date: Sun, 24 Jan 1999 18:25:27 -0500 (EST)
From: "Deborah A. DeMania" <dad3w@unix.mail.virginia.edu>
To: R A E Fosbury <rfosbury@eso.org>

Bob-

Thanks so much for your reply!  However, I have one clarifying question.

Your "Rayleigh's Fish" was quite a beautiful and accurate portrayl of the
"sunset effect", but again, as I mentioned in my letter, I see no green
(or even apple-green) in your fish.  Let me see if I can reason why this
is.

The red light that we see is caused by the absence of all other colors
(being scattered away before they get to our eyes due to the thick
atmosphere).  If the atmosphere were a little thinner, say, to allow red
and orange, we would see a redish-orange light.  If the atmosphere were a
little thinner still, we'd see red, orange, and yellow, and so a
redish-orangish-yellow light would be seen.  Now, if the atmosphere were
thinner still, we'd see red, orange, yellow, and GREEN.  This combination
of light doen't look green.  Red and green make yellow in the light color
wheel.  As we add more and more wavelengths, we get white, not green.

The above is why there is no green in sunsets, nor in your Rayleigh Fish.
I've seen greenish hues in the sky before a big storm (some people I know
back in Ohio where I'm from always say that the sky turns greenish just
before a tornado strikes) - this is the only time I've ever seen green in
the sky.  I'm not sure why this would be, unless is is caused by ice
crystals forming in the upper atmostphere, so that it is a combined
refraction/scattering pheonmenon (remember those hail-balls the size of
grapefruit? They have to start somewhere!).

Please let me know if my above logic is correct for the green being absent
- if not, I'd really like to see a picture of a sunset with some green in
it - I've never seen one!

Thanks again,
Deborah

Deborah A. DeMania
MSB 211
Materials Science and Engineering Department
University of Virginia
Charlottesville, VA  22906

phone: (804)-982-5691
email: dad3w@virginia.edu