From: Marc Afifi (marc_afifi@yahoo.com)
Date: Tue Mar 28 2000 - 15:24:31 PST
Message-ID: <20000328232431.17109.qmail@web204.mail.yahoo.com> Date: Tue, 28 Mar 2000 15:24:31 -0800 (PST) From: Marc Afifi <marc_afifi@yahoo.com> Subject: Blue Skies Etc.
Hi everyone,
What an excellent discussion. I had several good
questions from students today (i.e. ones I can't
answer well) and I'm hoping that someone can help me.
1. I have read that the reason the sky is blue is
because oxygen preferentially scatters blue light.
What color does nitrogen preferentially scatter and
why doesn't the abundance of nitrogen in the
atmosphere make the sky some other color (if it
scatters a color other blue)? From Paul's last post
about his thesis I wonder if the type of molecule
makes any difference.
2. I have read that the reason the sunset is red is
because water molecules scatter green light
preferentially and the combination of water and oxygen
in the atmosphere scatter enough blue and green such
that only red light reaches our eyes. Also, sunsets
are much redder if there is smog or smoke in the sky
presumably because the particles are bigger and
therefore able to scatter the longer wavelength green
photons better. I seem to remember that the scattering
effect is true for all wavelengths but that the amount
of scattering varies inversely with the fourth power
of the wavelength. Am I totally out to lunch on that
one? So, is the sky really an x-ray or UV sky but we
only see the blue because the x-ray and UV are
absorbed through ionization processes? Or is it
because of the sun's temperature and its black body
spectrum producing more blue photons than x-ray that
we see the blue sky?
3. In terms of the critical angle for refraction, what
happens when a photon's angle of incidence is exactly
at the critical angle? If the photon travels along the
interface then, taken to the extreme, the photon would
be in two media simultaneously with two different
speeds. Wouldn't it then turn into the slower speed
medium? And, what is the thickness of a photon? I
remember Paul D. saying visible photons were eight
feet long (which is weird enough) but are they thick
enough that they could be in two media simultaneously?
If so, what is going on?! And, if not, then they don't
really travel along the interface at all, do they?
Rather, it would seem they should be in one medium or
the other, and thus there is really no chance at all
of them being refracted at 90 degrees, implying that
photons never have an angle of incidence exactly equal
to the critical angle. Does this question make sense?
I love my students dearly but it seems they keep
asking for the truth, and the truth is certainly "more
complicated than that."
-Marc
=====
Marc Afifi
Chemistry, Physics, Marine Science
Pacific Grove High School
Pacific Grove, CA
http://www.pghs.org/staff/afifi/d5hp.html
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