From: pauld@exploratorium.edu
Date: Tue Mar 28 2000 - 21:46:55 PST
Message-Id: <200003290546.VAA13044@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole Blue Skies Etc. Date: Wed, 29 Mar 2000 05:46:55 GMT
Hi Marc
Liquid oxygen is a pretty blue, but this contributes almost nothing to the blue
sky.
1. Nitrogen, Oxygen, water and Argon gas have small enough absorptions in
visible light that they are essentially colorless even over 10's of miles of
path length. However, they all have a resonance in the ultraviolet, they all
absorb UV. Because their electron clouds resonate with UV they all shake
mightily will Blue and less so with more distant frequency red. The shaking
electron clouds scatter light (proportional to the fourth power of frequency or
the inverse fourth power of wavelength.) So for these molecules the type makes
little difference to the color.
2. Water liquid absorbs red and blue becoming green at depth. Once again this
has nothing to do with sunsets. All the molecules in the air scater much blue,
and some green, over enough distance the scattering removes enough blue/green
that the remaining light is red. Some molecules actually do absorb blue light
like nitrous oxide and so their presence colors the sky red. Some dust particles
also absorb blue and are larger than a wavelength of light so that they scatter
red adding to the "red sky at night LA's delight" effect. It would be a violet
or UV sky if the sun's blackbody radiation were hotter and produced more of
those wavelengths. (The resonance that makes the scatter also absorbs light
that's why far UV is called vacuum ultraviolet since air absorbs it. Air also
ionizes and absorbs x-rays. so air wouldn't be x-ray in "color.")
3. Your students aren't going to like the answer.
Really they aren't.
In the theory of quantum electrodynamics we now use, Photons take every path in
the universe going from one place to another. Some of the photon going near the
critical angle went centimeters away, other bts went to Mars...
They travel as probability waves and only become particulate when absorbed and
emitted. However most of the photons probability amplitude wave is within a
region one wavelength in diameter and 3 meters long.
Paul D
>
> 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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