From: Ronald Wong (ronwong@inreach.com)
Date: Mon Apr 28 2003 - 20:53:05 PDT
Message-Id: <l03102800bad37eff101a@[209.209.19.201]> Date: Mon, 28 Apr 2003 20:53:05 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: re: small circular rainbow question
--- Debbie Berlin <debbie_berlin@yahoo.com> wrote:
>
> I was flying home to Boston yesterday and I noticed
> a rainbow outside the plane's window. It was
> circular, which makes complete sense (I was viewing
> it above the clouds, thus against a white
> background, so I could perceive all of it). Here's
> what I don't get. It was much much smaller than a
> typical rainbow. In diameter, it was about one fist
> held at arm's length. I ruled out some obvious
> explanations but was then stuck. Any answers?
> Other than "get a new pair of glasses?" :)
>
> Thanks, Debbie
Debbie:
Your observation that "It was much much smaller than a typical rainbow"
meant that the phenomena was NOT a rainbow - a nice observation on your
part.
Rainbows become a possibility when drops of water fall through the air
(i.e. rain, waterfalls, sprinkling systems, etc.). These drops are quite
large relative to the wavelengths of visible light and, under these
conditions, the effects of refraction and internal reflection dominate.
Individuals who are standing with their backs to the sun while the sun's
light strikes these falling drops of water will see the classic rainbow
arching across the sky with a solid angle of about 84° - an angle that is
considerably greater than that of a glory.
Clouds - and fog - appear in the air when molecules of water vapor begin to
stick together to form droplets of water. These droplets are quite small
and are far closer in size to the wavelengths of light than a typical drop
of rain. Under these conditions, the wave properties of light come into
play in which the surface waves that Paul referred to interfere in a way
that produces the circular spectrum that you saw in the form of a glory.
Refraction and internal reflection are still going on but their
contribution is too small to be seen.
Some fine points that I wish to add to Paul's message:
1. The size of the glory depends on the size of the droplets. Since
the effect is produced in a manner very similar to diffraction
(the angle of dispersion depends on the wavelength), the smaller
the size of the droplet, the larger effect.
2. The fact that you saw a spectrum of color means that the droplets
in the cloud were uniform in size.
3. When the droplets in the cloud/fog vary in size, the spectrums
of the individual droplets overlap and you see a white halo
instead of a well defined spectrum.
4. The glory is sometimes referred to as the "Specter of the
Brocken". The Brocken is a mountain in Germany and people who
climb that mountain frequently see it on the fog below them.
That's why, as a mountain climber, Paul is very familiar with
glories. The next time you climb a mountain (or any place where
the fog/clouds are below you), check it out.
5. If you should ever see one again while flying, see if the plane
is so close to the cloud that it's shadow is quite large. If you
can, take a walk to the far end of the plane and check it out
again. You'll see that the glory has "moved" to your end of the
plane. It "knows" where you are (pilot's see the glory as a
colored ring/white-halo around the nose of the shadow of their
airplane). In other words, the glory is all yours and no one
else's (the same is true with rainbows).
Cheers.
ron
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