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> ...there's also backscattering preferentially in exactly the
direction
> of the incident light, plus a probably small additional brightening due
> to this backscatter being coherent.
> The phenomenon is usually referred to as "heiligenschein". If
there
> are water droplets invovled (from bedewed ground or fog) you can also
> see colored rings from diffraction, and this is called a "glory" or the
> "spectre of Brocken".
Now you're confusing different phenomena here:
- "Opposition effect": an apparent brightening directly opposite the
Sun, due to disappearance of shadows. No water drops are needed here,
ann that's needed is a rough surface of some kind: a wood, a corn
field, a savannah, whatever.
- "Helilgenschein": this is a refraction+reflection phenomenon due
to large (i.e. large compared to the wavelength of light) water drops
resting on a reflecting surface. The water drops focus the incident
light, which is reflected by the surface, and the water drops then
also focuses the reflected light back towards the incident direction.
This can nicely be simulated with many small glass balls, or round
glass beads, on e.g. a piece of white paper.
- "Spectre of Brocken", "Glory", "Anti-Corona": these are different
names of the same phenomenon, which is due to diffraction in small
water droplets floating freely in the air. It's typically seen on a
cloud if you're above the cloud. This phenomenon, which is quite
complex, is probably related to the rainbow. We all know the
appearance and the cause of the ordinary rainbow, right? If the
water drops get smaller, diffraction of light will cause visible
effects. First one will see extra (or supernumerary) rainbows,
immediately inside the primary raindow, with colors like
red-blu-red-blue/etc up to three or even 4-5 times. If the water
droplets are even smaller, the primary rainbow vanishes and only the
supernumerary bows are seen, but their colors are now so well mixed
that we only see them as white -- this is the "fog-bow". The fog-bow
is a few degrees smaller than the normal primary rainbow. Now, if
the water droplets get even smaller than that, the fogbow will become
smaller still, and fainter -- until it all "converges" into a spot a
few degrees in diameter, directly opposite the Sun. Since all the
light now is concentrated in a fairly small spot, this "degenerated
fog-bow" once again becomes visible, as the anti-corona, or glory.
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch_at_saaf.se psr_at_net.ausys.se paul_at_inorbit.com
WWW: http://www.raditex.se/~pausch/ http://spitfire.ausys.se/psr/
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