🚀 go-pugleaf

In article <1099@kepler1.rentec.com> Andrew Mullhaupt (andrew@rentec.com)
asked:
>Why is the sky blue?

   In the ravel of thread that followed, a consensus developed for the
standard "correct" answer of Rayleigh scattering.  As has been noted in
many posts, this perception is not just a question of physics.  In fact,
the standard answer would yield not a blue sky but a PURPLE one, if our
eyes were equally sensitive throughout the spectrum of "visible" lights.
(It's why we speak of ultra-_violet_, and not "ultra-blue".)
   It is well known that our eyes are in fact most sensitive in the
_green_.  My understanding (though I am not a biologist) is that the
increasing sensitivity with wavelength, and the decreasing intensity of
Rayleigh-scattered light, are the dominant factors combining to produce
the observation of a blue sky.  Of course, the complex way that we combine
information from three kinds of cones in our retinas to obtain a color
judgement, as well as the frequency-dependence of the solar radiation
filtered to the earth's surface, are also factors.
   The question of blue animals, apart from blue birds, has also arisen.
Clearly, they are less common, and some of the bright examples have been
given.  No one has mentioned people, however.  There are two instances
I can think of, apart from those found in Jazz and in a movie made by a
certain 1960's rock quartet.
(1) The Blue Berbers:  I don't know if this tribe persists, or if it keeps
to its old sartorial habits, but once the group was wont to wear robes dyed
a deep blue with indigo.  The color "rubbed off" (more likely sweated in)
and these people had appreciable amounts of indigo dye in their blood-
streams.  Their skin had a slight blue tinge.
(2) Pale-skinned people of partial color:
 (a) With veins close to the skin.
 (b) With the "Mongolian spot," (although, either through biology or
     through the use of one term for too many phenomena, this is not in all
     cases blue).  (For more details, please ask someone who actually has
     or had this pigmentation phenomenon.  I only know from veins.)

>P. S. I have an idea, but I'd like to collect answers from physicists.
>I sometimes use as a philosophical example 'go ask a physicist why the
>sky is blue, and he'll likely say...' and I would like to be honest
>about it. It's been a while since I actually asked any physicists this
>question, so here goes.

P.S.  Why does everybody forget blueberries when talking about blue foods?
Some crabs?  Aren't blue whales blue?  Do blue-fin tuna have blue fins?
Also, insects are animals, a few of these blue.  They might even be edible,
if you're hungry enow.

P.P.S.  The question was raised, why leaves are green.  Presumably this is
because most chlorophyll absorbs in the red.  But will someone explain
"chlorophyll a" and "chlorophyll b," and the algae colors?

Al_Kriman> cat flames >/dev/null.