Re: UV was Re: IR film & X-Rays ?

["Dave" <gannet@xxxxxxxxxx>]

Warning: long and possibly off-topic reply.

I just subscribed to the list (no bio because I have little or no
IR/UV experience to speak of <G>), but perhaps I can contribute a
little on this one question because it touches on another of my
hobbies, saltwater aquaria and the reproduction of corals in
aquaria. Even though corals are animals, most species receive most of
their nutrition from a symbiotic algae.  Because of this, coral
researchers have done a great deal of research on biological effects
of underwater spectra.  I offer this to the list, rather than email,
in case anyone else wants to do underwater IR/UV photography, and
also because I subscribed too recently to have the original message
and, hence, the poster's address. :)

> >10. And now a question for my sister:  How deep does IR penetrate
> >in freshwater?  in saltwater?  How about UV in freshwater?  in
> >saltwater? What about different colors of visible light?  (she is
> >interested in fish vision and when she asked her physics professor
> >he said he didn't know and wasn't sure where to look it up).

I have one book which is the "standard reference" for captive coral 
breeding: "The Reef Aquarium" by Delbeek and Sprung, ISBN 
1-883693-12-8.  On page 185 they give a chart that shows transmission 
of visible light.  By looking at the falloff curves, we can perhaps 
extrapolate to the IR/UV ranges.

The shortest wavelength shown is about 380nm, and they list a depth 
of 15m.  For 760nm they show about 8m.  In general, the tranmission 
curve is more-or-less bell-shaped and peaks at about 425nm, with a 
depth of 250m or more.  Light towards the red end of the spectrum 
falls off much more rapidly than blue light, hence the fact that 
underwater scenes of any depth appear blue to our eyes.

It would seem likely that fishes that live at shallow depths have 
near-human spectral sensitivity, but those who live at great depths 
are possibly blue-adapted.  Pure speculation on my part.  I do note 
that marine reef fishes and animals are the most exotically colored 
and patterned species on earth - they wouldn't expend the effort if 
it wasn't doing them some good, hence I conclude they can see it.

Delbeek and Sprung list the table as "After Thurman and Webber 1984". 
The bibiography (very extensive) lists the following: 

"Marine Biology", Thurman and Webber, Scott, Foresman and Co.  

Other references in the bibliography that seem like they may be of
interest are: 

"Ultraviolet radiation in the sea", Jerlov, Nature 166:111

"Solar ultraviolet radiation and coral reef epifauna", Science 
207:1069-1071

"Light harvesting by wavelength transformation in a symbiotic coral 
of the Red Sea twilight zone", Mar. Bio. 91:403-407

Finally, I suggest asking on the Usenet newsgroups (not all at once
<G>):

sci.aquaria
sci.bio.fisheries
sci.engr.color
sci.engr.lighting
sci.geo.oceanography
sci.optics
sci.physics.electromag

Hope this helps, and my apologies to the list for a long and 
borderline off-topic first post. :)

Dave

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Topic No. 15