Re: goldfish scales

[brian whatcott <inet@INTELLISYS.NET>]

At 11:35 4/15/99 -0400, George Spagna wrote:
> In my mis-spent youth, I recall an assigned problem in thin-film
> interference, in which the thicknesses and indices of refraction for several
> layers in a fish scale were given, and the question asked was "What color is
> the fish?"
>
> We'd like to work it through as an in-class exercise, but can't seem to
> locate the data.  Does anybody have the appropriate data, or a notion where
> we might find it?
>
> Thanks!

> George Spagna

I don't recall seeing many responses to this question.
 A web search did not reveal discussion of fish scale
interference films.

Not quite relevant, I DID find this on the web at
 http://www.dejanews.com/=zzz_maf/
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Subject: Re: Need scale information
From: fnkwp@aurora.alaska.edu
Date: 1998/08/26
Newsgroups: sci.bio.entomology.lepidoptera

> A book on irridescence in nature describes scales on wings like Morpho
> as each one being somewhat transparent but each one being "christmas"
> tree in shape.

That book may be "The Splendor of Iridescence: Structural Colors in the
Animal World' by Hilda Simon (Dodd, Mead 1971).

        The blue iridescence in _Morpho_ is produced by interference in
light scattered from the 'branches' in the 'christmas tree' structures
on the scale surface--the branches act as diffraction gratings. There
is also brown pigment within the scales, which accounts for the underlying
brown coloration when the light passes through the wing, or reflects off
the wing surface at very low angles.

                                                        Ken Philip
fnkwp@uaf.edu


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A more general search on thin film interference threw up this
physics demo page from Carnegie-Mellon at Pittsburg:

 http://bednorzmuller87.phys.cmu.edu/demonstrations/

This is still a 'work in progress' by all accounts.

  It reminded me that Carnegie, the British born
 U.S industrialist from the Victorian era was also responsible for
erecting 2505 public libraries, among many other public benefactions
 - notably, a pension fund for U.S college professors - one of
the first.  His Carnegie Institute at Washington, like compatriot
 Smithson's Institution there, added more luster to his inheritance.

It is interesting to compare various techniques in  radio frequency
and optical devices. One method of matching two transmission lines
of different characteristic impedances is to insert a matching section
of line whose characteristic impedance is the root mean square value
of the two.
   The analogy with the optical film of intermediate refractive index
between that of air and glass is evident.

Fish scales would presumably interpose heavily mismatched layers whose
thickness comprises odd multiples of a quarter wave of a particular
color.

    A method for matching a balanced pair transmission line to some
 balanced antenna of higher impedance is simply to gradually increase
the line impedance by increasing the conductor seperation, the so called
delta match. In the same vein, if such a transmission line is divided
far enough to provide the characteristic impedance of free space, an
effective directional antenna called the 'rhombic' results.
(A mirror image section and a terminator completes this design.)

   The analogy with the same highly directional property of the graded
 optical index fiber is instructive.

   It's been more than ten years now since I noted
(in one of my exceedingly rare technical conference papers  :-)
the direct optical control of an optical signal at 27GHz at Bell Labs
 (as it then was).

It seems we are ready for a creative leap into devices for direct
optical control of logic signals. This innovation has probably been
damped by the continuing success of the semiconductor makers at
providing more and more computing power on silicon, for less and
 less money.

   The comparably chilling effect of the wildly successful
thermionic tube industry in the first half of the 20th century
 on semiconductor device innovation is not lost on me....

Brian
brian whatcott <inet@intellisys.net>
Altus OK