next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects
Index of Subjects
Jean: After using the words reflexively without really thinking
reflectively, I think that 'reflective' is a general word that simply
implies any structure that sends back to your eye a fairly large
fraction of the light that was incident upon it, rather than just
absorbing or scattering that light. At least sometimes it implies some
directionality of the returning light (as in a mirror), otherwise if
the light scatters off in all directions from a rough surface, you
would not be struck by the idea that you had a reflective surface
there. Obviously this is not a strict definition, since it would be
perfectly fine to say that the newspaper reflects light back to you so
you can read it, and therefore has a reflective surface (which is rough
and so actually scatters light quite well). It doesn't say anything
about the specific wavelengths that are reflected, though one would
normally think of it in terms of "white" light being reflected
(difficult to define term -- broad band- or broad spectrum- light would
be better).
There are several ways colour things, for instance by having
red-orange pigments in leaves that selectively reflect or absorb
certain ranges of wavelengths, as in red-yellow fall colours seen at
present. Irridescence likewise results in specific colours returning
to the eye, usually noticed as an intense colored reflection of
incident sunlight which itself is broad-band (has a full range of
visible wavelengths present, though not all equally intense). It is a
reflection too but is more specific, characterized by high intensity
reflection at a particular viewing angle, intense 'pure' colors, and
strong directionality (disappears e.g. from specific feathers at most
viewing angles, and reappears on other feathers). In all the things
I've seen where this general English word is used, it is a so-called
structural colour, resulting from some kind of submicroscopic repeating
structure built into the feather, scale, beetle carapace or whatever.
If a series of grooves or ridges are raised on a reflecting surface,
constructive interference in the reflected light beam enhances those
specific wavelengths where the physical spacing is 1/4 of the
wavelength that you see reflected, while other wavelengths on either
side of this destructively interfere, reducing the reflected intensity
for these. This depends on the actual physical distance apart of the
ridges, so a specific periodic structure might reflect green, while the
blue on one side and yellow-red on the other side are muted ('green' is
not a physical quantity but a private human percept, inappropriate
here, so one should write 'reflects a wavelength band ~100 nanometers
wide centered on 500 nm' or some such, but most just say 'reflects
green'). If you look at the irridescent reflector from a different
angle, not straight on or normal incidence, the spacing seen at your
position changes and so the light reflected back towards you will
appear to change colour a bit, before it vanishes altogether at steeper
angles. More usually, the beetle carapace or whatever is not flat but
curved, so as you move around it, a different part of the carapace now
lights up -- the irridescence shifts around.
There are physics people on this list who can doubtless elaborate
further and more competently on related periodic things like photonics
crystals that can generate structural colours, or tell me I'm wrong.
The spot reflectors on these male wings are reflectors but are not
irridescent -- in 'white' light illumination their appearance is bright
white. The structures that commonly do this in animals, and have been
examined, are usually found to have have layered arrays of crystals
that are known to be crystalline guanine in some cases. Examples are
the back of the eyes of scallops, eyes of some crustaceans and some
moths at night (in the day, black absorbing pigments migrate in front
of the tapetum, so the eye glow disappears), fish scales apparently,
and (I'm not certain but think...) the eyes of alligators, cats etc.
The wing spots in this fly not only reflect also fluoresce fairly
strongly in UV-green, which makes me think that there may be a
reflector layer down there, perhaps guanine, but I didn't think flies
did this. But the transparent bristles covering the spot may be
involved somehow too, since they light up strongly (to repeat a bit of
an interchange about fibre optics I've been having with Dave off-line).
Steve
On 5-Oct-06, at 2:52 PM, Jean Timpa wrote:
> Is reflective the same as irridescence on some wings, especially
> dragonflies and
> damselflies? Probably not. JET
>
next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects