An Electron Microscopic Study
I. There are many displays of iridescence in nature, found
in many different climates, for presumably, many different reasons. Iridescence
is simply coloration due to a microscopic physical geometry, rather than
the pigments that are usually considered to be sources of coloration. The
wings of Blue Morpho butterfly,the Urania ripheus butterfly,
the Rainboy Butterfly, and the surface of the mother of pearl, sea shell,
all share a similar mode of coloration: iridescence. The apparent bend of
the light comes from interference caused by small surface grooves on the
surface. There are approximately 4 striations on one micrometer of Blue
Morpho wing, and they are supposed to be made of chitin, the same material
marine creature's shells are made of. Looking at objects of this size is
well within the magnification range of the microscope. The purpose of this
lab is to compare the sources of iridescence in 3 butterfly and one shelled
II. The samples will be gold coated and placed under the
microscope, without any risk to the substructure's integrity. The butterfly
samples were aquired through Randi Jones, a butterfly artist form Kingston,
III. The experiment will focus on the microscopic structure
responsible for this visual phenomenon. All of the bitterfly samples will
generally share the same features, as iridescence is produced by very specific
factors common to all the samples, and a brief explanation of this optical
process is necessary. There are very small, evenly spaced groves, much like
those on a vinyl record, only smaller, on the wing of a butterfly. Light
is basically bunches of small particles traveling as a wave. As white light
waves hit the surface grooves from a certain angle, some of them are reflected
back to the eye "in phase", and some are "out of phase".
These two terms simply refer to the position of the traveling waves in relation
to other, parallel, traveling waves. If the crests and the troughs of the
waves are aligned, or "in phase", they will cause constructive
interference and iridescence. This happens when the one light wave hits
the first groove, and a second light wave travels half of a wavelength to
an other groove and is reflected back in phase with the first. If the crest
of one wave meets the trough of an other wave ("out of phase"),
they will cancel each other out, demising the overall light intensity. Because
iridescence is a very specific process, the angle of a flapping butterfly
wing to an observer may change the distance the light has to travel between
the grooves, giving the butterfly the appearance of changing from a sparkling
Blue, to a dull brown, as the light waves flicher "in" and "out
of phase." The same idea of in or out of phase applies to the sea shell,
but there a phenomenon called thin film interference that causes the coloration.
The symmetry and sizes of various surface grooves will be observed with
SEM using a backscatter imaging technique. Comparisons to known wavelengths
of certain light will be used to verify the results and teories proposed.
IV. Sources of Information
Platt, Meridith E. 1996. Iridescence
in Insects. Milwaukee Public Museum Web Page: 18.104.22.168/collect/insect.htm
Tada, Haruna et all. 1998.
Effects of butterfly scale on the iridescent color observed at different
angles. Applied Optics. Vol 37, # 9, March 20, 1998.
Vulinec, Kevina. 1997. Iridescent
Dung Beetles: a Different Angle. Florida Entomologist. Vol. 80, No. 2: 132-138.
Wu, C. 1997. Butterfly Sparkle
Characterized for Chips. Science News Online: www.sciencenews.org/sn_arc97/12_13_97/fob3.htm
Iridescence proved to be caused by multiple slit interference
in the butterfly species studied. The different visible colors, shimmery
blue, green, and dull blue, were only observed on one side of the wing with
the naked eye. The other side varied considerably from specie to specie,
but each reinforced the multiple slit hypothesis. The actual structure of
the wing was a clear material covered in melanin, and lined with microscopic
scales on both sides, that looked like dust to the naked eye. By definition,
the blue coloration observed in the shell was caused by iridescence, but
a different mode of interference explains the effect. Blue color is prevalent
among the species analyzed, which seem to be representative of the physical
coloration's in nature found by other researchers. The functionality of
iridescence is not known exactly at this point, but may have something to
do with the environmental conditions such as predator and mate eye capabilities,
temperature, color or other unknown factors.
Morpho menelaus menelaus (shimmery
Iridescence is bright, and fluctuates when the sample is
tilted or rotated with respect to the eye, resulting in a shimmery effect.
This effect may be used to confuse predators, as each flap of the wing would
produce a bright blue flash, like a strobe light. The overall structure
can be seen in the following progression of micrographs:
Figure 1 - 3 are micrographs of the same section of a Morpho
wing at increasing magnifications, which are displayed over each photo.
Figure 1 shows the a 5x3 mm section of wing. The parallel arrangement of
tiny scales can be seen. Some scales have been removed and the dark wing
(believed to be melanin) they grew out of is exposed. Figure 2 shows the
individual scales. They are approximately 70x200 um in size. 3 parallel
veins from figure two are seen close up in Figure 3, which has been photographed
at a 40 degree angle.
Sunlight is a white light because it is a combination of
light of different colors blended together, which all correspond to different
wavelengths. As white light hits the slits, most of the wavelengths of light
approach in phase, but are put out of phase when they bounce off the slits,
as shown in Figure 6. This is destructive interference. Blue light has a
wavelength range from 400-480 nm, and is the only wavelength that is interfered
with constructively by the slits, which are 200 nm apart. Blue light waves
travel exactly half of a wavelength between slits, and the presence of many
slits adds more "in phase" blue light to reflected image, strengthening
the appearance of the color blue. The dark material which the slit structure
is attached to (figure 3) is made of melanin, a material that absorbs light,
further strengthening the blue image. The following photo shows the site
where the scales are attached to the wing.
Figure 7 shows the pore from which the scales producing
the iridescence grew out of and differentiated into the complex structure
that was observed in the previous slides.
The underside of the wing is brown and lackluster. Its
apparent coloration does not change with viewing angle as it is due to chemical
pigments, rather than a physical structure. The iridescence is more useful
on the top side of the butterfly, where it can be used to elude their main
predators; birds. These scales are for protecting the wing from physical
Figure 8 and 9 are the scales and the microstructure on
the underside of the wings. The slits on the scales are 160 nm apart, which
is not a multiple of half the wavelength of visible, and thus do not cause
The iridescence displayed on this butterflies wings is
seen as thin green stripes on a black background. The black background is
colored by chemical pigmentation, and the green color is a result of a similar
physical construction as described for Morpho, but the distance between
the slits are half of a wavelength of green light. This samples iridescence
is not as vivid as that of Morpho, because there are not as many
slits reinforcing the effect. On this sample (figure 10), there are 2 to
3 slits in one given spot as opposed to the 4 to 5 (figure 3) on Morpho.
Figure 10 is the microstructure of Urania
ripheus, aka Sunset Moth.
Blue patches of color dot the black wings of this butterfly,
but the blue is duller than the green and blue of the previous samples,
and is unaffected by angular rotations. The backside of blue dot is clear,
allowing the an unobstructed view of the blue dot from both sides. Since
the back of the blue dot is white, the color on the top surface must be
caused by physical, rather than chemical phenomenon. The examination of
a chemically colored dot, that is green on both sides, supports this assumption,
but the mode of physical coloration is unclear. A careful comparison of
the Morpho slits producing the brightest iridescence (figure 3) and
the Urania slits of lesser brightness (figure 10) reveals a shift
to an increasing angle between the slits and the surface of the scales.
The dull nature of the rainbow's blue dots can be explained using this correlation.
Figure 11 displays parallel striations approximately 220 nm from edge to
edge. This distance is half the wavelength of blue light. The large angle
between the striations and the surface of the wing causes the dull color,
and because the valleys between the ridges are rounded, it is easier to
view the striations from any angle, without a change in color or brightness.
Figure 11 is a micrograph of a rainbow butterfly's
microstructure. The arrangement of the grooves gives rise to a dull blue
color that can be observed with the naked eye.
Figure 12 is a rare micrograph of an overturned and uprooted
scale of a rainbow butterfly lying next to a relatively intact scale. The
scales are so small it is extremely difficult to manipulate them into position.
The lack of grooves indicates that the physical coloration has only been
useful to the evolution of the butterfly on the front side, where it can
be seen by animals the butterfly interacts with.
Mother of Pearl (sea shell)
The sea shell shows an other mode of iridescence, clearly
demonstrating the thin film model (figure 14). Here waves of light are slowed
down by the thin layers of chitin (figure 13), which are layer on top of
each other. If the wavelength of light is refracted (slowed down) by the
change in traveling medium, from air to chitin, some wavelengths of light
are reflected off the bottom surface of the thin film, and placed back in
phase with light that has not penetrated the thin film, but reflected off
the top surface. This particular piece displayed a blue iridescence, but
other sections of the shell, displayed a range of colors. The color's displayed
are dependent on the thickness of the layer of chitin or its index of refraction,
which is just how much a material slows down light as it enters.
Figure 13 is cross section of a piece of a
sea shell. It is made of chitin, and the thickness of the layers is shown.
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