Naturally Occurring Iridescence
From a microscopic perspective
Iridescence can produce some of the purest and most intense colors found anywhere in the world, greatly overshadowing even the finest dyes. Many insects, and various creatures found on every continent, display iridescence, whether it be on their wings, shells, antennae, abdomen, feathers, hairs, or eye coverings. This is for many purposes ranging from mating to defense. Most of the creatures famous for their beautiful coloring, such as peacocks, butterflies, and Japanese beetles, achieve such ornate patterns through reflections from iridescent structures rather than pigments, a common misconception. It was actually on insects and peacock feathers, that Sir Isaac Newton first studied the phenomenon of iridescence in the later years of the 17th century. It was discovered that the phenomenon occurs due to the interference of light through multiple reflections within the structure of a material. Many layers of film, like on the shell of the Japanese beetle, or millions of microscopic scales, like on the wings of a butterfly, create repeated interference that causes the colorful, intense light characteristic of reflections from iridescent surfaces.
Even if you have yet to experience a sighting of any one of these creatures, surely you have seen the swirling colored pattern an oil film makes on the surface of a puddle. The difference in the indices of refraction between the oil and the water creates an optical interface able to produce iridescent patterns. Since the phenomenon is caused by a combination of reflections and interference, it has a critical dependence on ray angles, or better described as incident and viewing angles. This is why as you walk around a puddle with an oil film on a sunny day, the colors and intensities change dramatically in a beautiful, dynamic pattern.
This experiment studies the iridescent structures of the exoskeleton of an insect leg, the wing covering of the Japanese tortoise beetle, and the shell of a fresh water mollusk. Each of these utilizes transparent layers of chitin, an extremely abundant material found in nature, most notably the cellular walls of animal cells. The insect leg was obtained through Brian McIntyre, Senior Laboratory Engineer for the Optics Electron Microscopy Facility. The fresh water mollusk was collected on the southern coast of New Jersey, and the Japanese tortoise beetle shell was provided courtesy of the University of Rochester Biology department. The samples are microtomed or cleaved, to create a clean cross-sectional cut for viewing. They are coated with approximately 100 nanometers of gold to protect the sample from charging. This thickness is within the bounds necessary to preserve the topographical integrity of the samples. The microscopic structures of each sample are studied in the Zeiss-Leo DSM982 scanning electron microscope.
The iridescent material
It was said that the main material which comprises these iridescent structures is chitin. Chitin is a polysaccharide chain composed of carbon, hydrogen, oxygen, and nitrogen in the following proportions: (C8H13O5N)n
Figure 1 shows its schematic chemical composition. Chitin is a naturally occurring polymer that is secreted by many creatures found all over the world. It is secreted for numerous different purposes, such as defense, detoxification, and vital mineral storage. Only a slight change in its exact chemical composition and thickness can cause noticeable differences in its optical properties. This is why secretion of the many different layers causes such random and beautiful iridescent patterns.
Plot 1 shows the x-ray spectrometry analysis of the mollusk shell. This is consistent with what the shell was thought to be made of. Hydrogen and Nitrogen cannot be seen by the spectrometer. Carbon and oxygen are both present in large quantities in chitin. The gold and palladium are the coating that was sputtered onto the sample. Calcium is secreted with the chitin, and it forms a bond with the chitin compound that makes the structure of the shell much stronger than chitin is alone.
The Japanese Tortoise Beetle
This beetle has two wing coverings each shaped like half of a tortoise shell. They have geometrical scale-like features on them that shine different colors at different angles. This is most recognizable in rather intense light. These shells are extremely tough and rigid, and offer a protective covering for the delicate wings. The iridescence is used as a defense mechanism, employed to startle predators.
Figures 2 and 3 show a cleaved
section of the wing covering of the Japanese tortoise beetle. Although
the cleaved section looked like a clean cut to the naked eye, at high magnifications
we can see that the fracture caused the many layers to split and fray.
This provides an opportunity to witness the number of layers that these shells
are composed of. These layers are extremely thin, in fact they seem to
be on the order of about 1/5 of a micron, well within the optical range.
These layers combine to form numerous interfaces. When white light, composed
of the entire visible spectrum, strikes the shell, the different layers can
pass the light to the next layer, absorb the light, or reflect it. Each
color of the spectrum corresponds to a specific wavelength, and due to the varying
thickness and refractive index of the each layer, one wavelength may be allowed
to pass while another is absorbed, or one absorbed while another is reflected,
etc. This is one cause of the colorful patterns that are visible
in the macroscopic world.
Figure 4 Figure 5
Figure 4 shows a microtomed section of the same shell. Although the microtomy blade makes an extremely clean cut, it can be seen from this high magnification micrograph that the tension of the material caused a slightly jagged cut to occur. The finer structure of the layers can be seen in each one of these "flaps," apparent at the center of the image. This image shows how ordered and structured the layers are, with an almost seamless transition between each layer. This allows the light to pass virtually uninhibited between interfaces.
Figure 5 demonstrates interesting structure. It seems to me that this is an artifact of the interference pattern. The layers appear to be curling in towards the center of the image. I believe this would be a section of the shell that would have either a dark spot, or a swirling iridescent effect.
Figure 6 is a zoomed out image of Figure 4. At this magnification, some interesting structure appears. It seems that above the "flaps" of the layers is some vertical repeating structure, followed again by horizontal repeating structure.
Iridescence has angular dependence because the incident angle determines a portion of the optical path length within each layer. The angular dependence is also due to scattering, refraction, and reflection angles from the optical layers within the structure.
If there are also vertical layers, as Figure 6 is suggesting, the light would strike these layers at some angle, and then the scattering, refraction, and reflection angles from a vertical axis orientation would also have to be taken into consideration. This would further enhance the angular dependence of the iridescence.
The Insect Exoskeleton
The exoskeletons of insects are often hard, shiny, and iridescent. Blue and green are the most prominent colors. Although it won't be discussed in depth here, these specific color choices have to do innate characteristic breeding mechanisms. The insects that display certain colors also seem to be born with an instinctive attraction to them. It's interesting that certain species of insects which display a certain color on their exoskeleton all somehow "know" to secrete a chitin compound of a specific refractive index and thickness, with rather small tolerances. This specific insect demonstrated green and blue-green iridescence (due to the destruction of the original whole sample, the species is unknown).
Figure 7 Figure 8
Figures 7 through 9 show the
rather minute structure of the insect shell. The high magnification micrographs
displayed as Figure 7 and 9 show that there may even be several
layers within the layers shown in Figures 1 and 2. Figure 7 seems
to show a fractured section of one of the layers, perhaps like those in Figure
8, since the lighter portion at the bottom of the image is a "shelf" of the
next layer. Within Figures 7 and 9, these narrow layers appear to
be on the order of 1/10 of a micron or less. These different thicknesses
of the secretions have interesting optical properties, since they cause an array
of optical path lengths. Varying numbers of reflections and optical
path lengths cause variations in phase. This causes constructive
and destructive interference to occur. If two waves of light meet peak
to trough, destructive interference occurs and the total beam power is lessened.
If the light meets peak to peak, constructive interference occurs and the total
beam power, and thus that specific color, is enhanced. This is yet another
integral part the iridescence phenomenon.
The Fresh Water Mollusk Shell
Mollusks are experts and secreting chitin. In fact, the word chitin comes from the Latin work for mollusk, "chiton." Many shells, in both fresh water and salt water, display beautiful arrays of iridescence. Pearls, a much desired artifact for jewelry purposes, is a good example of this. Also the mother of pearl sea shell. Most of this iridescence is on the interior of the shell, and is not used for defense or for mating purposes. It is simply the compound that is secreted during detoxification, for vital mineral storage simply to be absorbed later, and to smooth over the rough edges of the shell. The sample below is from the interior of a fresh water mollusk shell. The species in unknown.
Figure 10 Figure 11
Figure 12 Figure 13
Figures 10 through 12 show the layers of the mollusk shell as you zoom in towards them. Even at 2,000 X, the lowest magnification pictured here, we are studying microscopic structures. The shell appears extremely smooth to the naked eye, and is even shiny. Figure 10, if studied closely, reveals that the layers, including each individual layer, are not of a uniform thickness. This is crucial to forming different colors of iridescence, which this shell clearly demonstrates. The micrograph pictured in Figure 13 was taken on the opposite side of the shell as that pictured in Figure 12. This was done to show that the shell consists of the layers, in other words, it is built by the layers. These stacked "films" we could say do not appear just in a certain section.
Figure 13 has measurements superimposed on it. The thickness of mollusk shells vary by a large amount, both from species to species and even within species. You cannot tell how old a mollusk is by the number of layers in its shell like you can rings in a tree. The mollusk secretes the chitin on an irregular basis, and in varying thicknesses. The average mollusk shell is between 2.1 mm and 7.0 mm thick. At the thicknesses measured in Figure 13, this means that there are well over 15,000 of these layers per shell. Each layer has unique optical properties. This allows for extremely interesting optical effects, and more importantly, iridescence.
Plot 2 is a plot produced by a program that randomly calculated the electron flight patterns for a specific material. In this case the material is chitin bonded with calcium, with a gold-palladium coating on it. The total blue area is the interaction volume for electrons accelerated at 5kV. It is clear from this image that many of the electrons passed through the material.
The red area is the interaction volume for x-rays with the calcium in the material. Since the photon absorption energy for calcium is about 3 kV, and 5 kV is close to this, the interaction volume is very small. The photons do not penetrate very far before they are absorbed.
Physics Review. 1998. Iridescence in Lepidoptera. Philip Allan Publishers: http://newton.ex.ac.uk/research/emag/butterflies/iridescence_in_nature.html
Chitin. 2007. Wikipedia. http://en.wikipedia.org/wiki/Chitin.
Hecht, Eugene, Optics, 2nd Ed, Addison Wesley, 1987.