Nature's Iridescence

An Electron Microscopic Study

 

 

Proposal

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 sea creature.

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, WA.

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: 192.206.48.3/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

 

Experiment Overview

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.

 

The Experiment

Morpho menelaus menelaus (shimmery blue)

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

Figure 2

 

 

 

Figure 3

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 contact.

   

 Figure 8

 Figure 9

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 interference.

 

Urania ripheus

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.

 

Rainbow Butterfly

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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