On Iridescence

(excepts from two pieces, by Gould & Lee and by Platt, plus a meditation on photography and iridescence)

source: Kevin S. Gould and David W. Lee, "Physical and Ultrastructural Bases of Blue Leaf Iridescence in Four Malaysians Understory Plants," American Journal of Botany 83. 1 January 1996: 45-50. p. 45: "Iridescence in plants has been attributed to a single physical effect, the constructive interference of reflected blue light," especially the effect created by special structures in cells near the surface of the leaf. p. 49: These cells have their cell walls "composed of multiple layers of parallel microfibrils.... The orientation of microfibrils in each layer is set at a small angle from that in adjacent layers." Such structures are called "helicoidal structures" and "share some properties of cholesteric crystals, a group of liquid crystals...." [on liquid crystals, see Peter Collins' book Liquid Crystals QD 923.C638 1990]

Can photographs [like the one above, a close-up of a rufous hummingbird's iridescent throat feathers] capture iridescence? It doesn't appear so. The camera's single eye sees some of the colors but not the shimmering, moving effect that two human eyes can see. If "constructive interference" and "layering" is needed for iridescence, maybe binocular vision is a requirement as well. Each eye's own `view" is layered image upon image within the brain, creating the rainbow-in-motion effect that the camera's cyclops eye is blind to. Brainwork's convolutions and rugae as constructive interference.... they hold the rainbow, not the retina. So what does the fly's compound eye see? Iridescence to the power of 40? Iridescence as essence of iris (iris: rainbow [Latin; stem irid]) incense for the eye

Iridescence in Insects by Meredith E. Platt [excerpts] Not all color [in Nature is] ... produced by pigments. Some of the most interesting colors in nature occur because of a phenomenon known as interference. The iridescent colors you see in a layer of oil on a wet pavement or in a soap bubble are produced by interference. So are the metallic colors found in some insects. These colors are produced by an object's surface structure, rather than by incorporated pigment molecules and they are often referred to as "structural colors." The principle of interference, in which light reflected from a lower surface interferes with light reflected from an upper surface, is easily illustrated using a soap bubble. When an incoming ray of light strikes the outer surface of a bubble, part of the light ray is immediately reflected, while the other part is transmitted into the soap film. After reaching the inner surface of the film, this transmitted light ray is reflected back toward the outer surface. When it leaves the bubble, it travels in the same direction as the ray that was immediately reflected and is, therefore, parallel to that ray. If the two rays of light are reflected back so that their wavelengths are "out of phase" with each other, the second ray will partly cancel out the reflection of the first ray. This is called destructive interference and the result is a reduction in color intensity. If, however, the wavelengths of the two reflected rays are "in phase", they will enhance each other. This is called constructive interference and it produces iridescent color. Whether the rays are in or out of phase with each other depends on the amount of added distance through the film that the second ray must travel before joining the first ray back on the surface. It is structural color that is responsible for the beautiful shimmering blue of neotropical Morpho butterflies and the glistening green of many tropical beetles. The situation with insects is much more complex, though, than the soap bubble example just described. Each microscopic butterfly scale is made up of not just one, but numerous angled, ridged surfaces. Rather than having just one reflecting surface, many are involved. The result of this multitude of reflecting surfaces is an intense iridescence..... NOTE: This excerpt is from LORE magazine, © 1996 Milwaukee Public Museum, Inc. For the full article, visit the following site: www.mpm.edu/collect/insect.html     cloud chamber patterns

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