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LESSONS: Science - Light and Iridescence
from The Shining Cloth: Dress and Adornment that Glitter by Victoria Z. Rivers and The Splendor of Iridescence by Hilda Simon Dodd, Mead & Co, New York, 1971
How many times have you stared at a colorful swirl of colors reflected in a parking lot puddle? The luminous colors are caused when a thin film of oil and water reflects some light from its shiny surface. The rest of the light strikes and is reflected by the lower surface of the pavement. As the light enters the film it is bent and reflected, producing rainbows of light. What about some seemingly glowing mother-of pearl or abalone shell, or vibrant feathers, or certain brilliant, jewel-like beetle wings? These lovely visual phenomenon are examples of iridescence, the display of multiple bright colors. Iridescence takes its name from Iris, the Greek goddess of the rainbow and messenger to the gods. The visual effects of iridescence in natural materials have been very appealing to many people around the world. Because the colors are so magical in appearance, many believed in their supernatural origins. Iridescence is caused by how light responds to the surface of a substance. White light contains all the colors of the spectrum, which are divided into wavelengths with red the longest and violet the shortest in the visible spectrum. We see a color when its wavelength is reflected, while the surface or structure of a substance absorbs all the other colors. Some animals get their colors from the presence of chemical substances called pigments. For example, the pigment that produces red and yellow birds comes from caratinoids, which are absorbed from foods the birds eat. Other animals get their colors through how light enters and exits the cellular structures of their outer bodies. Pigments contain chemical substances that can reflect light rays, but they can fade and when certain pigment colored animals die, many of those colors fade with the animal. Structural coloring is more rare than pigment coloring, and is usually found in feathers, insects, and shells. Structural colors do not fade, because the reflected light is caused by the structure of the animals' surfaces. As light bounces off the surface of an animal, the wavelengths hit sub-microscopic materials and are bent and scattered. Birds produce feathers, which are composed of a fingernail-like material called keratin. The cells of some birds' feathers are filled with a dark substance called melanin. The melanin absorbs all light waves except for a few. Therefore, the color not absorbed, like blue, becomes extremely bright. Did you know that most green birds are really structurally blue? Hilda Simon, author of The Splendor of Iridescence saysthat the structural blue phenomenon in birds produces most green colored birds. 99% of green birds are actually blue birds with a dash of yellow pigment or yellow birds with blue-reflecting keratin structures. The green color results from the visual mixing of the bird's structure and pigmentation to produce unlimited ranges of greens. Iridescence, the multiple rainbow color effect, is caused by what is called interference. Interference colors are the brightest and purest of colors. These colors do not change or fade, even when the colorful insect or bird dies. Interference results when equal wavelengths of light meet and either amplify or cancel out each other, due to how they enter and pass through a substance. Some of the most brilliant birds are hummingbirds. Motion is important because the birds flash and change as they move. At one angle a hummingbird can look black, then at another angle it can appear brilliant yellow-green. Some hummingbirds flash violet, blue, green, gold-copper or ruby red due to interference, where the structure of the feather tissues causes light waves to reflect, eliminate or amplify each other. Hummingbird feathers are arranged like shingles, but only the tips can reflect brilliant colors. The structure on the tips contain about 8 to 10 thin layers of stacked air and melanin. Melanin, the animal pigment that produces grays, browns, and blacks absorbs light because it is dark. Light is greatly slowed, bent and partially absorbed as it enters through the stacked layers. Peacocks also reflect brilliant colors due to how the light waves are absorbed and eliminated in their very complex feather structures. Under great magnification, the peacock's feather structure is very complex and contains many rows and dots of melanin interspaced with keratin. The distance and spacing of the melanin rods determines the angle at which light rays bend and reflect. In the lifetime of an adult peacock, these complicated feather structures are recreated in every feather every time the animal sheds and grows a new feather train. Just think about it- billions of individual particles are necessary to create the colors of even a single peacock feather! Pearly shells are also iridescent. Mollusks create shells by secreting calcium carbonate, a chemical in their blood. Mother-of-pearl is the luminous inside of the shell produced by certain oyster and mussel shells. The glow is an optical effect created by light rays reflecting from many thin layers of the shell's inner layers. Iridescent colors result from how light passes through the layers. Some light is reflected through the top surface, while other rays travel through the layers and reflect back from the lower surfaces. The light is bent and slowed, compared to light that is immediately reflected from a surface. Some insects exhibit interference, due to the physical structure of their light-weight but sturdy exoskeletons composed of chitin. Multiple layers of this cuticle substance are composed of minute spacings that allows light waves to reinforce, weaken, or eliminate each other. Interference colors change with shifting rays of light and the animal's movements. The seemingly magical coloration of metallic beetles, mother-of-pearl, and certain feathers makes them extremely fascinating and beautiful.
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Word bank: Some animals reflect rainbow-like, ____________________colors caused by how light reflects off the surface of their structures. Feathers are composed of _____________________, a tough protein similar to hair or nails, while insects' exoskeletons are composed of ____________________. Birds' colors can come from ___________________,which are absorbed from the birds' diets. _____________________ contains all colors of the spectrum. Chemical pigment colors that produce browns, blacks, and grays are called ____________________. When equal light waves pass through a substance with irregular spacings, and the light waves are amplified, while other light waves are diminished or cancelled out, the resulting brilliant color is caused by _________________________. Nacre is the inner layer of shell manufactured by soft animals called _______________________. |