Iridescence
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Newton observed that iridescence (change in color with the direction of observation) is caused by optical interference. The colors are produced by the interference or the light diffracting characteristics of the microstructures and not by the selective reflection or absorption of light, as in the case of pigments. Photonic band structures consist of periodic arrangement of dielectric materials even though they are transparent, have a bandgap resulting in inhibition of certain wavelengths. Light with frequencies in the gaps between the bands get reflected resulting in the specific color.
The structural colors of light can be seen in the animal world and minerals. For example, opal, a sedimentary gem does not have crystalline structure, but consists of tiny spheres of silica packed together. The diffraction of light through these structures results in spectacular colors determined by the size of the spheres and their periodicity. The spines of the sea mouse Aphrodite are covered with natural photonic structures that produce iridescence resulting in brilliant colors depending on the direction of the incident light and the direction of observation.
In the case of the butterfly, Morpho rhetenor, the metallic blue color is produced by the periodic structure of its wings. Its wings have layers of scales, each of about 200 mm long and 80 mm wide and about 1300 thin parallel structures per mm which form a diffraction grating.
The bird feathers viewed through an optical microscope show the structural components which are responsible for their coloration. The structural details of the peacock's eye pattern employing the scanning electron microscopy show photonic structures on peacock barbules. Barbules are the strap like branches on the peacock feathers. The four principal colors on the peacock feathers blue, green, yellow, and brown were identified wth the specific structures. The interference between the light reflected from the front surface to that reflected from the back was shown to be responsible for the brown color in the peacock's eye. The photonic structure of the green barbule has a periodic structure of melanin cylinders with a spacing of about 150 nm whereas the blue barbule has a periodic structure of 140 nm, and yellow barbules have a periodicity of 165 nm.
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