Opals iridescence

Tyndall scattering also causes the blinding effect of shining a car headlight directly into a thick bank of fog or mist it also yields the beautiful iridescent colours on the wing of a butterfly or peacock tail, and an opal and mother of pearl. A good potter can reproduce some of these optical effects with so-called iridescent glazes, which comprise colloidal materials. 

Figure 15.6 (a) an opal bracelet and (b) the iridescent colours of the Madagascan sunset moth. The... 

Figure 3.14 Detail of some natural opals from the Natural History Museum ofVienna. The iridescence of this semiprecious stone of hydrated silica is due to the ordered packaging of the silica microspheres as shown on the left bottom comer. This structure can be artificially reproduced and... 

Opalescence is a pearly or milky internal reflection, most common in white or light-colored minerals such as some opal or moonstone (albite or ortho-clase feldspar). This is also known as adularescence. Like iridescence and play of color, these properties are not consistent within any natural material, and can only be used to describe individual specimens. Some synthetic materials have been created specifically to display these properties and so always show them. 

Two-dimensional PBG crystals can be thought of as a two-dimensional array of atoms in a transparent medium, and opal is an example of a three-dimensional PBG crystal. The fight reflected by an opal gives a measure of the photonic band gap of the gemstone. Many insects also use photonic crystal structures for the production of vivid colours, and a study of insect colours, especially of iridescent butterfly wings, has led to advances in understanding structures similar to those in PBG materials. 

Iridescence. The best known form of iridescence is opalescence. The reason opals show different colors when viewed in different directions is that the silica spheres are... 

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