Biomedical Engineering Reference
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blue color of wavelength about 402 nm would be
predominantly diffracted in the visible spectrum.
Increasing the value of D to 275 nm would mean
that the red color of wavelength about 692 nm
would be predominantly diffracted in the vis-
ible spectrum. If the silica spheres are too large
( D > 350 nm), the play of colors would be invis-
ible to human eyes because it would occur in
the infrared regime; however, tilting the opal to
reduce sin θ would shift the Bragg phenomenon
to shorter wavelengths, and the play of colors
may reappear--initially with red hues.
Precious opals are rare in nature, but common
opals are not. Common opals do not contain the
ordered structure of precious opals and, there-
fore, do not diffract visible light. A typical exam-
ple is the fire opal, a transparent orange variety,
the vivid colors of which come from fine traces
of iron oxide. Common opals are primarily built
from random accumulation of silica nanograins
with different effective diameters that average
25 nm [72] . Thus, the primary cause of the lack
of play of color in common opals is the aspheric-
ity in shape and polydispersity in size of the
constituent nanograins.
FIGURE 11.13 The play of colors in a precious opal. (For
interpretation of the references to color in this figure legend,
the reader is referred to the web version of this topic.)
a positive integer. For a hexagonal opal, D = 160
nm implies that d = 130.6 nm; additionally, with
n eff 1.54 as the typical value for silica, we see that
FIGURE 11.14 SEMs ( × 30,000) of the fracture surface of a precious opal reveals closely packed silica spherical particles.
(a) Cubic packing. (b) Hexagonal packing [70] . Reproduced with permission of the International Union of Crystallography
( http://dx.doi.org/10.1107/S0567739468000860) .
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