Callister - An introduction - 8th edition

854 • Chapter 21 / Optical Properties
wavelength over the visible spectrum, which accounts for the colorlessness of this
material. However, strong absorption peaks (or minima) occur for the ruby, one in
the blue-violet region (at about 0.4 m), and the other for yellow-green light (at
about 0.6 m). That nonabsorbed or transmitted light mixed with reemitted light
imparts to ruby its deep-red color.
Inorganic glasses are colored by incorporating transition or rare earth ions
while the glass is still in the molten state. Representative color–ion pairs include
Cu 2 , blue-green; Co 2 , blue-violet; Cr 3 , green; Mn 2 , yellow; and Mn 3 , purple.
These colored glasses are also used as glazes and decorative coatings on
ceramic ware.
Concept Check 21.6
Compare the factors that determine the characteristic colors of metals and transparent
nonmetals.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
21.10 OPACITY AND TRANSLUCENCY IN
INSULATORS
The extent of translucency and opacity for inherently transparent dielectric materials
depends to a great degree on their internal reflectance and transmittance
characteristics. Many dielectric materials that are intrinsically transparent may
be made translucent or even opaque because of interior reflection and refraction.
A transmitted light beam is deflected in direction and appears diffuse as a
result of multiple scattering events. Opacity results when the scattering is so extensive
that virtually none of the incident beam is transmitted, undeflected, to
the back surface.
This internal scattering may result from several different sources. Polycrystalline
specimens in which the index of refraction is anisotropic normally appear translucent.
Both reflection and refraction occur at grain boundaries, which causes a
diversion in the incident beam. This results from a slight difference in index of
refraction n between adjacent grains that do not have the same crystallographic
orientation.
Scattering of light also occurs in two-phase materials in which one phase is
finely dispersed within the other. Again, the beam dispersion occurs across phase
boundaries when there is a difference in the refractive index for the two phases;
the greater this difference, the more efficient the scattering. Glass-ceramics (Section
13.3), which may consist of both crystalline and residual glass phases, will appear
highly transparent if the sizes of the crystallites are smaller than the wavelength of
visible light, and when the indices of refraction of the two phases are nearly identical
(which is possible by adjustment of composition).
As a consequence of fabrication or processing, many ceramic pieces contain
some residual porosity in the form of finely dispersed pores. These pores also
effectively scatter light radiation.
Figure 21.10 demonstrates the difference in optical transmission characteristics
of single-crystal, fully dense polycrystalline, and porous (~5% porosity) aluminum
oxide specimens. Whereas the single crystal is totally transparent, polycrystalline
and porous materials are, respectively, translucent and opaque.