GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59
Revised Pages774 • Chapter 19 / Optical Properties19.10 OPACITY AND TRANSLUCENCY
IN INSULATORS
The extent of translucency and opacity for inherently transparent dielectric ma-
terials 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 refractionn
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 bound-
aries when there is a difference in the refractive index for the two phases; the greater
this difference, the more efficient is the scattering. Glass–ceramics (Section 13.5),
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 19.10 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.Figure 19.10 Photograph showing the light transmittance of three aluminum oxide
specimens. From left to right: single-crystal material (sapphire), which is transparent; a
polycrystalline and fully dense (nonporous) material, which is translucent; and a
polycrystalline material that contains approximately 5% porosity, which is opaque.
(Specimen preparation, P. A. Lessing; photography by S. Tanner.)