Thin disk specimens of aluminum oxide

Photograph of three thin disk specimens of aluminum oxide, which have been placed over a printed page in order to demonstrate their differences in light-transmittance
characteristics. The disk on the left is transparent (that is, virtually all light that is reflected from the page passes through it), whereas the one in the center is translucent (meaning that
some of this reflected light is transmitted through the disk). And, the disk on the right is opaque—i.e., none of the light passes through it. These differences in optical properties are a
consequence of differences in structure of these materials, which have resulted from the way
the materials were processed. (Specimen preparation, P. A. Lessing; photography by S. Tanner.)