Callister - An introduction - 8th edition

21.11 Luminescence • 855
Figure 21.10 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.)
For intrinsic polymers (without additives and impurities), the degree of translucency
is influenced primarily by the extent of crystallinity. Some scattering of visible
light occurs at the boundaries between crystalline and amorphous regions, again
as a result of different indices of refraction. For highly crystalline specimens, this
degree of scattering is extensive, which leads to translucency, and, in some instances,
even opacity. Highly amorphous polymers are completely transparent.
Applications of Optical Phenomena
21.11 LUMINESCENCE
luminescence
fluorescence
phosphorescence
Some materials are capable of absorbing energy and then reemitting visible light
in a phenomenon called luminescence. Photons of emitted light are generated from
electron transitions in the solid. Energy is absorbed when an electron is promoted
to an excited energy state; visible light is emitted when it falls back to a lower energy
state if 1.8 eV hv 3.1 eV. The absorbed energy may be supplied as higher-energy
electromagnetic radiation (causing valence band–conduction band transitions,
Figure 21.6a) such as ultraviolet light; other sources such as high-energy electrons;
or heat, mechanical, or chemical energy. Furthermore, luminescence is classified according
to the magnitude of the delay time between absorption and reemission
events. If reemission occurs for times much less than one second, the phenomenon
is termed fluorescence; for longer times, it is called phosphorescence. A number of
materials can be made to fluoresce or phosphoresce; these include some sulfides,
oxides, tungstates, and a few organic materials. Ordinarily, pure materials do not
display these phenomena, and to induce them, impurities in controlled concentrations
must be added.
Luminescence has a number of commercial applications. Fluorescent lamps consist
of a glass housing, coated on the inside with specially prepared tungstates or
silicates. Ultraviolet light is generated within the tube from a mercury glow
discharge, which causes the coating to fluoresce and emit white light. The picture
viewed on a television screen (cathode ray tube screen) is the product of luminescence.The
inside of the screen is coated with a material that fluoresces as an electron
beam inside the picture tube very rapidly traverses the screen. Detection of x-rays
and g-rays is also possible; certain phosphors emit visible light or glow when
introduced into a beam of the radiation that is otherwise invisible.