Callister - An introduction - 8th edition

Summary • 867
Transparent nonmetals have band gaps greater than 3.1 eV.
For nonmetallic materials that have band gaps between 1.8 and 3.1 eV, only
a portion of the visible spectrum is absorbed; these materials appear
colored.
• Some light absorption occurs in even transparent materials as a consequence of
electronic polarization.
• For wide-band-gap insulators that contain impurities, decay processes involving
excited electrons to states within the band gap are possible with the emission of
photons having energies less than the band gap energy (Figure 21.6).
Color
• Transparent materials appear colored as a consequence of specific wavelength
ranges of light that are selectively absorbed (usually by electron excitations).
• The color discerned is a result of the distribution of wavelength ranges in the
transmitted beam.
Opacity and Translucency in Insulators
• Normally transparent materials may be made translucent or even opaque if the
incident light beam experiences interior reflection and/or refraction.
• Translucency and opacity as a result of internal scattering may occur
(1) in polycrystalline materials that have anisotropic indices of refraction;
(2) in two-phase materials;
(3) in materials containing small pores; and
(4) in highly crystalline polymers.
Luminescence
• With luminescence, energy is absorbed as a consequence of electron excitations,
which is subsequently reemitted as visible light.
When light is reemitted in less than a second after excitation, the phenomenon
is called fluorescence.
For longer reemission times, the term phosphorescence is used.
• Electroluminescence is the phenomenon whereby light is emitted as a result of
electron–hole recombination events that are induced in a forward-biased diode
(Figure 21.11).
• The device that experiences electroluminescence is the light-emitting diode
(LED).
Photoconductivity
• Photoconductivity is the phenomenon whereby the electrical conductivity of some
semiconductors may be enhanced by photo-induced electron transitions, whereby
additional free electrons and holes are generated.
Lasers
• Coherent and high-intensity light beams are produced in lasers by stimulated
electron transitions.
• With the ruby laser, a beam is generated by electrons that decay back into their
ground Cr 3 states from metastable excited states.
• The beam from a semiconducting laser results from the recombination of excited
electrons in the conduction band with valence band holes.