Callister - An introduction - 8th edition

21.9 Color • 853
21.9 COLOR
color
Transparent materials appear colored as a consequence of specific wavelength
ranges of light that are selectively absorbed; the color discerned is a result of the
combination of wavelengths that are transmitted. If absorption is uniform for all
visible wavelengths, the material appears colorless; examples include high-purity
inorganic glasses and high-purity and single-crystal diamonds and sapphire.
Usually, any selective absorption is by electron excitation. One such situation
involves semiconducting materials that have band gaps within the range of photon
energies for visible light (1.8 to 3.1 eV). Thus, the fraction of the visible light having
energies greater than E g is selectively absorbed by valence band–conduction
band electron transitions. Of course, some of this absorbed radiation is reemitted
as the excited electrons drop back into their original, lower-lying energy states. It
is not necessary that this reemission occur at the same frequency as that of the
absorption. As a result, the color depends on the frequency distribution of both
transmitted and reemitted light beams.
For example, cadmium sulfide (CdS) has a band gap of about 2.4 eV; hence, it
absorbs photons having energies greater than about 2.4 eV, which correspond to
the blue and violet portions of the visible spectrum; some of this energy is reradiated
as light having other wavelengths. Nonabsorbed visible light consists of photons
having energies between about 1.8 and 2.4 eV. Cadmium sulfide takes on a
yellow-orange color because of the composition of the transmitted beam.
With insulator ceramics, specific impurities also introduce electron levels within
the forbidden band gap, as discussed previously. Photons having energies less than
the band gap may be emitted as a consequence of electron decay processes involving
impurity atoms or ions, as demonstrated in Figures 21.6b and 21.6c. Again, the color
of the material is a function of the distribution of wavelengths that is found in the
transmitted beam.
For example, high-purity and single-crystal aluminum oxide or sapphire is colorless.
Ruby, which has a brilliant red color, is simply sapphire to which has been
added 0.5 to 2% chromium oxide (Cr 2 O 3 ). The Cr 3 ion substitutes for the Al 3 ion
in the Al 2 O 3 crystal structure and, furthermore, introduces impurity levels within
the wide energy band gap of the sapphire. Light radiation is absorbed by valence
band–conduction band electron transitions, some of which is then reemitted at specific
wavelengths as a consequence of electron transitions to and from these impurity
levels. The transmittance as a function of wavelength for sapphire and ruby is
presented in Figure 21.9. For the sapphire, transmittance is relatively constant with
Figure 21.9 Transmission of light radiation as a
function of wavelength for sapphire (single-crystal
aluminum oxide) and ruby (aluminum oxide
containing some chromium oxide). The sapphire
appears colorless, whereas the ruby has a red tint
due to selective absorption over specific
wavelength ranges. (Adapted from “The Optical
Properties of Materials,” by A. Javan. Copyright
© 1967 by Scientific American, Inc.
All rights reserved.)
Transmittance (%)
90
80
70
60
50
Violet Green Orange
Blue Yellow Red
Sapphire
Ruby
40
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Wavelength, (m)