Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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Revised Pages

772 • Chapter 19 / Optical Properties

1.0

0.8

0.6

0.4

0.2

0

Fraction of radiant energy

0.2 0.3 0.4 0.5 0.6

Reflected

0.7 0.8 1.0 1.5 2.0 2.5 3.0

Absorbed

Wavelength ( m)

Transmitted

visible

Figure 19.8 The variation with wavelength of the fractions of incident light transmitted,

absorbed, and reflected through a green glass. (From W. D. Kingery, H. K. Bowen, and D. R.

Uhlmann,Introduction to Ceramics,2nd edition. Copyright©c1976 by John Wiley & Sons,

New York. Reprinted by permission of John Wiley & Sons, Inc.)

to Equation 19.5. Also, each of the variablesR,A, andTdepends on light wavelength.

This is demonstrated over the visible region of the spectrum for a green glass in Figure

19.8. For example, for light having a wavelength of 0.4μm, the fractions transmitted,

absorbed, and reflected are approximately 0.90, 0.05, and 0.05, respectively. However,

at 0.55μm, the respective fractions have shifted to about 0.50, 0.48, and 0.02.

19.9 COLOR

Transparent materials appear colored as a consequence of specific wavelength ranges

color of light that are selectively absorbed; thecolordiscerned is a result of the combi-

nation 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 thanEgis 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 above. 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 19.6band 19.6c. Again, the color