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

GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59

Revised Pages

19.9 Color • 773

Wavelength, (m)

Transmittance (%)

0.3 0.4 0.5 0.6

Ruby

Sapphire

Blue Yellow Red

Violet Green Orange

0.7 0.8 0.9 1.0

40

50

60

70

80

90

Figure 19.9 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, while 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©c1967 by Scientific American, Inc. All rights

reserved.)

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 color-

less. Ruby, which has a brilliant red color, is simply sapphire to which has been added

0.5 to 2% of 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 lev-

els. The transmittance as a function of wavelength for sapphire and ruby is presented

in Figure 19.9. For the sapphire, transmittance is relatively constant with wavelength

over the visible spectrum, which accounts for the colorlessness of this material. How-

ever, strong absorption peaks (or minima) occur for the ruby, one in the blue-violet

region (at about 0.4μm), and the other for yellow-green light (at about 0.6μm).

That nonabsorbed or transmitted light mixed with reemitted light imparts to ruby its

deep-red color.

Inorganic glasses are colored by incorporating transition or rare earth ions while

the glass is still in the molten state. Representative color–ion pairs include Cu^2 +,

blue-green; Co^2 +, blue-violet; Cr^3 +, green; Mn^2 +, yellow; and Mn^3 +, purple. These

colored glasses are also used as glazes, decorative coatings on ceramic ware.

Concept Check 19.6

Compare those factors that determine the characteristic colors of metals and trans-

parent nonmetals.

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]