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

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

Revised Pages

19.5 Refraction • 765

(a) (b)

Fermi

energy

Empty

states

Filled

states Photon

absorbedPhoton emitted

Energy

ΔE

Energy

Fermi

energy

ΔE

Figure 19.4 (a) Schematic representation of the mechanism of photon absorption for

metallic materials in which an electron is excited into a higher-energy unoccupied state. The

change in energy of the electronEis equal to the energy of the photon. (b) Reemission of

a photon of light by the direct transition of an electron from a high to a low energy state.

highly reflective over the entire range of the visible spectrum. In other words, for the

reflected beam, the composition of these reemitted photons, in terms of frequency

and number, is approximately the same as for the incident beam. Aluminum and

silver are two metals that exhibit this reflective behavior. Copper and gold appear

red-orange and yellow, respectively, because some of the energy associated with light

photons having short wavelengths is not reemitted as visible light.

Concept Check 19.3

Why are metals transparent to high-frequency x-ray andγ-ray radiation?

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

Optical Properties of Nonmetals

By virtue of their electron energy band structures, nonmetallic materials may be

transparent to visible light. Therefore, in addition to reflection and absorption, re-

fraction and transmission phenomena also need to be considered.

19.5 REFRACTION

Light that is transmitted into the interior of transparent materials experiences a

decrease in velocity and, as a result, is bent at the interface; this phenomenon is

termedrefraction.Theindex of refractionnof a material is defined as the ratio of

refraction

index of refraction

the velocity in a vacuumcto the velocity in the mediumv,or

Definition of index of

refraction—the ratio

of light velocities in a

vacuum and in the

medium of interest

n=

c

v

(19.7)

The magnitude ofn(or the degree of bending) will depend on the wavelength of the

light. This effect is graphically demonstrated by the familiar dispersion or separation