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

GTBL042-19 GTBL042-Callister-v2 September 17, 2007 17:39

Revised Pages

762 • Chapter 19 / Optical Properties

Sometimes it is more convenient to view electromagnetic radiation from a

quantum-mechanical perspective, in which the radiation, rather than consisting of

photon waves, is composed of groups or packets of energy, which are calledphotons.The

energyEof a photon is said to be quantized, or can only have specific values, defined

by the relationship

E=hν=

hc

λ

(19.3)

For a photon of

electromagnetic

radiation,

dependence of

energy on frequency,

and also velocity and

wavelength wherehis a universal constant calledPlanck’s constant,which has a value of 6.63×

Planck’s constant

10 −^34 J-s. Thus, photon energy is proportional to the frequency of the radiation, or

inversely proportional to the wavelength. Photon energies are also included in the

electromagnetic spectrum (Figure 19.2).

When describing optical phenomena involving the interactions between radi-

ation and matter, an explanation is often facilitated if light is treated in terms of

photons. On other occasions, a wave treatment is more appropriate; at one time or

another, both approaches are used in this discussion.

Concept Check 19.1

Briefly discuss the similarities and differences between photons and phonons.Hint:

you may want to consult Section 17.2.

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

Concept Check 19.2

Electromagnetic radiation may be treated from the classical or the quantum-

mechanical perspective. Briefly compare these two viewpoints.

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

19.3 LIGHT INTERACTIONS WITH SOLIDS

When light proceeds from one medium into another (e.g., from air into a solid

substance), several things happen. Some of the light radiation may be transmitted

through the medium, some will be absorbed, and some will be reflected at the inter-

face between the two media. The intensityI 0 of the beam incident to the surface of

the solid medium must equal the sum of the intensities of the transmitted, absorbed,

Intensity of incident and reflected beams, denoted asIT,IA, andIR, respectively, or

beam at an interface

is equal to the sum of

the intensities of

transmitted,

absorbed, and

reflected beams

I 0 =IT+IA+IR (19.4)

Radiation intensity, expressed in watts per square meter, corresponds to the energy

being transmitted per unit of time across a unit area that is perpendicular to the

direction of propagation.