C h a p t e r 21 Optical Properties Awhite light beam experiences both refraction and dispersion as it passes through the triangular

glass prism. Refraction occurs when the direction of the light beam is bent at both glass-air prism interfaces (i.e., as it passes into and out of the prism). And dispersion (chromatic) occurs when the degree of bending depends on wavelength (i.e., the beam is separated into its component colors). (© PhotoDisc/Getty Images.) When materials are exposed to electromagnetic radiation, it is sometimes important to be able to predict and alter their responses. This is possible when we are familiar with their optical properties and understand the mechanisms responsible for their optical behaviors. For example, in Section 21.14 on optical fiber materials, we note that the performance of optical fibers is increased by introducing a gradual variation of the index of refraction (i.e., a graded index) at the outer surface of the fiber. This is accomplished by the addition of specific impurities in controlled concentrations. WHY STUDY the Optical Properties of Materials? 21.1 INTRODUCTION By “optical property” is meant a material’s response to exposure to electromagnetic radiation and, in particular, to visible light. This chapter first discusses some of the basic principles and concepts relating to the nature of electromagnetic radiation and its possible interactions with solid materials. Next to be explored are the optical behaviors of metallic and nonmetallic materials in terms of their absorption, reflection, and transmission characteristics. The final sections outline luminescence, photoconductivity, and light amplification by stimulated emission of radiation (laser), the practical utilization of these phenomena, and optical fibers in communications.

Basic Concepts 21.2 ELECTROMAGNETIC RADIATION In the classical sense, electromagnetic radiation is considered to be wave-like, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation (Figure 21.1). Light, heat (or radiant energy), radar, radio waves, and x-rays are all forms of electromagnetic radiation. Each is characterized primarily by a specific range of wavelengths, and also according to the technique by which it is generated. The electromagnetic spectrum of radiation spans the wide range from -rays (emitted by radioactive materials) having wavelengths on the order of m ( nm), through x-rays, ultraviolet, visible, infrared, and finally radio waves with wavelengths as long as m. This spectrum, on a logarithmic scale, is shown in Figure 21.2. 105

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g Learning Objectives After careful study of this chapter you should be able to do the following: 1. Compute the energy of a photon given its frequency and the value of Planck’s constant. 2. Briefly describe electronic polarization that results from electromagnetic radiation-atomic interactions.

Cite two consequences of electronic polarization. 3. Briefly explain why metallic materials are opaque to visible light. 4. Define index of refraction. 5. Describe the mechanism of photon absorption for (a) high-purity insulators and semiconductors, and (b) insulators and semiconductors that contain electrically active defects. 6. For inherently transparent dielectric materials, note three sources of internal scattering that can lead to translucency and opacity. 7. Briefly describe the construction and operation of ruby and semiconductor lasers. _ H Position _ Figure 21.1 An electromagnetic wave showing electric field and magnetic field H components, and the wavelength l. e 21.2 Electromagnetic Radiation • W59 Visible light lies within a very narrow region of the spectrum, with wavelengths ranging between about 0.4 ( m) and 0.7 The perceived color is determined by wavelength; for example, radiation having a wavelength of approximately 0.4 appears violet, whereas green and red occur at about 0.5 and 0.65 respectively. The spectral ranges for the several colors are included in Figure 21.2. White light is simply a mixture of all colors. The ensuing discussion is concerned primarily with this visible radiation, by definition the only radiation to which the eye is sensitive. All electromagnetic radiation traverses a vacuum at the same velocity, that of light—namely, m/s (186,000 miles/s). This velocity, c, is related to the electric permittivity of a vacuum and the magnetic permeability of a vacuum through (21.1) Thus, there is an association between the electromagnetic constant c and these electrical and magnetic constants. Furthermore, the frequency and the wavelength of the electromagnetic radiation are a function of velocity according to (21.2) Frequency is expressed in terms of hertz (Hz), and 1 Hz cycle per second. Ranges of frequency for the various forms of electromagnetic radiation are also included in the spectrum (Figure 21.2). _ 1 c _ ln n l c _ 1 1_0 m0

_0 m0

3 _ 108

mm mm, mm 4 _ 10_7 mm.

108 Energy (eV) Wavelength (m) 1 angstrom (Å) Violet Blue Green Yellow Orange Red 0.7 _m 0.6 _m 0.5 _m 0.4 _m Visible spectrum wavelength 1 nanometer (nm) 1 micrometer (_m) 1 millimeter (mm) 1 meter (m) 1 kilometer (km) Frequency (Hz) 106 104 102 100 10–2 10–4 10–6 10–8 10–10 1022 1020 1018 1016 1014 1012 1010 108 106 104 10–14 10–12 10–10 10–8 10–6 10–4 10–2 100 102 104 _-Rays X-Rays Ultraviolet Infrared Microwave Radio, TV –Visible Figure 21.2 The spectrum of electromagnetic radiation, including wavelength ranges for the various colors in the visible spectrum. For electromagnetic radiation, relationship among velocity, wavelength, and frequency For a vacuum, dependence of the velocity of light on electric permittivity and magnetic

permeability Sometimes it is more convenient to view electromagnetic radiation from a quantum-mechanical perspective, in which the radiation, rather than consisting of waves, is composed of groups or packets of energy, which are called photons. The energy E of a photon is said to be quantized, or can only have specific values, defined by the relationship (21.3) where h is a universal constant called Planck’s constant, which has a value of 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 21.2). When describing optical phenomena involving the interactions between radiation 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 21.1 Briefly discuss the similarities and differences between photons and phonons. Hint: you may want to consult Section 19.2. [The answer may be found at www.wiley.com/college/callister (Student Companion Site).] Concept Check 21.2 Electromagnetic radiation may be treated from the classical or the quantummechanical perspectives. Briefly compare these two viewpoints. [The answer may be found at www.wiley.com/college/callister (Student Companion Site).]