GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59
Revised Pages788 • Chapter 19 / Optical Properties19.7The index of refraction of quartz is
anisotropic. Suppose that visible light is pass-
ing from one grain to another of different
crystallographic orientation and at normal in-
cidence to the grain boundary. Calculate the
reflectivity at the boundary if the indices of
refraction for the two grains are 1.544 and
1.553 in the direction of light propagation.
Absorption
19.8Briefly explain why the magnitude of the ab-
sorption coefficient (βin Equation 19.18) de-
pends on the radiation wavelength.
Transmission
19.9The transmissivityTof a transparent mate-
rial 15 mm thick to normally incident light is
0.80. If the index of refraction of this material
is 1.5, compute the thickness of material that
will yield a transmissivity of 0.70. All reflec-
tion losses should be considered.
Color
19.10Briefly explain why some transparent materi-
als appear colored while others are colorless.Opacity and Translucency in Insulators
19.11Briefly explain why amorphous polymers are
transparent, while predominantly crystalline
polymers appear opaque or, at best, translu-
cent.
Photoconductivity
Lasers
19.12In your own words, briefly describe the phe-
nomenon of photoconductivity.
19.13In your own words, describe how a ruby laser
operates.
Optical Fibers in Communications
19.14At the end of Section 19.14 it was noted that
the intensity of light absorbed while pass-
ing through a 16-kilometer length of optical
fiber glass is equivalent to the light intensity
absorbed through for a 25-mm thickness of
ordinary window glass. Calculate the absorp-
tion coefficientβof the optical fiber glass
if the value ofβfor the window glass is
10 −^4 mm−^1.DESIGN PROBLEM
Atomic and Electronic Interactions
19.D1Gallium arsenide (GaAs) and gallium phos-
phide (GaP) are compound semiconductors
that have room-temperature band gap en-
ergies of 1.42 and 2.25 eV, respectively, and
form solid solutions in all proportions. Fur-
thermore, the band gap of the alloy increasesapproximately linearly with GaP additions
(in mol%). Alloys of these two materials are
used for light-emitting diodes wherein light
is generated by conduction band-to-valence
band electron transitions. Determine the
composition of a GaAs–GaP alloy that
will emit red light having a wavelength of
0.68μm.