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Renewable Energy
In this exercise, you will connect and apply course topics and concepts to a real-world scenario. Then, you will critically reflect and articulate some thoughts about your perceived learning, which can be a powerful bridge in the learning process. After reviewing your submission, your instructor will provide you with personalized feedback to further your learning and contribute to your understanding and application of the concepts.
Activity Instructions
Formulate, using your own words and thoughts, a two-paragraph written discussion that concisely* fulfills the following requirements:
Paragraph 2: Based on your discussion in paragraph 1, formulate a discussion that reflects on your learning thus far in the course. In doing so, consider answering the following questions:How did the knowledge you’ve acquired prepare you for developing paragraph 1?How has the knowledge you’ve acquired affected your interest in the topics and concepts presented?How will the knowledge you’ve acquired apply to ventures beyond this course?In formulating thoughts on the connections and applications to concepts and reflections on your learning, consider framing them within the context of the course and weekly learning objectives. Researchers, Inc.
130 CHAPTER 6 WAVES AND ELECTROMAGNETIC RADIATION
GURE 6-8 Two waves originating
om different points create
interference pattern. Bright
gions correspond to constructive
erference, while dark regions
respond to destructive
erference.
RE 6-9 Cross sections of
ering waves illustrate the
mena of (a) constructive and
tructive interference.
Constructive
interference
(a)
(b)
Destructive
interference
I
One easy way to think about what happens is to imagine
that each part of each wave carries with it a set of instructions
for the water surface-“move down 2 inches,” or “move up 1 inch.” When two waves arrive
simultaneously at a point, the surface responds to both sets of instructions. If one wave says to
move down 2 inches and the other to move up 1 inch, the result will be that the water surface
will move down a total of 1 inch. Thus each point on the surface of the water moves a different
distance up or down depending on the instructions that are brought to it by the two waves.
One possible situation is shown in Figure 6-9a. Two waves, each carrying the command “go
up 1 inch,” arrive at a point together. The two waves act together to lift the water surface to the
highest possible height it can have. By the same token, if two waves troughs, each 1-inch deep,
meet then the net change will be a trough 2 inches deep. This effect is called constructive inter-
ference, or reinforcement. On the other hand, you could have a situation like the one shown in
Figure 6-9b, where the two waves arrive at a point such that one is giving an instruction to go up
1 inch and the other to go down 1 inch. In this case, the two waves cancel each other out and the
STOP & THINK! Today’s scientists are much more con-
cerned about the ethical treatment of animals than were
naturalists of the eighteenth century. How might you con-
duct an experiment on the hearing of bats without injuring
the animals?
– Amplitude 1 inch
IC
Interference
Waves from different sources may overlap and affect each
other in the phenomenon called interference. Interference
describes what happens when waves from two different sources
come together at a single point-each wave interferes with the
other, and the observed height of the wave-the amplitude-is
simply the sum of the amplitudes of the two interfering waves.
Consider the common situation shown in Figure 6-8. Suppose
you and a friend each throw rocks into a pond at two separate
points as in the figure. The waves from each of these two points
travel outward and eventually will meet. What will happen
when the two waves come together?
Amplitude 1 inch
ELECTROMAGNETIC WAVE
Amplitude 1 inch
Amplitude 1 inch
131
Amplitude 2 inches
m
Wave amplitudes add
Wave amplitudes cancel
zero amplitudetons, which focuses
tina, where the
converted into nerve
signals are carried to
the optic nerve.
(adjusts focus)
of frequency slices or “banas exist, and many more people w
can do sa
undergo total internal reflection each time it comes to the edge of the glass. Light entering
longer ultraviolet waves can cause a chemical change in
skin pigments, a phenomenon known as tanning. This
lower-energy portion of the ultraviolet is not particularly
harmful by itself.
Shorter-wavelength (higher-energy) ultraviolet radia-
tion, on the other hand, carries more energy-enough
energy that this radiation, if absorbed by your skin cells.
can cause sunburn and other cellular damage. If the ultra-
violet wave’s energy alters your cell’s DNA. it may increase
your risk of developing skin cancer (see Chapter 23). In
fact, because ultraviolet radiation can damage living cells,
hospitals use it to sterilize equipment and kill unwanted.
bacteria.
The Sun produces intense ultraviolet radiation in both
longer and shorter wavelengths. Fortunately, our atmos-
phere absorbs much of the harmful short wavelengths and
thus shields living things. Nevertheless, if you spend much
time outdoors under a bright Sun. you should protect
exposed skin with a Sun-blocking chemical, which is trans-
parent (colorless) to visible light but reflects or absorbs
harmful ultraviolet rays before they can reach your skin
(Figure 6-26).
The energy contained in both long and short ultraviolet wavelengths can be absorbed by
atoms, which in special materials may subsequently emit a portion of that absorbed energy as
visible light. (Remember, both visible light and ultraviolet light are forms of electromagnetic
radiation, but visible light has longer wavelengths, and therefore less energy, than ultraviolet
radiation.) This phenomenon, called fluorescence, provides the so-called black light effects so
popular in stage shows and nightclubs. We’ll examine the origins of fluorescence in more detail
in Chapter 8.
X-rays
X-rays are electromagnetic waves that range in wavelength from
about 100 nanometers down to 0.1 nanometer, smaller than a
single atom. These high-frequency (and thus high-energy) waves
can penetrate several centimeters into most solid matter but are
absorbed to different degrees by all kinds of materials. This fact
allows X-rays to be used extensively in medicine to form visual
images of bones and organs inside the body. Bones and teeth
absorb X-rays much more efficiently than skin or muscle, so a
detailed picture of inner structures emerges (Figure 6-27). X-rays
are also used extensively in industry to inspect for defects in
welds and manufactured parts.
6.3 THE ELECTROMAGNETIC SPECTRUM 143
a
The X-ray machine in your doctor’s or dentist’s office is
something like a giant lightbulb with a glass vacuum tube. At
one end of the tube is a tungsten filament that is heated to a
very high temperature by an electrical current, just as in an
incandescent lightbulb. At the other end is a polished metal
plate. X-rays are produced by applying an extremely high volt-
age-negative on the filament and positive on the metal plate-
so electrons stream off the filament and smash into the metal
plate at high velocity. The sudden deceleration of the negatively
charged electrons releases a flood of high-energy electromag-
netic radiation-the X-rays that travel from the machine to you
at light speed.
Philip and Karen Smith/lconica/Getty Images
FIGURE 6-26 When you spend
time outdoors under a bright Sun,
you should protect your skin with
sunblock, which is transparent to
visible light, but reflects or absorbs
harmful ultraviolet rays.
FIGURE 6-27 Internal structures
are revealed because bones and
different tissues absorb X-rays to
different degrees.Longitudinal
Imagine that a chip of bark or a piece of grass is
throw a rock into the water. When the ripples go by, the b
it move up and down: they do not move to a different spot. A
wave crest moves in a direction parallel to the surface of the
kind of wave, where the motion is perpendicular to the direction of the wave, is e
motion of the wave is different from the motion of the medium on which the
verse wave (Figure 6-4a).
Transverse
(a)
Longitudinal
(b)
DIIIIIIII I9000000
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еееееееееее
eeeeeeeeeee eeee
llele eeeee
You can observe (and participate in) this phenomenon if you ever go to a sporting event in a
crowded stadium where fans “do the wave. Each individual simply stands up and sits down, but
the visual effect is of a giant sweeping motion around the entire stadium. In this way, transverse
waves can move great distances, even though individual pieces of the transmitting medium
hardly move at all.
Not all waves are transverse waves like those on the surface of water-we used the example
of a pond simply because it is so familiar and can be visualized. Sound is a form of wave that
moves through the air. When you talk, for example, your vocal cords move air molecules back
and forth. The vibrations of these air molecules set the adjacent molecules in motion, which
sets the next set of molecules in motion and so forth. A wave moves out from your mouth, and
that wave looks similar to ripples on a pond. Sound waves differ, however, because in the air the
wave crest that is moving out is not a raised portion of a water surface, but a denser region of air
molecules. In the language of physics, sound is a longitudinal wave. As a wave of sound moves
through the air, gas molecules vibrate forward and back in the same direction as the wave. This
motion is very different from the transverse wave of a ripple in water, where the water molecules
move perpendicular to the direction of the waves (see Figure 6-4b). Note that in both longitudinal
and transverse waves, the energy always moves in the direction of the wave.
STOP&THINK! How would you do a longitudinal wave in a stadium?
SCIENCE BY THE NUMBERS
The Sound of Music
VIAMUH
The speed of sound in air is more or less constant for all kinds of sound. The way we perceive
a sound wave, therefore, depends on its other properties: wavelength, frequency, and ampli-
tude. For example, what we sense as loudness depends
on both the amplitude of a sound and its frequency-the
greater the amplitude, for example, the louder the sound.
Similarly, we hear higher-frequency sound waves (sound
with shorter wavelengths) as higher pitches, while we
perceive lower-frequency sound waves (with longer wave-
lengths) as lower-pitched sounds.
You can experience one consequence of this contrast
when you listen to a symphony orchestra. The highest
notes are played by small instruments, such as the piccolo
and violins, while the lowest notes are the domain of the
massive tuba and double basses (Figure 6-5). Similarly, the
size of each of a big pipe organ’s thousands of pipes deter-
mines which single note it will produce. An organ pipe
6.1 THE NATURE OF WAVES 127
еееее
eeeeeeeeeee
eeeeeeeee 00000000
FIGURE 6-4 Transverse (a) and
longitudinal (b) waves differ in the
motion of the wave relative to the
motion of individual particles.
FIGURE 6-5 Different-sized
instruments in a band play in differe
ranges. The larger string bass in the
saxophones play in a higher range.
back plays lower notes, while thes section of the
e path of light,
the protective
ough the
changes the
Tough which
rolling the
g the eye.
and change
hich focuses
ere the
TROMAGNETIC RADIATION
ed into nerve
are carried to
: nerve.
Doug Martin/Photo Researchers, Inc
(b)
cal energy, as is done in this emergency flare. (c) Chemical reactions also produce the light given off by a fire
(a) A variety of chemical reactions, including fire, produce light energy. (b) One way of producing light is to
Light waves enter the eye through a clear lens whose thickness can be chang
of muscles around it. The direction of the waves is changed by refraction in
they are focused at receptor cells located in the retina at the back of the eye
is absorbed by two different kinds of cells, called rods and cones (the names.
shape, not their function). The rods are sensitive to light and dark, including le
they give us night vision. Three kinds of cones, sensitive to red, blue, and gre
to see colors.
The energy of incoming light triggers complex changes in molecules in th
initiating a series of reactions that eventually leads to a nerve signal that trav
nerve to the brain (see Chapter 5).
Lens
Cornea
Pupil
Darwin Dale/Photo Researchers
Ultraviolet Radiation
600
At wavelengths shorter than visible light, we begin to find waves of E
therefore high energy and potential danger. The wavelengths of ultravio
from 400 nanometers down to about 100 nanometers in length. The
Light-
(c)
Iris
Muscles
(adjusts focus)
vi nivquvu
can do so.
Muscle
(for moving eye)
boyl
Retina
Vitreous
humor
Blind spot-
O O
To
O
O
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Optic nⒸsciencephotos/Alamy
If, on the other hand, you are standing in back of the moving
light source, the distance between crests will be stretched out and
it will look to you as if the light had a lower frequency. We say that
it is redshifted. In Chapter 15 we will see that the redshifting of light
from distant moving sources is one of the main clues that we have
about the structure of the universe.
The Doppler effect also has practical applications much closer to
your home. Police radar units send out a pulse of electromagnetic
waves that is absorbed by the metal in your car, then reemitted. The
waves that come back will be Doppler shifted, and by comparing
the frequency of the wave that went out and the wave that comes
back, the speed of your car can be deduced. Similar techniques are
used by bats, who rely on the Doppler shift to detect the motion of
their insect prey, and by meteorologists, who employ Doppler radar
to measure wind speed and direction during the approach of poten-
tially damaging storms (see Chapter 18).
Transmission, Refraction,
Absorption, Reflection, and
Scattering
The only way we can know about electromagnetic radiation is to
observe its interaction with matter. Our eyes, for example, interact
with visible light and send nerve impulses to our brain-impulses
that are interpreted as what we “see.” When an electromagnetic
wave hits matter, one of five processes takes place:
(a)
6151 HB
Sound
Lower
frequency
(b)
Redshift
(c)
Emitter motion
1. Transmission. The wave will often pass right through matter,
as does the light that passes through your window or Earth’s
atmosphere. This process is called transmission. Transpar-17
ent materials do not affect the wave other than slowing it down a bit while it is in transit.
2. Refraction. This slowing down of light as it passes through a transparent material can cause
the light to change its direction slightly as in a lens or a glass of water-an important pro-
cess called refraction. Lenses that bend and focus light are an important application of
refraction. And when the different colors that make up white light bend differently, those
colors of the spectrum are spread out in a beautiful display. The rainbow you see in the sky
is formed by the interaction of light with falling raindrops (Figure 6-14).
BOLONHOST
NETIC SPECTRUM 137
6.2 THE ELECTROMAGNETIC WAVE 135
FIGURE 6-14 A pencil in a glass of water appears bent, illustrating the phenomenon of
refraction. Light passing through a prism is spread out.
Emitter
motion
Higher
frequency
→ Blueshift
FIGURE 6-13 The Doppler effect
occurs whenever a source of waves is
moving relative to the observer of the
waves. (a) When sound waves spreac
out from a fixed source in all directio
stationary listeners will hear the same
pitch. (b) Sound waves from a movin
source seem to increase or decrease
in pitch, depending on whether the
sound is approaching or receding
away from the listener. (c) The Dopp
shift for light waves causes a bluesh
for approaching light sources and a
redshift for receding light sources.
