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KEY CONCEPT
The Sun is the source of
most visible light.
BEFORE, you learned
NOW, you will learn
• Visible light is part of the
EM spectrum
• EM waves are produced both
in nature and by technology
• How visible light is produced
by materials at high
temperatures
• How some living organisms
produce light
• How humans produce light
artificially
VOCABULARY
THINK ABOUT
incandescence p. 569
luminescence p. 569
bioluminescence p. 569
fluorescence p. 571
Why is light
important?
This railroad worm has eleven
pairs of green lights on its sides
and a red light on its head. The
animal probably uses these
lights for illumination and to
frighten away predators. Almost every living organism, including
humans, depends on visible light. Think of as many different ways
as you can that plants, animals, and people use light. Then, think of
all the sources of visible light that you know of, both natural and
artificial. Why is light important to living organisms?
Light comes from the Sun and other
natural sources.
RESOURCE CENTER
CLASSZONE.COM
Learn more about
visible light.
It is hard to imagine life without light. Human beings depend on
vision in countless ways, and they depend on light for vision. Light is
the only form of EM radiation for which human bodies have specialized sensory organs. The human eye is extremely sensitive to light and
color and the many kinds of information they convey.
Most animals depend on visible light to find food and to do other
things necessary for their survival. Green plants need light to make
their own food. Plants, in turn, supply food directly or indirectly for
nearly all other living creatures. With very few exceptions, living
creatures depend on light for their existence.
check your reading
568 Unit 4: Waves, Sound, and Light
How is plants’ use of light important to animals?
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Most of the visible light waves in the environment come from the Sun. The Sun’s high
temperature produces light of every wavelength.
The production of light by materials at high
temperatures is called incandescence (IHN-kuhnDEHS-uhns). When a material gets hot enough, it
gives off light by glowing or by bursting into flames.
Other than the Sun, few natural sources of
incandescent light strongly affect life on Earth.
Most other stars give off as much light as the Sun,
or even more, but little light from stars reaches
Earth because they are so far away. Lightning
produces bright, short-lived bursts of light. Fire,
which can occur naturally, is a lower-level, longerlasting source of visible light. The ability to make
and use fire was one of the first light technologies,
making it possible for human beings to see on a
dark night or inside a cave.
check your reading
Why does little light reach Earth from stars
other than the Sun?
Some living things produce visible light.
Many organisms produce their own visible light, which they use
in a variety of ways. They produce this light through luminescence.
Luminescence is the production of light without the high temperatures needed for incandescence. The production of light by living
organisms is called bioluminescence. Bioluminescent organisms
produce light from chemical reactions rather than from intense heat.
Bioluminescence enables organisms to produce light inside their
tissues without being harmed.
VOCABULARY
Don’t forget to make
word frames for the
terms luminescence and
bioluminescence.
Bioluminescent organisms include insects, worms, fish, squid,
jellyfish, bacteria, and fungi. Some of these creatures have lightproducing organs that are highly complex. These organs might include
light-producing cells but also reflectors, lenses, and even color filters.
The firefly, a type of beetle, uses bioluminescence to attract mates.
A chemical reaction in its abdomen allows the firefly to glow at specific
intervals. The pattern of glowing helps fireflies of the same species
identify each other at night. Most often, the male flashes a signal while
flying, and the female responds with a flash. After they have identified
each other, the fireflies may continue to exchange flashes until the male
has located the female.
Chapter 17: Electromagnetic Waves 569
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The process of bioluminescence is very efficient.
Almost all of the energy released by the chemical
reactions of bioluminescence is converted into light.
Very little heat is produced. Researchers in lighting
technology wanted for years to imitate this efficiency,
and that became possible with the development of
light-emitting diodes (LEDs). LEDs produce little heat,
converting almost all of the incoming electrical energy
into light.
`check your reading
A female firefly responds
to a male’s signal.
How is bioluminescence different from
incandescence?
Human technologies produce visible light.
Human beings invented the first artificial lighting when they learned
to make and control fire. For most of human history, people have
made light with devices that use fire in some form, such as oil lamps,
candles, and natural gas lamps. After the discovery of electricity, people began to make light through a means other than fire. However, the
technique of using a very hot material as a light source stayed the
same until the invention of fluorescent lighting. In recent years, “cool”
lighting has become much more common.
Artificial Lighting
Is all artificial light the same?
Many types of artificial light sources are available.
These sources differ in the amount of light they
produce, the way the light beams are directed, and
the characteristics of the light itself.
SKILL FOCUS
Designing
experiments
MATERIALS
PROCEDURE
1 Design a procedure to discover and record differences among several different
types of artificial lighting. Your procedure should test how different colored
materials appear in different types of lighting. You should compare the
results with how these materials appear in direct sunlight.
2 Write up your experiment and carry it out.
WHAT DO YOU THINK?
• What differences did you discover among bulbs of
different types and sizes?
• How would you improve your design if you were
to repeat your experiment?
570 Unit 4: Waves, Sound, and Light
Artificial lighting
with a variety of
bulb types and
sizes
TIME
30 minutes
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Incandescent and Fluorescent Lighting
The development of the electric light bulb in the late 1800s made light
available at a touch. An ordinary light bulb is a sealed glass tube with
a thin tungsten wire running through it. This wire is called a filament.
When electrical current passes through the filament, the tungsten gets
hotter and begins to glow. Because these light bulbs use high temperatures to produce light, they are called incandescent bulbs.
Tungsten can become very hot—about 3500 degrees Celsius
(6300˚F)—without melting. At such high temperatures, tungsten
gives off a bright light. However, the tungsten filament also produces
much infrared radiation. In fact, the filament produces more infrared
light than visible light. As a result, incandescent bulbs waste a lot of
energy that ends up as heat. At such high temperatures, tungsten also
slowly evaporates and collects on the inside of the bulb. Eventually,
the filament weakens and breaks, and the bulb burns out.
check your reading
SUPPORTING MAIN IDEAS
List the characteristics of
incandescent lighting and
the different types that
are available.
What causes ordinary light bulbs to burn out?
Since the 1980s, halogen (HAL-uh-juhn) bulbs have come into
wide use. Halogen bulbs have several advantages over ordinary incandescent bulbs. They contain a gas from the halogen group. This gas
combines with evaporating tungsten atoms and deposits the tungsten
back onto the filament. As a result, the filament lasts longer. The
filament can also be raised to a higher temperature without damage,
so it produces more light. Halogen bulbs, which are made of quartz,
resist heat better than glass.
Incandescent Light Bulb
tungsten
filament
Halogen Light Bulb
tungsten
filament
Fluorescent Light Bulb
mercury
vapor and
other gases
halogen gas
mixture
phosphor
coating
quartz
bulb
glass bulb
electric
current
electrode
Many electric lights in use today are fluorescent. Fluorescence
(flu-REHS-uhns) occurs when a material absorbs EM radiation of
one wavelength and gives off EM radiation of another. Fluorescent
bulbs are filled with a mixture of mercury vapor and other gases that
give off ultraviolet light when an electric current passes through them.
Chapter 17: Electromagnetic Waves 571
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The insides of the bulbs are coated with a powder called phosphor
that fluoresces. Phosphor absorbs ultraviolet light and gives off visible
light. Because fluorescent lighting is cool and does not waste much
energy as heat, it is more efficient and more economical than incandescent lighting.
check your reading
LEDs are being used more
and more in place of
incandescent bulbs.
Why are fluorescent lights more efficient than incandescent lights?
Other Types of Artificial Lighting
Like fluorescent lights, many other artificial light sources use a gas in
place of a filament. For example, neon lights use gas-filled tubes to
produce light. However, instead of ultraviolet light, the gas gives off
visible light directly. The colors of neon lights come from the particular
mixtures of gases and filters used. Vapor lights, which are commonly
used for street lights, work in a similar way. In a vapor light,
a material such as sodium is heated until it becomes a gas,
or vapor. The vapor responds to an electric current by
glowing brightly.
One of fastest-growing types of artificial lighting is the
light emitting diode, or LED. LEDs do not involve bulbs,
filaments, or gases. Instead, they produce light electronically.
A diode is a type of semiconductor—a device that regulates electric current. An LED is a semiconductor that
converts electric energy directly into visible light.
LEDs have many advantages over traditional forms of
lighting. They produce a very bright light, do not break
easily, use little energy, produce little heat, and can last for
decades. Some technologists believe that LEDs will eventually replace most traditional forms of artificial lighting.
KEY CONCEPTS
CRITICAL THINKING
1. Describe natural, nonliving
sources of incandescent light.
4. Classify Make a chart summarizing the different types of
artificial lighting discussed in
this section.
2. What advantages does
bioluminescence have over
incandescence as a way for
living organisms to produce
light?
3. What are some advantages
and disadvantages of artificial
incandescent lighting?
572 Unit 4: Waves, Sound, and Light
5. Infer Why do you think
moonlight does not warm you,
even though the Moon reflects
light from the hot Sun?
CHALLENGE
6. Compare and Contrast
What does LED lighting have
in common with bioluminescence? How are the two
different?