Chapter 17: Electromagnetic Waves 573
VOCABULARY
transmission p. 573
absorption p. 573
scattering p. 575
polarization p. 576
prism p. 577
primary colors p. 578
primary pigments p. 579
BEFORE, you learned
Mechanical waves respond
to a change in medium
•Visible light is made up of
EM waves
EM waves interact with a new
medium in the same ways that
mechanical waves do
NOW, you will learn
How the wave behavior of
light affects what we see
How light waves interact
with materials
Why objects have color
How different colors
are produced
KEY CONCEPT
Light waves interact
with materials.
EXPLORE Light and Matter
How can a change in medium affect light?
PROCEDURE
Fill the container with water.
Add 10 mL (2 tsp) of milk to the water. Put
on the lid, and gently shake the container
until the milk and water are mixed.
In a dark room, shine the light at one side
of the container from about 5 cm (2 in.) away.
Observe what happens to the beam of light.
WHAT DO YOU THINK?
• What happened to the beam of light from
the flashlight?
• Why did the light behave in this way?
3
2
1
MATERIALS
clear plastic
container with lid
•water
measuring spoons
•milk
flashlight
Light can be reflected, transmitted,
or absorbed.
You have read that EM waves can interact with a material medium in
the same ways that mechanical waves do. Three forms of interaction
play an especially important role in how people see light. One form is
reflection. Most things are visible because they reflect light. The two
other forms of interaction are transmission and absorption.
(trans-MIHSH-uhn) is the passage of an EM wave
through a medium. If the light reflected from objects did not pass
through the air, windows, or most of the eye, we could not see the
objects. (uhb-SAWRP-shun) is the disappearance of an
EM wave into the medium. Absorption affects how things look, because
it limits the light available to be reflected or transmitted.
Absorption
Transmission
VOCABULARY
Don’t forget to make word
frames for transmission
and absorption.
Page 1 of 7
574 Unit 4: Waves, Sound, and Light
This stained-glass window
contains transparent,
translucent, and opaque
materials.
How Materials Transmit Light
Materials can be classified according to the amount and type of light
they transmit.
Tr ansparent (trans-PAIR-uhnt) materials allow most of the light
that strikes them to pass through. It is possible to see objects
through a transparent material. Air, water, and clear glass are trans-
parent. Transparent materials are used for items such as windows,
light bulbs, thermometers, sandwich bags, and clock faces.
Tr anslucent (trans-LOO-suhnt) materials transmit some light, but
they also cause it to spread out in all directions. You can see light
through translucent materials, but you cannot see objects clearly
through them. Some examples are lampshades, frosted light bulbs,
frosted windows, sheer fabrics, and notepaper.
Opaque (oh-PAYK) materials do not allow any light to pass
through them, because they reflect light, absorb light, or both.
Heavy fabrics, construction paper, and ceramic mugs are opaque.
Shiny materials may be opaque mainly because they reflect light.
Other materials, such as wood and rock, are opaque mainly
because they absorb light.
check your reading What is the difference between translucent and opaque materials?
3
2
1
translucent
opaque
transparent
1
2
3
Page 2 of 7
Chapter 17: Electromagnetic Waves 575
A light filter is a material that is transparent to some kinds of light
and opaque to others. For example, clear red glass transmits red light but
absorbs other wavelengths. Examples of light filters are the colored cov-
ers on taillights and traffic lights, infrared lamp bulbs, and UV-protected
sunglasses. Filters that transmit only certain colors are called color filters.
Scattering
Sometimes fine particles in a material interact with light passing
through the material to cause scattering. is the spreading
out of light rays in all directions, because particles reflect and absorb
the light. Fog or dust in the air, mud in water, and scratches or smudges
on glass can all cause scattering. Scattering creates glare and makes it
hard to see through even a transparent material. Making the light
brighter causes more scattering, as you might have noticed if you have
ever tried to use a flashlight to see through fog.
Scattering is what makes the sky blue. During the middle of the
day, when the Sun is high in the sky, molecules in Earths atmosphere
scatter the blue part of visible light more than they scatter the other
wavelengths. This process makes the sky light and blue. It is too bright
to see the faint stars beyond Earths atmosphere. At dawn and dusk,
light from the Sun must travel farther through the atmosphere before it
reaches your eyes. By the time you see it, the greens and blues are scat-
tered away and the light appears reddish. At night, because there is so
little sunlight, the sky is dark and you can see the stars.
Scattering
SUPPORTING MAIN IDEAS
Be sure to add to your
chart the different
ways light interacts
with materials.
Fine particles, such as
those in fog, scatter
light and reduce visibility.
Page 3 of 7
576 Unit 4: Waves, Sound, and Light
A polarizing filter reduces
glare, making it possible to
see objects under the water.
Light reflecting off the surface
of this pond causes glare.
Polarization
Polarizing filters reduce glare and make it easier to see objects.
(POH-luhr-ih-ZAY-shuhn) is a quality of light in which
all of its waves vibrate in the same direction. Remember that
EM waves are made of electric and magnetic fields vibrating at right
angles to each other. Polarization describes the electric fields of a light
wave. When all of the electric fields of a group of light waves vibrate
in the same direction, the light is polarized.
Light can be polarized by a particular type of light filter called
a polarizing filter. A polarizing filter acts on a light wave’s electric
field like the bars of a cage. The filter allows through only waves
whose electric fields vibrate in one particular direction. Light that
passes through the filter is polarized. In the illustration below, these
waves are shown in darker yellow.
What do you think happens when polarized light passes into a
second polarizing filter? If the direction of the second filter is the
same as the first, then all of the light will pass through the second fil-
ter. The light will still be polarized. If the second filter is at a right
angle to the first, as in the illustration above, then no light at all will
pass through the second filter.
Wavelengths determine color.
The section of the EM spectrum called visible light is made up of
many different wavelengths. When all of these wavelengths are
present together, as in light from the Sun or a light bulb, the light
appears white.
Polarization
unpolarized light waves
The fields of visible light waves
vibrate in all directions.
polarized light waves
A polarizing filter lets through
only waves vibrating vertically.
no light waves
A second filter lets through only
waves vibrating horizontally.
2
3
Page 4 of 7
Chapter 17: Electromagnetic Waves 577
Seen individually, different wavelengths appear as different
colors of light. This fact can be demonstrated by using a
prism. A is a tool that uses refraction to spread out the
different wavelengths that make up white light. The prism
bends some of the wavelengths more than others. The
lightwaves, bent at slightly different angles, form a color
spectrum. The color spectrum could be divided into countless
individual wavelengths, each with its own color. However, the
color spectrum is usually divided into seven named color
bands. In order of decreasing wavelength, the bands are red,
orange, yellow, green, blue, indigo, and violet. You see a color
spectrum whenever you see a rainbow.
Color Reflection and Absorption
The color of an object or material is determined by the wavelengths it
absorbs and those it reflects. An object has the color of the wavelengths
it reflects. A material that reflects all wavelengths of visible light appears
white. A material that absorbs all wavelengths of visible light appears
black. A green lime absorbs most wavelengths but reflects green, so
the lime looks green, as shown below.
The color that an object appears to the eye depends on another
factor besides the wavelengths the object absorbs and reflects. An
object can reflect only wavelengths that are in the light that shines on
it. In white light, a white object reflects all the wavelengths of visible
light and appears white. If you shine only red light on a white piece of
paper, however, the paper will appear red, not white, because only red
light is available to be reflected.
In summary, two factors determine the color of an object: first, the
wavelengths that the object itself reflects or absorbs, and second, the
wavelengths present in the light that shines on the object.
check your reading What color band or bands does a red apple absorb? a white
flower?
prism
The lime absorbs
all wavelengths
except green.
2
The lime reflects
mostly green, so it
appears green.
In this simplified
diagram, light of
all colors strikes
the lime.
1
Prisms split light into
colors by refracting
wavelengths in different
amounts.
SUPPORTING MAIN IDEAS
Describe the roles of
reflection and absorption
in color.
Page 5 of 7
Primary Colors of Light
The human eye can detect only three color
bands: red, green, and blue. Your brain per-
ceives these three colors and various mixtures
of them as all the colors. These three colors of
light, which can be mixed to produce all possi-
ble colors, are called When all
three colors are mixed together equally, they
appear white, or colorless. Whenever colored
light is added to a mixture, specific wavelengths
An example of the practical use of primary
colors is a color television or computer monitor.
The screen is divided into thousands of tiny
bundles of red, green, and blue dots, or pixels.
A television broadcast or DVD sends signals that
tell the monitor which pixels to light up and
when to do so. By causing only some pixels to give off light, the monitor
can mix the three colors to create an amazing variety of colorful images.
What does an equal mix of all three primary colors produce?
primary colors.
What is black ink made of?
PROCEDURE
Trim each of the filter papers to a disk about 10 cm (4 in.) in diameter. Make
two parallel cuts about 1 cm (.5 in.) apart and 5 cm (2 in.) long from the edge
of each disk toward the center. Fold the paper to make a flap at a right angle.
Use a different marker to make a dark spot in the middle of the flap on
each disk.
Fill each of the cups with water. Set one of the disks on top of each cup so
that the water covers the end of the flap but does not reach the ink spot.
After 15 minutes, examine each of the flaps.
WHAT DO YOU THINK?
•What did you observe about the effects of water on
the ink spots?
How do the three different samples compare?
CHALLENGE Write a hypothesis to explain
what you observed about the colors in a black marker.
4
3
2
1
Mixing Colors
Mixing Colors
SKILL FOCUS
Observing
MATERIALS
•3 coffee filters
scissors
•3 brands of
black felt-tip
marker
•3 cups
•water
TIME
30 minutes
Primary colors of light
combine to make the
secondary colors yellow,
cyan (light blue), and
magenta (dark pink).
578 Unit 4: Waves, Sound, and Light
Page 6 of 7
Primary Pigments
Remember that two factors affect an object’s color. One is the wave-
lengths present in the light that shines on the object. The other is the
wavelengths that the object’s material reflects or absorbs. Materials
can be mixed to produce colors just as light can. Materials that are
used to produce colors are called pigments. The
are cyan, yellow, and magenta. You
can mix primary pigments just as
you can mix primary colors to
produce all the colors.
The primary pigment colors are
the same as the secondary colors of
light. The secondary pigment colors
are red, blue, and green—the same
as the primary colors of light.
The effect of mixing pigments
is different from the effect of mixing
light. Remember that a colored
material absorbs all wavelengths
except those of the color it reflects.
Ye llow paint absorbs all wavelengths
except yellow. Because pigments
absorb wavelengths, whenever you
mix pigments, you are subtracting
wavelengths rather than adding them. Mixing colors by subtracting
wavelengths is called subtractive color mixing. When all three primary
pigments are mixed together in equal amounts, all wavelengths are
subtracted. The result is black—the absence of reflected light.
primary pigments
Chapter 17: Electromagnetic Waves 579
KEY CONCEPTS
1. What are some ways in
which materials affect how
light is transmitted?
2. How does a polarizing filter
reduce glare?
3. In order for an object to
appear white, which wave-
lengths must the light contain
and the object reflect?
CRITICAL THINKING
4. Apply Imagine that you are a
firefighter searching a smoke-
filled apartment. Would using
5. Predict Higher-energy EM
waves penetrate farthest into a
dense medium. What colors
are more likely to penetrate to
the bottom of a lake?
CHALLENGE
6. Synthesize If you focus a red
light, a green light, and a blue
light on the same part of a
black curtain, what color will
the curtain appear to be? Why?
The inks used to make the