Name
CHAPTER 16
Class
Date
Sound and Light
SECTION
31 Reflection and Color
KEY IDEAS
As you read this section, keep these questions in mind:
• What happens to light when it hits an object?
• Why can you see an image in a mirror?
• Why do we see colors?
What Happens When Light Hits an Object?
You may think of light bulbs, candles, and the sun as
objects that send light to your eyes. However, all of the
other objects that you see, including this book, also send
light to your eyes. Otherwise, you would not see them. In
order for you to see an object, light must travel from the
object to your eyes.
There are two main ways that light can get from an
object to your eyes. Some objects, such as the sun or
a light bulb, produce light. However, most objects,
including this book, only reflect light from other sources.
READING TOOLBOX
Compare After you read
this section, make a chart
comparing the images
created when light is reflected
from flat, concave, and convex
mirrors.
LIGHT AS A RAY
It can be hard to imagine how light waves travel. To
make it easier to understand and predict how light waves
behave during reflection, refraction, and other processes,
scientists use light rays.
A light ray is an imaginary line that shows the
direction in which light travels. The light ray’s direction
is the same as the direction the light wave travels. It is
also the same as the path the photons of light take. When
scientists draw light rays, they use single-headed arrows,
like the ones shown below.
READING CHECK
1. Define What is a light ray?
Light ray
Reflected
light ray
Refracted
light ray
Light rays cannot be used to model all the properties
of light. However, they can be useful for showing how
light behaves in many situations.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Reflection and Color continued
REFLECTION OF LIGHT
Remember that waves can reflect, or bounce off,
surfaces. All objects that you can see reflect at least
some of the light that hits them. The way the light reflects
affects what the surface looks like.
Many surfaces, such as paper and skin, look dull. This
is because light scatters off of them in many different
directions, as shown in the figure below. This reflection
of light into random directions is called diffuse reflection.
During diffuse reflection,
light rays reflect in many
different directions.
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2. Describe Would the
surface in the figure look dull
or shiny?
Smooth, polished surfaces, such as mirrors, look
shiny. Instead of scattering, the light rays that hit smooth
surfaces from a single direction all reflect in the same
direction. You can see this in the figure below.
Light rays that reflect off a
smooth surface all travel in
the same direction.
THE LAW OF REFLECTION
READING CHECK
3. Describe What is the law
of reflection?
Scientists describe reflection using angles. For
example, look at the image at the top of the next page.
The imaginary line that is perpendicular to the surface is
called the normal. The angle between the incoming light
ray and the normal is called the angle of incidence. The
angle between the reflected light ray and the normal is
called the angle of reflection.
According to the law of reflection, the angle of
incidence is equal to the angle of reflection. You can use
this law to predict the direction of a light ray that reflects
off a smooth surface.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Sound and Light
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Reflection and Color continued
Normal
According to the law of
reflection, the angle of
incidence is equal to the
angle of reflection.
Reflected light
Incoming light
Angle of incidence
Angle of reflection
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4. Apply Concepts On the
figure, draw an incoming
light ray that has a smaller
angle of incidence. Then,
draw the reflected light ray
for that incoming ray.
Why Can You See Yourself in a Mirror?
When you look into a flat mirror, you see an image of
yourself standing behind the mirror. Of course, there is
not really a copy of you behind the mirror. The light rays
that form the image seem to come from behind the mirror, but they actually don’t. Therefore, the image you see
in the mirror is a virtual image.
The ray diagram in the figure below shows the paths of
light rays striking a flat mirror. When a light ray reflects
off a flat mirror, the light ray obeys the law of reflection.
The angle of reflection equals the angle of incidence.
When the reflected rays reach your eyes, your brain
interprets the light as if it traveled in straight lines. So,
you see an image of yourself behind the mirror. In other
words, flat mirrors “fool” you into seeing objects that
aren’t actually there.
READING CHECK
5. Identify What kind of
image does a flat mirror
produce?
The man sees a virtual image behind
the mirror. However, no light rays
actually traveled from behind the
mirror to the man’s eyes.
This light ray
reflects off the
mirror and travels
to the man’s eyes.
His brain perceives
the light as if it
had traveled in a
straight line.
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6. Explain Why does the
man see an image of himself
in the mirror?
Visually, it appears
that the reflected
light ray traveled in
a straight line from
behind the mirror.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Reflection and Color continued
CURVED MIRRORS
READING CHECK
7. Describe What do curved
mirrors do to reflected
images?
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Think Critically In a small
group, think of situations
in which a curved mirror
would be more useful than
a flat mirror. Try to identify
examples, other than the
examples given here, of the
uses of curved mirrors in
everyday life.
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8. Compare How is a real
image different from a virtual
image?
You are probably most familiar with flat mirrors.
However, people sometimes also use curved mirrors.
Curved mirrors distort, or change, images they reflect.
Like flat mirrors, light reflects off of curved mirrors
according to the angle of reflection. However, the
surface of a curved mirror is not flat. Therefore, the
normal to the mirror points in different directions at
different places on the mirror. This results in the
distorted images we see in curved mirrors.
There are two main kinds of curved mirrors: convex
mirrors and concave mirrors. Mirrors that bulge out are
called convex mirrors. Convex mirrors produce virtual
images that are smaller than the actual object. People use
convex mirrors to see large areas easily. For example, the
mirror on the passenger side of a car is a convex mirror.
This mirror allows the driver to see a larger area of the
road behind the car.
Mirrors that curve inward are called concave mirrors.
People use concave mirrors to focus reflected light onto
a single point.
A concave mirror can form one of two kinds of
images. It may form a virtual image behind the mirror or
a real image in front of the mirror. Remember that light
rays do not actually pass through a virtual image. A real
image is an image that light rays pass through. The type
of image that forms depends on the location of the object
relative to the mirror.
Mirror
Light rays pass through a
real image. Therefore, a real
image would appear on a
piece of paper placed in
front of the mirror.
Object
Real
image
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9. Identify What determines
whether the image produced
by a concave mirror is virtual
or real?
Front of
mirror
Back of
mirror
Mirror
Object
Front of
mirror
Virtual
image
Light rays do not pass
through a virtual image. If
you placed a piece of paper
behind the mirror, no image
would form on the paper.
Back of
mirror
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Reflection and Color continued
How Do We See Colors?
White light from the sun contains all of the different
wavelengths of visible light. What color an object appears
to be depends on the wavelengths of light that come from
it and enter your eyes. Your brain interprets different
wavelengths of light as different colors.
For example, when white light strikes a leaf, the leaf
reflects only visible light with a wavelength of about
550 nm. The leaf absorbs light with other wavelengths.
When the light reflected from the leaf enters your eyes,
your brain interprets the light as green. Therefore, the
leaf looks green. If you shine only red light on the leaf,
the leaf looks black. This is because there is no green
light for the leaf to reflect.
READING CHECK
10. Identify What
determines what color an
object appears to be?
MIXTURES OF COLORS
Most of the colors that we see are not pure colors.
They are mixtures of colors created by combining light or
pigments. The additive primary colors of light are red,
green, and blue. Mixing light of two of these colors can
produce the secondary colors yellow, cyan, and magenta.
Mixing light of the three additive primary colors makes
white light.
Filters and pigments, or dyes, absorb light. Therefore,
mixing pigments or filters has a different effect on color
than mixing light. The subtractive primary colors—
yellow, cyan, and magenta—can be combined to create
red, green, and blue. If pigments or filters of all three
colors are combined in equal proportions, all visible light
is absorbed. No light gets to your eyes, so you see black.
Black is the absence of color.
Red
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White
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Green
11. Describe What color of
light is produced by mixing
red, green, and blue light?
:FMMPX
Magenta
Yellow
READING CHECK
Cyan
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Blue
Red, green, and blue light can
combine to produce yellow,
magenta, cyan, or white light.
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#MBDL
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12. Explain Why does a
mixture of yellow, cyan, and
magenta pigments look
black?
Cyan, magenta, and yellow filters
or pigments can combine to
produce red, green, blue, or black.
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Section 3 Review
Section Vocabulary
light ray a line in space that matches the direction of the flow of radiant energy
real image an image that is formed by the
intersection of light rays; a real image can be
projected on a screen
virtual image an image from which light rays
appear to diverge, even though they are not
actually focused there; a virtual image cannot
be projected on a screen
1.Explain Why do scientists use light rays to represent light waves?
2.Identify Give three examples of objects that produce diffuse reflection. Explain
how you know these objects produce diffuse reflection.
3.Identify Label the normal, angle of incidence, and angle of reflection in the diagram below.
4.Apply Concepts The angle between an incoming light ray and the normal is 25°.
What is the angle of reflection?
5.Infer What color would a blue object look if you shined red light on it? Explain
your answer.
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