Download Mirrors form images by reflecting light.

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KEY CONCEPT
Mirrors form images by
reflecting light.
BEFORE, you learned
NOW, you will learn
• EM waves interact with
materials
• Light can be reflected
• About the science of optics
• How light is reflected
• How mirrors form images
VOCABULARY
EXPLORE Reflection
optics p. 593
law of reflection p. 594
regular reflection p. 594
diffuse reflection p. 594
image p. 595
convex p. 596
concave p. 596
focal point p. 597
How does surface affect reflection?
PROCEDURE
1
Tear off a square sheet of aluminum foil.
Look at your reflection in the shiny side
of the foil.
MATERIALS
aluminum foil
2 Turn the foil over and look at your reflection
in the dull side.
3 Crumple up the piece of foil, then smooth it
out again, shiny side up. Again, look at your
reflection in the foil.
WHAT DO YOU THINK?
• How did the three reflections differ from one another?
• What might explain these differences?
Optics is the science of light and vision.
COMBINATION NOTES
Don’t forget to include
sketches of important
concepts in your
notebook.
Optics (AHP-tihks) is the study of visible light and the ways in which
visible light interacts with the eye to produce vision. Optics is also the
application of knowledge about visible light to develop tools—such as
eyeglasses, mirrors, magnifying lenses, cameras, and lasers—that
extend vision or that use light in other ways.
Mirrors, lenses, and other optical inventions are called optical
tools. By combining optical tools, inventors have developed powerful
instruments to extend human vision. For example, the microscope
uses a combination of mirrors and lenses to make very small structures visible. Telescopes combine optical tools to extend vision far into
space. As you will see, some of the latest optical technology—lasers—
use visible light in ways that do not involve human vision at all.
Chapter 18: Light and Optics 593
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Mirrors use regular reflection.
You have read that when light waves strike an object, they either pass
through it or they bounce off its surface. Objects are made visible by
light waves, or rays, bouncing off their surfaces. In section 3 you will
see how the light waves create images inside the human eye.
Light rays bounce off objects in a very predictable way. For example,
look at the diagram on the left below. Light rays from a flashlight
strike a mirror at an angle of 60° as measured from the normal, an
imaginary line perpendicular to the surface of the mirror. This angle is
called the angle of incidence. The angle at which the rays reflect off the
mirror, called the angle of reflection, is also 60° as measured from the
normal. The example illustrates the law of reflection, which states
that the angle of reflection equals the angle of incidence. As you can
see in the second diagram, holding the flashlight at a different angle
changes both the angle of incidence and the angle of reflection.
However, the two angles remain equal.
normal
angle of
incidence
60º
angle of
reflection
60º
40º 40º
The angle of reflection equals the angle of incidence.
If the surface of an object is very
smooth, like a mirror, light rays that come
from the same direction will bounce off in
the same new direction. The reflection of
parallel light rays all in the same direction
is called regular reflection.
The light rays striking
the mirror bounce back
by regular reflection.
Rays striking everything
else bounce back by
diffuse reflection.
594 Unit 4: Waves, Sound, and Light
If the surface is not very smooth—even
if it feels smooth to the touch, like a piece of
paper—light rays striking it from the same
direction bounce off in many new directions. Each light ray follows the law of
reflection, but rays coming from the same
direction bounce off different bumps and
hollows of the irregular surface. The reflection of parallel light rays in many different
directions is called diffuse reflection.
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The Law of Reflection
SKILL FOCUS
How can you use mirrors to see
around a corner?
Analyzing
PROCEDURE
1
To make a periscope, cut two flaps on opposite
sides of the carton, one from the top and one
from the bottom, as shown in the illustration.
MATERIALS
• paper milk or
juice carton
• scissors
• tape
• 2 mirrors slightly
smaller than the
bottom of the
carton
• protractor
2 Fold each flap inward until it is at a 45-degree
angle to the side cuts and tape it into place.
3 Attach a mirror to the outside surface of each of
the flaps.
4 Holding the periscope straight up, look through
one of the openings. Observe what you can see
through the other opening.
WHAT DO YOU THINK?
TIME
30 minutes
mirror
• Where are the objects you see when
you look through the periscope?
• How does the angle of the mirrors affect
the path of light through the periscope?
flap 1
tape
flap 2
fold
cut
CHALLENGE How would it affect what you see through
45°
the periscope if you changed the angle of the mirrors from
45 degrees to 30 degrees? Try it.
step 1
Shape determines how mirrors form images.
When you look in a mirror, you see an image of yourself. An image is
a picture of an object formed by waves of light. The image of yourself
is formed by light waves reflecting off you, onto the mirror, and back
toward your eyes. Mirrors of different shapes can produce images that
are distorted in certain ways.
VISUALIZATION
CLASSZONE.COM
See reflection in action.
Flat Mirrors
Your image in a flat mirror looks exactly like you. It appears to be the
same size as you, and it’s wearing the same clothes. However, if you
raise your right hand, the image of yourself in the mirror will appear
to raise its left hand. That is because you see the image as a person
standing facing you. In fact, your right hand is reflected on the right
side of the image, and your left on the left side.
Check Your Reading
If you wink your left eye while looking in the mirror, which eye
in the image of you will wink?
Chapter 18: Light and Optics 595
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The solid line shows the
actual path of light. The
broken line shows where
the light appears to be
coming from.
If you look closely at your image in a mirror, you will notice that
it actually appears to be on the far side of the mirror, exactly as far
from the mirror as you are. This is a trick of light. The solid yellow
arrows in the photograph above show the path of the light rays from
the boy’s elbow to the mirror and back to his eyes. The light rays
reflect off the mirror. The broken line shows the apparent path of
the light rays. They appear to his eyes to be coming through the
mirror from a spot behind it.
VOCABULARY
Try making sketches to help
you remember the new
terms on this page.
Concave and Convex Mirrors
Unlike light rays hitting a flat mirror, parallel light rays reflecting off
a curved mirror do not move in the same direction. A convex mirror
is curved outward, like the bottom of a spoon. In a convex mirror,
parallel light rays move away from each other, as you can see in the
diagram below on the left. A concave mirror is curved inward toward
the center, like the inside of a spoon. Parallel light rays reflecting off a
concave mirror move toward each other, as shown on the right.
Convex Mirror
Concave Mirror
focal point
light rays
596 Unit 4: Waves, Sound, and Light
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The rays striking a concave mirror cross and then move apart
again. The point at which the rays meet is called the focal point of
the mirror. The distance between the mirror and its focal point
depends on the shape of the curve.
The images formed in these mirrors depend on the curve of the
mirror’s surface and the distance of the object from the mirror. Your
image in a curved mirror may appear larger or smaller than you are,
and it may even be upside down.
Convex Mirror
Concave Mirror, Far Away
Concave Mirror, Up Close
Your image in a convex
mirror appears smaller
than you.
If you are standing far away,
your image in a concave
mirror appears upside down
and smaller than you.
If you are standing inside
the focal point, your image
in a concave mirror appears
right-side up and larger.
All rays parallel to a line through the center of the mirror are
reflected off the mirror and pass through the mirror’s focal point.
Rays from the top of the object are reflected downward and those
from the bottom are reflected upward.
Check Your Reading
How does your distance from the mirror affect the way your
image appears in a concave mirror?
KEY CONCEPTS
CRITICAL THINKING
1. Explain the term optics in your
own words.
4. Infer Imagine seeing your
reflection in a polished table
top. The image is blurry and
hard to recognize. What can
you tell about the surface of the
table from your observations?
2. How is diffuse reflection similar
to regular reflection? How is it
different?
3. Describe the path that light
rays take when they form an
image of your smile when you
look into a flat mirror.
5. Analyze Why do images
formed by concave mirrors
sometimes appear upside
down?
CHALLENGE
6. Synthesize Draw the
letter R below as it would
appear if you held the book
up to (a) a flat mirror and (b)
a convex mirror.
R
Chapter 18: Light and Optics 597