5.9
Using Curved Mirrors
You may not realize it, but curved mirrors are part of your everyday
life. Whether you’re shopping for CDs, riding a school bus, or learning
about solar heating, curved mirrors will be near. Figure 1 shows some
of the terms used when describing curved mirrors.
principal axis: a line through the centr e
of the mirror that includes the principal
focus
principal focus: the position where
reflected parallel light rays come together
focal length: the distance from the
principal focus to the middle of the mirror
concave mirror
parallel light rays
parallel
light rays
principal axis
principal axis
focal
length
principal focus: the position where
parallel light rays appear to reflect from
convex mirror
principal
focus
principal axis: a line through the centr e
of the mirror that includes the principal
focus
principal focal length: the distance from the
focus
principal focus to the middle of the mirror
focal
length
a A concave mirror. When the object is beyond the principal
focus, the type of image produced by a concave mirror will
be real because it is in front of the mirror and can be placed
on a screen.
b A convex mirror. Images in a convex mirror are always
virtual, because they are behind the mirror and cannot be
placed on a screen.
Figure 1
A concave mirror focuses parallel light rays, and a convex mirror spreads the light rays out.
light
from
distant
stars
Using Concave Mirrors
If you’ve ever looked through a reflecting telescope,
you’ve used a concave mirror. Figure 2 shows how a
concave mirror is used to gather light from distant
objects and bring it to a focus. The biggest telescopes
built, including space telescopes, use this design.
Figure 3 shows how a concave cosmetic mirror
is used to produce an enlarged image of a nearby
object. This happens when the person using the
mirror is closer to it than the principal focus.
A third use for concave mirrors is illustrated in
Figure 4. Concave reflectors can be used in devices
to send light in a beam.
plane mirror
concave mirror
eyepiece
observer
or camera
Figure 2
A reflecting telescope creates an image that can be
viewed, photographed, or recorded digitally.
object
image
principal
axis
principal focus
Figure 3
A concave mirror produces an upright, enlarged image when
the person using it is closer to the mirror than the principal
focus. Could this image be placed onto a screen?
286
Unit 5
Figure 4
In this flashlight, the filament of the light bulb is near
the principal focus of the concave mirror behind it. The
rays that reflect off the mirror are nearly parallel. This
produces a beam of light.
Using Convex Mirrors
Understanding Concepts
While shopping for CDs, you probably noticed large
surveillance mirrors in the store. A convex mirror can
be used to monitor a very large area because its curved
surface reflects light from all parts of a room to a person’s
eye. Images are always upright and smaller than the object,
no matter where the object is located. Figure 5 shows why
a convex mirror gives a much wider view than any other
kind of mirror, and how it produces an image. Figure 6
shows another common application of convex mirrors.
Can you think of others?
1. For each situation described
below, state whether the image
produced is real or virtual, and
how you know.
(a) A man is standing close to a
cosmetic mirror while shaving.
(b) An astronomer is looking at
an image of the Moon through
her telescope, which has a
concave mirror.
(c) A clerk in a drug store is looking
at the image of a customer in a
surveillance mirror.
field
of
view
plane
mirror
field
of
view
convex
mirror
2. Rewrite the following false
statements to make them true.
(a) The image in a convex mirror
is always real and upright.
(b) When an object is inside the
principal focus of a concave
mirror, the image is inverted
and real.
a The reflection in a convex mirror gives a much larger view
than the reflection in a plane mirror of similar size.
image
(c) Real images are always
located behind the mirror.
object
Making Connections
principal axis
principal
focus
focal
length
3. Curved mirrors can be used to
gather light from the Sun and
focus it for solar heating. Draw
a diagram illustrating how this
might work.
6C
b How a convex mirror produces an image. The characteristics of the
image are the same whether the object is near or far from the mirror.
Exploring
Figure 5
The convex mirror.
4. A helium-neon laser uses a
concave mirror at each end to
help create a narrow beam of
light. Research this type of laser,
and draw a diagram showing how
the mirrors control the light.
4A
Reflecting
5. What safety problems can occur
when using convex mirrors? List
situations in which a convex mirror
should not be used.
Figure 6
A convex mirror on the front of a school bus allows the driver to see children
both beside and in front of the bus. If a convex mirror is used to see behind, the
driver must be careful, because the objects are actually closer than their images
suggest they are.
SKILLS HANDBOOK: 6C Scientific & Technical Drawing
4A Research Skills
Which challenges would be best
for using concave and convex
mirrors?
Optics
287