Date
Period
Name
18-1
Concave and Convex Mirrors
Objectives
Demonstrate image
formation with
concave and convex
spherical mirrors.
Observe the properties of
images formed by
spherical mirrors.
Measure the focal length
of spherical mirrors.
Calculate the focal length
of a spherical mirror,
using the mirror
equation.
Spherical mirrors are portions of spheres and have one silvered side that
is a reflecting surface. If the inner side is the reflecting surface, the mirror
is concave. If the outer side is the reflecting surface, the mirror is convex.
The center of the sphere of which the mirror is a portion is the center of
curvature (C) of the mirror. An imaginary line that is perpendicular to the
center of the mirror (A) and that passes through C is the principal axis of
the mirror. The point halfway between C and A is the focal point (F) of the
mirror. The distance from F to A is the focal length (f ) of the mirror. The
distance of the object from the mirror, do, and the distance of the image from
the mirror, di, are related to the focal length by the lens/mirror equation
1
1
1
} 5 } 1 }.
f
di
do
Figure A illustrates these relationships.
Concave mirrors produce real images, virtual images, or no image,
depending upon how far the object is from the mirror. Real images can be
captured on a screen, and light rays actually pass through them. Convex
mirrors produce only virtual images. Virtual images cannot be captured
on a screen and appear to originate behind the mirror.
Copyright © by Glencoe/McGraw-Hill
Light rays that approach a concave mirror and are parallel to the principal axis reflect and converge at the focal point. If all the light rays
approaching a concave mirror are parallel to the principal axis, they will
meet at or near the focal point and form an image of the object.
Figure A
The center of the curvature C is located at a distance of twice the
focal length f from the center of the mirror, along the principal axis.
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Laboratory Manual 129
18-1
Materials
Name
Physics Lab
Procedure
A. Focal Length of a Concave Mirror
concave mirror
convex mirror
2 metersticks
cardboard screen
holders for mirror, screen,
and light source
masking tape
Arrange your mirror, metersticks and screen as shown in Figure B. If
the sun is visible, the focal length can be determined by projecting a
focused image of the sun onto a screen and measuring the distance
from the mirror’s center to the screen. CAUTION: Do not look directly
at the sun or you may severely damage your eyes. Another method is to
point the mirror at a distant object (more than 10 m away) and move
the screen along the meterstick until you obtain a sharp image of
the distant object on the screen. The distance between the mirror
and the screen is the approximate focal length of the mirror. Record
in Table 1 the focal length of your mirror.
metric ruler
light source
Figure B
Finding the focal length of a mirror
B. Concave Mirror
Figure C
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Laboratory Manual
2. Place the light source at a distance greater
than C from the mirror. Measure the height
of the light source and record this value in
Table 1. Move the screen back and forth
along the meterstick until you obtain a
sharp image of the light source. Determine
the distance of the image from the mirror,
di, by measuring from the mirror’s center to
the screen. Record in Table 2 your measurements of di, do, and hi and your observations
of the image.
Physics: Principles and Problems
Copyright © by Glencoe/McGraw-Hill
1. The center of curvature of the mirror, C, is
twice the focal length. Record this value in
Table 1. Arrange the two metersticks, mirror,
light source, and screen as shown in Figure
C. Use masking tape to hold the metersticks
in place.
18-1
Name
Physics Lab
3. Move the light source to C. Move the screen back and forth until you obtain a sharp image. Record
in Table 2 your measurements of di, do, and hi and your observations of the image.
4. Move the light source to a position between F and C. Move the screen back and forth until you
obtain a sharp image. Record in Table 2 your measurement of di, do, and hi and your observations
of the image.
5. Move the light source to a distance F from the mirror. Try to locate an image on the screen.
Observe the light source in the mirror. Record your observations in Table 2.
6. Move the light source to a position between F and A. Try to locate an image on the screen.
Observe the image in the mirror. Record your observations in Table 2.
C. Convex Mirrors
Place the convex mirror in the holder. Place the light source anywhere along the meterstick. Try to
obtain an image on the screen. Observe the image in the mirror. Move the light source to two other
positions along the meterstick and try each time to produce an image on the screen. Observe the image
in the mirror. Record your observations in Table 3.
Data and Observations
Table 1
Focal length of mirror, f
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Center of curvature of mirror, C
Height of light source, ho
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Laboratory Manual 131
18-1
Name
Physics Lab
Table 2
Position of object
Beyond C
Between C
and F
At C
At F
Between F
and A
do
di
hi
Type of image: real,
none, or virtual
Direction of image:
inverted or erect
Table 3
Trial
Position of
Position of
object
image
Type of image:
real or virtual
Image size
compared to
object size
Direction of
image: inverted
or erect
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1
2
3
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Physics: Principles and Problems
18-1
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Analysis and Conclusions
1. Use your observations from Table 2 to summarize the characteristics of images formed by concave
mirrors in each situation.
a. The object is beyond the center of curvature.
b. The object is at the center of curvature.
c. The object is between the center of curvature and the focal point.
d. The object is at the focal point.
e. The object is between the focal point and the mirror.
Copyright © by Glencoe/McGraw-Hill
2. Use your observations from Table 3 to summarize the characteristics of images formed by convex
mirrors.
3. For each of the real images you observed, use the lens/mirror equation to calculate f. Do your calculated values agree with each other?
Physics: Principles and Problems
Laboratory Manual 133
18-1
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Physics Lab
4. Average the values of f that you calculated for question 3 and compute the relative error between
the average and the measured value for f recorded in Table 1.
Extension and Application
1. The image of an illuminated lightbulb in the apparatus in Figure D can
be projected onto the empty socket on top of the box. Describe how you
would orient a large, spherical mirror to reflect the illuminated lightbulb
in the box onto the socket on top.
Figure D
Illusion apparatus
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Physics: Principles and Problems