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M o d e l N o . O S - 8 5 1 5 B
Experiment 12: Focal Length and
Magnification of a Thin Lens
Required Equipment from Basic Optics System
Light Source
Bench
Converging lens of unknown focal length
Screen
Other Equipment
Metric ruler
Optics Caliper (optional, for measuring image sizes), PASCO part OS-8468
1
Instructors: see note on page 51.
Purpose
The purpose of this experiment is to determine the focal length of a thin lens, and to
measure the magnification for a certain combination of object and image distances.
Theory
For a thin lens:
(eq. 12.1)
where f is focal length, d
distance between the image and the lens. By measuring d
be determined.
Magnification, M, is the ratio of image size to object size. If the image is inverted, M
is negative.
Part I: Object at Infinity
In this part, you will determine the focal length of the lens by making a single mea-
surement of d
with
d
i
o
Procedure
1.
Hold the lens in one hand and the screen in the other hand. Focus the image of a
distant bright object (such as a window or lamp across the room) on the screen.
2.
Have your partner measure the distance from the lens to the screen. This is the
image distance, d
.
i
d
= _______________
i
Analysis
1.
As d
approaches infinity, what does 1/d
o
®
E x p e r i me n t 1 2 : F o c a l L e n g t h a n d M a g n if ic a ti o n o f a Th in L e n s
1
1
1
1
-- -
=
---- -
+
--- -
f
d
d
o
i
is the distance between the object and the lens, and d
o
≅
∞
.
approach?
o
is the
i
and d
the focal length can
o
i
31
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