SIMG 215 - Laboratory
Lens Magnification
Objective: To measure the magnifying properties of lenses and a telescope, and to test
the geometric theory of lens magnification.
Equipment: This experiment requires the "Optics Discovery Kit" by OSA. Your
instructor will provide other equipment needed for the lab.
Part (I) Magnifying Glasses and Lens Power:
Set up the small lens "B" as shown in Figure 1. Move the 1 cm black test target
up to the lens so it is almost touching. Put your eye close to the lens and move the test
target back until it is in best magnified focus. Measure the distance, d, between the test
target and the lens.
Figure 1: Magnifying Power
lens
1 cm
Measure separation, S
Right eye
Left eye
marker
23 cm
marker
Left eye sees
the two markers
Together, the brain can
combine them to see both.
Right eye sees
1cm target
Place two markers 23 cm in front of the lens. Look through the lens at the black
test target with one eye while holding the other eye open to see the two markers. Move
the markers left and right to make it appear that they exactly bracket the test target. This
is challenging because it requires your brain to merge two images. People with some
types of vision abnormalities may not be able to do this.
Once you have the test target bracketed by the markers, measure the separation in
cm, S, between the markers. The width of the test target is w = 1 cm. Calculate the
visual magnifying power, X, as shown in equation (1). Measure X in this way for both
lenses A and B.
X
w
S
(1)
Part (II) Predicting Magnifying Power
Theory says the magnifying power of a lens, X, is related to the focal length of the
lens, f in cm, by equation (2). Use the values of f for lenses A and B that you found in
the previous experiment to calculate X for the two lenses. Do the calculated values and
the measured values agree?
X
23
1
f
(2)
Part (III) Build and Characterize a Telescope:
Assemble a telescope as illustrated in Figure 2. To focus the telescope, look
through the small lens and slide the large lens along the cardboard until you see a focused
image. It will be an inverted image. The telescope should come to focus with the lenses
somewhere between 10 and 20 cm. To get acquainted with its use (especially focusing),
try viewing objects a few feet away and also a very long distance away.
Figure 2: Lenses A and B form a telescope when mounted on a strip of cardboard.
a) Focal Distance: Focus the telescope on a distant object. Your instructor will
set up the object. Measure the distance between the two lenses. The distance should be
close to the sum of the focal lengths of the two lenses
b) Telescope Magnification: View the distant object through the telescope with
one eye. With the other eye, view the object without the telescope. You should be able
to see the real object and the image of the object at the same time, as illustrated in figure
3. Your lab partner can help you mark the top and bottom of the virtual image on the
wall where the real object is located. Measure the distance between the marks, y', as
shown in Figure 3, and also the size of the real object, y. The ratio, Mt = y'/y, is the
telescope magnifying power. Theory says the value of Mt for the telescope should also
be equal to the ratio of the focal lengths of the lenses, Mt = fa/fb. Use your measured
values of focal length form the previous experiment to calculate the ratio fa/fb. Compare
this to the value of Mt you measured experimentally. Do our results agree?
Figure 3: The test pattern and the inverted image seen through the telescope.