SP212 Lab: Nine Thin Lenses
Version: April, 2014
Physics II
Lab 9
SP212
Thin Lenses
I. Introduction
A. Thin lenses have many applications; they are used in microscopes,
binoculars, eyeglasses, contact lenses, magnifying glasses, cameras and
refracting telescopes. Consequently, we encounter them frequently in
everyday life.
B. The physics of a thin lens is relatively simple since a lens only forms an
image. However, images can be either real or virtual. Complicating
things further, the image formed by a thin lenses can even be a real or
virtual object for another lens. It is therefore worthwhile spending some
time firming up our understanding of thin lenses.
C. The laboratory is an excellent place to study lenses since a real image
can be displayed on a screen. We can measure where a real image is
relative to a lens and compare its actual position to the position
predicted by our equations.
D. We can also carry out experiments that involve virtual images. Since
virtual images can't be displayed on a screen, we will study
combinations of lenses where the important role of the virtual image
can be inferred.
E. If an object is a distance p (object distance) from a thin lens of focal
length f and forms an image at a distance i (image distance) from the
thin lens, the relationship between p, i and f is 
1/p + 1/i = 1/f
F. Magnification of the object is given by the equation: m= -i/p
M = M1 M2 for a combination of lenses.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
II.
Needed Equipment
A. The equipment consists of a light source, lenses (blue dot, red dot, and yellow dot),
spreadsheet for calculations, a viewing screen and an optics bench. The optics bench
is a slotted aluminum track for fixing the light source, lenses and screen. The "object"
will be the illuminated drawing (concentric circles, perpendicular arrows and scale)
on the front of the light source. A real image of the object is visible on the screen in
the next picture.
III.Objectives
At the end of this activity, you should:
1. Find the focal length of a converging lens and see why a different technique is
required for a diverging lens.
2. Practice using lens equation to predict the image location given the object distance
and the focal length of the lens.
3. Predict the magnification of an object based on the object distance and the
theoretical image distance.
4. Observe the location of the image and its magnification and see if they match
predicted.
5. Observe the results of a two lens system similar to homework problems.
6. Observe how a virtual image can still be used as the object for another lens.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
IV. Your instructor will demonstrate how to operate the instrument and
take the data.
V. Turn in your Homework/Pre-Lab Assignment if assigned.
VI. Procedure
A. Preliminary Measurements
Note: You cannot find the focal length via the below method for a diverging lens.
1. Estimate of Focal Lengths of Positive Lenses Using Image of
Distant Object
a. Since only converging lenses can form real images of real objects, we will
use this fact to decide which of the lenses with colored dots are
converging and which is diverging. Using the technique shown in the
following diagram, attempt to form an image of the ceiling lights (real
objects) with each of the lenses. Separate the lenses into converging and
diverging and record which are which.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
b. Ask your instructor if they want you to do procedure (i) or (ii). Their
decision is probably based on how sunny it is outside. I learned last year,
that taking a class outside when it is cloudy is a waste of their time.
i. If the object distance is much, much larger than the focal length, then
the image distance is very nearly equal to the focal length. According
to Google Earth, the houses across the Severn River are about 1 km
from Chauvenet Hall. Find the image distance for each of your lenses,
using a house across the Severn as an object, and record the image
distance as the lens’s focal length. You must do this for each of the two
converging lenses and record their focal length. Skip step ii and Go to
step c.
ii. Your instructor will place an image on the overhead projector, stand in
the back of the room as far away from the screen with the optics bench
on your shoulder (almost like a backwards “bazooka”). Move the
converging lens until you are able to form a clear image on the small
screen attached to you optics bench. You must do this for each of the
two converging lenses and record their focal length.
2. Diverging Lens: Since virtual images cannot be displayed on a
screen, we must use a different technique to determine the
focal length of a diverging lens.
Warning: Do not shine the laser beam into anyone's eyes. Permanent vision damage may
result.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
a. Put a diverging (negative) lens on the optics bench and a laser on the lab
jack as your instructor demonstrates. It is necessary to adjust the laser so
that the laser beam lies on the lens’s optical axis. There is only one ray
that is both parallel to the optical axis and is also a vertex ray, and that is a
ray that lies along the optical axis. Therefore, adjust the lab jack, and the
laser on the jack, until (1) the beam is normal to the lens, and (2), the
beam does not divert when it goes through the lens. If both of these occur,
the beam must coincide with the optical axis.
b. Note the position of the image of the laser beam when it is aligned along
the lens’s axis.
c. Now, hold a ruler next to the lab jack, and turn the screw to raise (or
lower) the jack and laser a centimeter or so. Measure the distance you
shifted the laser carefully, and call this distance e. Also measure the
distance by which the image shifts, and call this distance y.
d. Finally, measure the distance, L, from the center of the lens to the screen
on which the image is projected. Since the laser beam is still parallel to the
optical axis, it is refracted through the lens’s focal point. A little geometry
involving similar triangles shows that f = L*e/y. Use this to calculate the
focal length of the yellow lens.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
B. Taking Data: The Thin Lens Equation: 1/p + 1/i = 1/f.
1. Now we are ready to take data.
a. Use the lamp as the object for this experiment, as your instructor
demonstrates. Set the object at x = 0.000 m on your optical bench. Set the
screen at x = 1.000 m; this is where the image will be located. Since the
lens will be between object an image, for this experiment p + i = 1.000 m,
which is convenient. Note that the diameter of the outer ring on the object
is 0.0200 ± 0.0001 m.
b. Now, place the blue lens on the bench, and adjust its position until you
obtain a sharp image on the screen. Note that there are two such positions,
one producing an enlarged image, the other producing a reduced image.
First, record the lens position that produces the enlarged image. Also
measure the image diameter.
c. Repeat for the position that produces the reduced image.
d. Use the results of your measurements to make the calculations indicated
on the spreadsheet template and test whether or not the focal length
determined in this way agrees with your previous measurement. Also
check to see whether both ways of determining the magnification agree.
i
2. Repeat the experiment (steps a-c above) using the red lens.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
C. A combination of lenses:
1. In a multi-lens optical instrument, the lens closest to the object is called
the “objective,” and we will call the second lens the “ocular,” or “eyepiece,”
even though we will not look through it. Instead we will cast its image on
the screen, and observe it there. The image of the objective is the object
for the eyepiece, and the overall magnification of the instrument is the
product of the magnifications of each lens.
2. The object lamp should still be at x = 0.000 m. Place the red lens on the
optical bench at x = 0.200 m. Adjust the screen until you find a nice, sharp
image, and record the image location. Also measure and record the image
diameter. Use your data to calculate the focal length, and compare with
your earlier measurements, and compare the magnifications calculated
both ways.
3. Now, place the yellow lens at x = 0.300 m, and adjust the screen to find the
image location. Record the image position, and measure and record the
image diameter. Use these data to test the lens equation and
magnifications as before.
4. Finally, find the product of the individual lens magnifications, and
calculate the overall magnification as the ratio of the final image diameter
to the original object diameter, and discuss agreement.
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SP212 Lab: Nine Thin Lenses
Version: April, 2014
D. Lab Report to hand in:
1. Spreadsheets, one each for Parts A, B1, B2, and C with
discussions.
Don’t forget uncertainties, units, annotations and discussions.
VII.
Clean-Up
A. Golden Rule: “Do unto others as you desire them to do unto you.”
This applies as much here in the lab as it does in the Fleet. As potential Naval
Officers, how can you expect your enlisted sailors to maintain a clean work area if
your stateroom, work areas, mess area, etc is a “pig sty?” So as officers it is
imperative that we clean up after ourselves not only to follow the Golden Rule, but
also to lead by example for the enlisted personnel under our charge.
1. End of Lab Checkout: Before leaving the laboratory, please tidy up the
equipment at the workstation and quit all running software.
2. The lab station should be in better condition than when you arrived and more
importantly, should be of an appearance that you would be PROUD to show to
your legal guardians during a “Parents Weekend.”
3. Have your instructor inspect your lab station and receive their permission to
leave the Lab Room.
4. You SHALL follow this procedure during every lab for SP212!
Many thanks to Dr. Huddle and Dr. Fontanella for their assistance in producing this Laboratory
procedure; specific references can be supplied on request. LCDR Timothy Shivok
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