Phy 122L/132L
Pre-Lab: Thin Lenses
Name:
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Choose the best answer. *** See Thin Lens Notes for sign conventions. *** See lab manual for variable
definitions. Note: There is a minor typo in the lab manual. Equation 2 should read m = hi/ho = -di/do
(10 pts total)
1. According to the lab procedure, how are we to determine the focal length of the lens or lens
combination? Read all of the steps of the procedure carefully before answering.
a. by observing an enlarged image of the light source and using the thin lens equation (eq. 1)
b. by observing the image height and the object height
c. by observing a reduced image of the light source and using the thin lens equation (eq. 1)
d. by observing a distant object (f ≈ di in this case)
e. Answers a, b, and c are all correct.
f. Answers a, c, and d are all correct.
2. The focal length for a diverging lens is defined to be __________.
a. negative
b. positive
c. No sign is necessary.
3. For a distant object, the object distance approaches (can be treated as approximately equal to) infinity.
We can use this idea to estimate the focal length of our lens, as is step one of the procedure. In this
case of a distant object, the focal length of the lens is approximately equal to _______.
a. infinity
b. 1/infinity
c. the image distance
d. 1/image distance
e. the object height (size)
Questions 4-8: READ THE FOLLOWING DESCRIPTION. SEE EXAMPLE CALCULATIONS.
During the lab, the object, the lens, and the screen where the image forms will be mounted on a meter
stick. Below is data for the positions (location on the meter stick) of the lens and the image that is formed
on the screen. The position of the object is fixed at 3 cm. The object distance is the distance between the
object being observed and the lens. The image distance is the distance between the lens and the position
of the screen where the image is formed. Image height is negative if image inverted because the image is
below the optic (principal) axis. Answer questions 4-8 using your lab manual and the sign conventions
listed in the Thin Lenses Notes. Make sure you put positive (+) or negative (-) signs as appropriate based
on the sign conventions. I will NOT assume that no sign means positive.
Positions ON meter stick of object, lens and screen, and size of object and image
Position of object Position of lens Position of screen Size of image
Size of object
Lens
on meter stick
on meter stick
on meter stick
(image height), (object height),
xo (cm)
xl (cm)
xs (cm)
hi (cm)
ho (cm)
R
3.0
16.2
37.8
-2.6
+1.6 cm
4. What is the object distance? Include units and positive/negative sign as appropriate. SHOW WORK
do =
5. What is the image distance? Include units and positive/negative sign as appropriate. SHOW WORK
di =
CSU Pomona
Typo Update 10/26/16
Dr. Julie J. Nazareth
Phy 122L/132L
6. Calculate the transverse magnification using object distance and image distance (answers to
questions 4 and 5) to the thousandths place (ie., #.###). Include positive or negative sign as
appropriate. SHOW YOUR WORK.
–di/do =
7. Calculate the transverse magnification using object height and image height to the thousandths place
(ie., #.###) Include positive or negative sign as appropriate. SHOW YOUR WORK.
hi/ho =
8. Calculate the focal length of the R lens using the object distance and image distance (answers to
questions 4 and 5). Include units and positive or negative sign as appropriate. SHOW YOUR WORK.
9. You put your P (purple) lens together with the G (green) lens together touching to make a compound
lens. Assume the focal length of the P (purple) lens is +8.19 cm and the G (green) lens is +17.1 cm.
Calculate the focal length of the PG (purple-green) compound lens. Include units and positive or
negative sign as appropriate. SHOW YOUR WORK.
10. During a previous thin lens experiment, data was taken for image distance and object distance for an
enlarged image using a PW (purple-white) compound lens. The focal length of the PW (purplewhite) compound lens was calculated to be +17.1 cm. Using the focal length of the P (purple) lens (fP
= +8.19 cm) and the focal length of the PW (purple-white) compound lens (fPW = +17.1 cm), calculate
the focal length, fW, of the W (white) lens. Include units and positive or negative sign as appropriate.
SHOW YOUR WORK.
Extra credit (Answer on an attached sheet of paper): Two lenses in combination but NOT touching
each other (so NOT a compound lens). The objective and eyepiece of a compound microscope are
both converging lenses and have focal lengths of fo = +15.0 mm and fe = 25.5 mm. A distance of 61.0
mm separates the lenses. The microscope is being used to examine an object placed do1 = 24.1 mm in
front of the objective lens. Calculate the first image distance, di1, the virtual object distance, do2, and
the second (final) image distance, di2. Show your work to receive credit.
[3 pts extra credit possible]
CSU Pomona
Typo Update 10/26/16
Dr. Julie J. Nazareth