PHO 101 Photonics Concepts
Three Rivers Community College ONLINE
Home Lab #5
The Thin Lens Equation: Fractions!
Purpose: In this lab you will use the thin lens equation to measure the focal lengths
of the two converging lenses in the OSA kit. You will check the focal lengths
with the "quick" method, using an object "infinitely" far
away.
Materials: small flashlight, two converging lenses from the OSA kit (labeled A and B),
small piece of Aluminum foil, clothespins or other supports for components,
card or heavy paper for screen, ruler or tape measure with metric (cm) markings
Background: To measure the focal lengths of the lenses, you will use the thin lens
equation:
(equation 1)
Solving
this equation for 1/f (by subtraction 1/do from both sides),
(equation 2)
If an object is placed to one side of the lens (do) and the corresponding image
distance is found by placing a screen at the position of the sharpest image (di), equation (2) can be used
to find the focal length of the lens. Remember
that in this equation, the object distance is a negative number.
Procedure:
I. QUICK CHECK OF FOCAL LENGTH (Object very far away)
In a semi-darkened room, stand away from the window and hold lens
"A" vertically and focus the light coming through the window onto a
sheet of paper also held vertically. Move the paper back and forth (toward and
away from the lens) until the image of the world outside becomes clear. The
focal length is the distance between the lens and the paper "screen".
Repeat for lens "B".
I. USING THE THIN LENS EQUATION
The thin lens equation method allows you to control the object distance.
The set up for this lab is shown below, photographed from above:
The relative positions of object, lens and image. The distance from the end of the
flashlight to the lens is do.
The distance from the lens to the focused image on the "screen" is di.
1. To prepare the object, cut a small narrow triangle of aluminum foil and
tape it to the center of the face of the flashlight. The triangle serves as the
object (its orientation is readily observed.) Be sure it is small enough so
that light can shine around it on all sides.
The object, lens and screen are supported by clip-type clothespins. It is
important that the object, lens and screen be at the same height from the table top, as well as in a straight line along the ruler.
That is, the optical axis, the line from the center of the object through
the center of the mirror to the center of the screen, should be level and
straight. (CD jewel cases make good bases for components!)
2. Begin with lens A. (Lens B is much more difficult!) Set the object so
that it is 30 cm from the lens. Measure this distance (from the triangle on
face of flashlight to center of lens support) as accurately as possible. Locate
the position of the image on a screen placed on the other side of the lens.
Measure the distance from the lens to the image location as accurately as
possible. Record the image distance on the data table (below). Note whether the
image is upright or inverted, and in the column labeled "size",
whether the image is larger or smaller than the object.
3. Repeat the procedure of step 2 for lens A with the object placed 25 cm
and 20 cm from the lens.
4. Replace lens A with lens B. Find the image distances for lens B when the
object is placed 15 cm, 10 cm and 5 cm from the lens. Again, record the
orientation of the image and whether it is larger or smaller than the object.
This lens will take some patience because it has a VERY short focal length. Do
the best you can.
Calculations
For each of the six measurements, calculate the focal length of the lenses
from equation (2). Find the average focal length for lens A
and the average for lens B. If any one measurement is very different from the
others, you may want to repeat it.
REPORT
Fill out the data/results table completely. Show a sample calculation for
finding the focal length, using your data. Answer the following questions:
1. What did you notice about the orientation of the images?
2. Were these images real or virtual?
3. Suppose you have an object and lens aligned so that a clear image is
projected onto a screen. If you cover the top half of the lens with a piece of
paper, what happens to the image? (This question was answered
incorrectly by the majority of college physics students who were surveyed!
You can try the experiment and find out for yourself!)
DATA/OBSERVATIONS/RESULTS
PART I: Quick Check
LENS A focal length___________________
LENS B focal length___________________
PART II: Thin lens equation
LENS A Data
object distance |
image distance |
orientation |
size |
calculated focal length |
-30 cm |
|
|
|
|
-25 cm |
|
|
|
|
-20 cm |
|
|
|
|
average focal length
=_____________________
LENS B Data
object distance |
image distance |
orientation |
size |
calculated focal length |
-15 cm |
|
|
|
|
-10 cm |
|
|
|
|
-5 cm |
|
|
|
|
average focal length
=_____________________
© J Donnelly 2001