Experiment 2: Thin Lens Equation
Objectives:
 Verify lens equation
 Determine focal length of a lens or combination of lenses
Pre Lab:
Write down the thin lens equation and give the approximations made to obtain it.
Write down the definitions for: Focal length, virtual image, real image, object distance,
image distance.
What kind of lens do you have if there is –120 written on it?
If the focal length of a lens is f = 100. You place the object at a distance S = 250 mm.
Where will you find the image? Is it a real or virtual image? (give formula and
explanation)
Draw the ray diagrams for the following case. Give the 3 specific rules that you used to
find the image.
Part A: Measuring the Focal Length-I (positive lens)
Equipment Required:
Light source
2 Target Assembly (TA) with index cards (object and screen)
Lens Chuck Assembly (LCA)
Experiment set up:
For each lens, write down all the distances in a table as followed:
Object
125 mm
150 mm
200 mm
400 mm
distance (S)
Image
distance (S’)
Distance
between grid
lines
Experimental
value of the
focal length
(f)
Magnification
(M)
600 mm
1. On the index card, construct a square grid of lines 5 mm apart that will serve as your
object. Add some arrows to enable you to determine if the image is upside down or
right side up.
2. Set up the different elements as shown in Fig.1
Fig.1: Set up for the positive lens experiment
3. Take a positive 100 mm focal length lens and mount it in the LCA.
4. Place the lens about 125 mm from the object (TA-I), measure and write down the
exact distance. This is the first object distance.
5. Place the TA-II at the end of the breadboard and slowly move it toward the lens until
an image is seen. Move the TA-II to produce the sharpest image. Measure the
distance from the lens to TA-II. Record this value along with the object distance.
6. Measure the distance between a specific number of grid lines. Is the image right side
up? Give explanation.
7. Redo the steps 3 trough 6 with the object distance set close to the following values:
150 mm, 200 mm, 400 mm, 600 mm.
8. Calculate the focal length of the length using each of the data sets. Compare these
values to the real value. Are these values close? Give the sources of error.
9. Calculate the magnification for each data set.
10. Plot a graph of 1/S’ vs 1/S
11. Get a value of f from this graph (explain how you can find this value on the graph).
12. Explain what happens if the object distance is smaller than the focal length.
Part B: Measuring the Focal Length-II (negative lens)
Equipment Required:
Light source
2 Target Assembly (TA) with index cards (object and screen)
2 Lens Chuck Assembly (LCA)
Note: be careful with the sign convention!!
Fig.2: Set up for the negative lens experiment
Experiment set up:
13. Why are we using a positive lens combined with the negative lens to determine the
focal length for the negative lens?
14. Place a negative focal length lens (-25.4 mm) in the LCA with its concave side facing
the object and measure the distance from the object to the lens.
15. Next place the positive lens used in the previous experiment (100 mm) more than 100
mm beyond the negative lens. Obtain a sharp image and measure the image distance
from the positive lens.
16. Since you know the focal length of the positive lens and the image distance, you can
calculate the object distance for this lens if only a positive lens were present. The
object of the positive lens is the image of the negative lens. Calculate the image
distance for the negative lens and then calculate the focal length for the negative lens.
17. Redo (3 times) the previous steps but change the object distance (write down all your
results in a table)
18. Compare the experimental values for the focal length of the negative lens with the
real value. Are they consistent?
19. Plot a graph of 1/S’ vs 1/S.
20. Get a value of f from this graph (explain how you can find this value on the graph).
Part C: Combinations of lenses
21. Use a combination of lenses (for example, a 100 mm lens and a 200 mm lens). Mount
the 2 lenses next to one another.
22. Find the theoretical value of the focal length for this combination of lenses. Measure
the focal length and compare it to the theoretical value.
23. Repeat these measurements using a positive lens and a negative lens of your choice
(don’t forget to write down on your report the ones you used). Compare the
experimental and theoretical values of the focal length.
24. Use the 100 mm lens and the 25.4 mm lens. Locate the 100 mm lens 200 mm from
the object and record the exact object distance. Record the image distance.
25. Place the 25.4 mm lens about 60 mm beyond the image location and move TA-II to
find the image of the combination. Record the separation between the lenses,
magnification, and orientation of the final image.
26. Draw a ray diagram explaining the results of this experiment.