Lab Activity Phys223
It’s All About Image and Perception]
Names:
____________________________________________________________________________________
It’s All About Image and Perception
In this lab activity you will apply the lens (and mirror) equation to single lens and
lens/mirror combination experiments.
You will
A. find the focal length of a converging lens (single lens setup).
B. find the focal length of a diverging lens (two component setup).
C. explore a lens/mirror combination.
D. explain the pig mystery.
Part A: Measuring the Focal Length of a Converging Lens
1. For five different values of the object distance do, measure the resulting image distance
di, and calculate the resulting value for the lens’s focal length f.
do (cm)
di (cm)
f (cm)
Questions (answers in boldface)
a. What is your average result for the focal length of the lens?
b. Find the relative error using the manufacturer’s listed focal length for the lens.
c. What type of image did you use (real or virtual) and why?
d. What is the minimum object distance you can use in this experiment and why?
Lab Activity Phys223
It’s All About Image and Perception]
Names:
____________________________________________________________________________________
Part B: Measuring the Focal Length of a Diverging Lens
In this part you will need to find and use a method that allows you to find the focal length
of a diverging lens.
The following questions (answers in boldface if typed) are supposed to be hints. 
a. What type of image (real or virtual) do you need in order to make measurements in an
experiment?
b. Show mathematically that the lens equation “If I do, I die”, when using a diverging
lens (f < 0) and a real object (do > 0) requires a virtual image (di < 0).
c. Suppose you use a diverging lens with focal length f= -A, and you want to have a
positive value for the image distance di, find the range of allowable values for do.
1
1
1
=
+
−𝐴 𝑑𝑜 𝑑𝑖
d. Describe your method for finding the focal length f for a diverging lens. Include a
sketch if it helps.
Lab Activity Phys223
It’s All About Image and Perception]
Names:
____________________________________________________________________________________
e. Present your data and your result for f. Find the relative error by comparing the
experimental result to the manufacturer’s data.
Part C: explore a lens/mirror combination.
Using your knowledge of the previous parts set up an experiment using one lens and one
spherical mirror to produce a real image.
a. Sketch a ray tracing diagram of your experiment here:
b. Using “If I do, I die”, set up an experiment that will produce a real image and predict
the image distance and image height.
c. Set up an experiment to verify your predictions. Note that you will most likely have to
tilt the mirror from the optical axis in order to be able to ‘catch’ the image without
blocking rays with your screen. Ignore that effect in parts (a) and (b). Present your
measured data and show that your predictions and experimental data are consistent.
Lab Activity Phys223
It’s All About Image and Perception]
Names:
____________________________________________________________________________________
Part D: You Pig
Our ‘pig-toy’ produces several images. You will need to find how many in part a but only
investigate the main image (the upright pig at the opening) in what follows after part a.
Questions:
a. How many images does our ‘pig-toy’ produce? Describe what they are and where they
are located (use a sketch if necessary).
b. Experimentally find the focal length of the two concave mirrors used by the toy.
Describe your method (sketch) and present your data and result for f.
c. How does the focal length f compare to where you find the upright pig at the opening?
d. Use words and a ray diagram to explain the pig mystery (only for the upright pig at the
opening). What does the top mirror do? What does the bottom mirror do?