Date __________
Lab Time ______
Name ___________________________
Lenses
Objective
This experiment is to help the student determine the focal length of a converging lens and a
diverging lens. In addition, students will observe how each lens magnifies the image, compared to
the object.
Background
Lenses form the basis of eyeglasses and contacts as well as instruments such as microscopes and
telescopes. In order to design any kind of optical system the focal length of the lens (or lenses) is
(are) required. In general two kinds of lenses are of practical use—converging lenses and
diverging lenses.
Converging lenses cause parallel light rays to converge to a single point, called the focus. The
distance between the converging lens and the focus is called the focal length. Converging (or
convex) lenses are commonly used to correct for farsightedness since the defective eye focuses the
image behind the retina. With proper vision correction, a converging lens “brings together” the
light rays so they focus on the retina.
Diverging lenses, on the other hand, are used to correct for nearsightedness, since a defective eye
focuses the image in front of the retina. A diverging lens “spreads out” the light rays so they focus
properly on the retina, allowing the person to see clearly.
Materials needed
Optical bench
Light source
Object
Screen
Lens holder
Converging lens
Diverging lens
Figure 1
______
Lenses
1
Date __________
Lab Time ______
Name ___________________________
Procedure
Part I – Converging Lens
Method I
1. The focal length of the converging lens can be found one way by measuring the image distance due to
an object at a great distance from the lens. To do this, mount the lens and screen on the optics bench.
Take the optical bench to a window and point it at an object far way (i.e. a tree or building). Move the
screen until that object appears clearly as an image on the screen. The focal length (f ∞ ) is the distance
between the lens and the screen.
Method II
2. Another way to calculate the focal length of a converging lens is to use objects that are close to the lens.
To do this, mount an object (card with arrows) between the light source and the lens on the optical
bench (see Figure 1). With the object distance (distance between the object and lens) do set at 3 times
the focal length (3 f ∞) you determined in Step 1, move the screen until a focused image appears.
Record d0 and di (the distance between the lens and the image) for this image.
3. For all observations, record relative size and orientation of the image (cross-hair with arrows) on your
data sheet as it appears on the screen.
4. Using d0 and di, calculate the focal length (f) of the converging lens with the following relation:
1 1 1
= +
f do di
5. Repeat Steps 2 - 4 and calculate the focal length (f) for the converging lens if d0 = 2 f ∞ and 1.5 f ∞.
Method III
6. For the Dual Images, separate the object and the screen by a distance of 30 cm (or more than four times
the focal length of the lens). Start with the lens near the object and slowly move it toward the screen in
order to find two locations where a focused image appears on the screen (one big and one small).
Record d0 and di for each image produced. Note the size and orientation of the images based on the
location of the lens from the object.
Part II – Diverging Lens
1. Determine the focal length of the diverging lens by combining it with the converging lens. Set d0 to at
least 12 cm and then obtain and record di for the image produced by the combination of lenses.
Calculate the focal length of the combination of lens, fc , in the same manner as in Step 3 of Part I.
Note the size and orientation of the images based on the location of the lens from the object.
2. Using
1 1 1
= +
fc f fd
Note: f represents the focal length of your original converging lens, f ∞.
calculate the focal length of just the diverging lens, fd.
______
Lenses
2
Date __________
Lab Time ______
Name ___________________________
Report Sheet – Data Analysis
Part I - Converging Lens
Method I
Object distance infinity.
[d0 = infinity]
di = _________ cm
f ∞ = _________ cm
Method II
** For all the following observations, sketch the relative size and orientation of the image.
For d0 = 3 f ∞ :
d0 = ______ cm,
di = _______ cm
and
f = _________ cm
d0 = ______ cm,
di = _______ cm
and
f = _________ cm
d0 = ______ cm,
di = _______ cm
and
f = _________ cm
Calculations:
For d0 = 2 f ∞ :
Calculations:
For d0 = 1.5 f ∞ :
Calculations:
Method III: Dual Images
** For all the following observations, sketch the relative size and orientation of the image.
Image 1:
d0 = ______ cm,
di = ______ cm and
f = _______ cm
Image 2:
d0 = _______ cm,
di = ______ cm and
f = _______ cm
Calculations:
______
Lenses
3
Date __________
Lab Time ______
Name ___________________________
Part II - Diverging Lens
** For all the following observations, sketch the relative size and orientation of the image.
d0 = _______ cm,
di =______ cm and
fc = _______ cm
Calculations:
fd = _______ cm
Calculations:
Draw a schematic diagram, to scale, of an object – lens – image combination for one setup of your
data.
______
Lenses
4
Date __________
Lab Time ______
Name ___________________________
Post lab questions:
1. How did the image size produced on the screen relate to the object distance (d0)?
2. How was the orientation of the image produced on the screen related to the object distance (d0)?
3. Why was it necessary to combine the diverging and converging lenses in order to determine the
focal length of just the diverging lens?
4. Calculate the average focal length (favg) from all your calculations for f in Part I and compare
favg to f ∞ . What is your percent difference between f ∞ and favg?
Value 1 − Value 2
% difference =
* 200%
Value 1 + Value 2
5. Does the focal length (f ) of a converging lens depend on the object distance (d0) or is it always
constant? What proof do you have to support your answer?
______
Lenses
5
Date __________
Lab Time ______
Name ___________________________
Summary/Conclusions:
______
Lenses
6