Lenses - Determination of Focal Length
INTRODUCTION
Just as mirrors are used to obtain real or virtual images of an object by reflection, lenses are used to obtain
images by refraction. The difference is that the light is transmitted through a lens – it is refracted once
entering the lens and again as it passes out again.
As with mirrors, there are two types of lenses: converging and diverging. Convex lenses cause parallel
rays of light to converge to a real focal point, while concave lenses cause a parallel beam of light to
diverge and appear to emerge from a virtual focal point.
OBJECTIVE
In this experiment you will use lenses to produce real images and investigate how lens characteristics and
the position of the object influence the appearance, orientation and size of real images. Finally you will
determine the relationship between object distance, image distance, focal length and magnification in real
images produced by the converging lenses.
MATERIALS
Light source, two different double convex lenses, one double concave lens, white screen, aperture disc,
figure “4” and letter “L” shaped objects, optical track.
PROCEDURE
Part I: Object placed very far away from the lens
1. Mount the 10 cm double convex lens and the white screen on the optical track as in Figure 1, and
point the lens at a window to the outdoors so the light from distant objects can pass through the lens
and strike the screen.
2. Change the lens-screen distance until you obtain a sharp image of the objects seen through the
window.
3. Mount the variable aperture disc holder on the track in front of the lens toward the window and
investigate the effect of the size and shape of the aperture on the image. Repeat your observations
with the aperture disc on the other side of the lens.
4. Record in Table 1 your observations regarding the following characteristics of the image:
• Position relative to the lens compared to position of the objects to the lens
• Size of the image compared to the size of the objects
• Orientation of the image compared to that of the objects
• Nature of the image: real or virtual
5. Repeat steps 1 through 4 with the 20 cm double convex lens.
6. Repeat steps 1 through 4 with the -15 cm double concave lens.
Table 1: Image characteristics when the object is placed very far away from the lens
Lens Type
10 cm
Double Convex
20 cm
Double Convex
-15 cm
Double Concave
Image Characteristics
Position:
Size:
Orientation:
Nature:
Position:
Size:
Orientation:
Nature:
Position:
Size:
Orientation:
Nature:
The number (expressed in cm) on each lens holder equals the value of the focal length of that lens. How
do the three image positions in the Table 1 compare to these three numbers?
Part II: Object placed closer to the lens on the optical track
1. Mount the light source, the 10 cm double convex lens and the screen on the optical track as in Figure
2. Turn the light source wheel until the number “4” is visible in the opening; this will be your object.
Move the lens and/or screen until you obtain a sharp real image of the object on the screen.
2. Mount the variable aperture disc holder on the track on each side of the lens and investigate the effect
of the size and shape of the aperture on the image in each case.
3. Measure and record in the Table 2 the following distances and heights:
a. Distance from object to lens (do )
b. Distance from image to lens (di )
c. Height of the object (ho )
d. Height of the image on screen (hi )
4. Denoting by f the focal length of the lens, use the lens equation:
1 1 1
= +
f d0 di
and determine the value of f (cm).
5. Change the distance from object to lens and repeat steps 3 and 4.
6. Repeat step 5 two more times.
do (cm)
di (cm)
f (cm)
ho (cm)
hi (cm)
-hi/ho
-di/do
7. Calculate the values for the last two columns of the table. How do they compare?
8. Plot the graph of 1/di versus 1/do and determine the focal length of the lens from the value of
the intercept of the line with the y-axis.
1
di
1
d0
9. Compare the value of focal length determined from this graph with the value estimated in
Part I in Table 1.