Activity 9: Thin Lenses
Date
Performed
SCOR
E
/20
Activity No.9
THIN LENSES
INTRODUCTION
Converging lenses are thicker at the center than at the rim and converge a beam of
parallel light to a real focus. Diverging lenses on the other hand are thinner at the center
than at the rim and diverge a beam of parallel light from a virtual focus.
Figure 9.1
The principal focus of a thin lens with spherical surfaces is the point F where rays
parallel to and near the principal axis x are brought to a focus; this focus is real for
converging lens and virtual for a diverging lens. The focal length is the distance of the
principal focus from the lens.
The object and image relation for the converging and diverging lenses is
1
1 1
  ,
s s' f
(1)
Where s is the distance of the object from the lens, s’ is the distance of the image from the
lens, and f is the focal length of the lens. The lens is assumed to be thin to adopt the
following conventions:
s=
positive for real object
negative for a virtual object
.
f=
positive for a converging
lens
negative for a diverging lens
.
Linear magnification m 
size of
image

s'
s
(2)
size of object
Given an upright and real object, a converging lens form a real and inverted image of
an object located outside of the principal focus. For an object located inside the principal
focus, the image is virtual (on the same side of the lens as the object), upright and
enlarged. With the same object, a diverging lens produces only virtual, upright and smaller
image.
When two thin lenses having focal lengths f1 and f2 are in contact, the focal length of
the combination is given by the equation
VE
OBJECTI
1 1 
 
f
f1
Activity 9: Thin Lenses
1
.
f2
(3)
To measure the focal lengths of converging and diverging lenses.
Activity 9: Thin Lenses
APPARATUS
Converging lens, diverging lens, lens holder, meter stick, colored chalk, rug, image
screen,
candle
PROCEDURE
A. Use of distant landscape
1. Fit a converging lens in a lens holder. Mount a white board image screen on a holder
making sure it is in upright position.
2. Place the lens on one end of a table facing a window. The window serves as the object.
Place the mounted screen on the end of the table with the lens in between. Adjust the
position of the lens
from the
until an image of the window is sharply outlined on the screen. Measure the
distance s's
lens to the screen. The distance should be from the middle of the lens holder to the screen.
s's =
(1 point)
Note: If the weather permits, use the sun as object and measure s's . (This is more
accurate).
Show why s's should be equal to the focal length of the lens. (2 points)
B. Use of thin lens equation
1. Using colored chalk, draw a straight line across a table.
2. Place a lighted candle on one end of the drawn line.
3. Place the converging lens at a distance a little greater s's in Part A but less than 2 s's
than
from the candle. NOTE: The height of the candle should be such that its light is in line with
the center of the lens.
4. Place the screen on the other end of the line. Adjust its position until an image of the
candle light is sharply outlined on the screen. Measure the distance s of the candle light to
the lens and the distance s’ of the screen to the lens.
5. Repeat Steps 3 and 4 for s satisfying the conditions as shown in the following table. Fill
up Table
9.1 with the indicated data.
Table 9.1. (10 points)
Condition
s
for s
s's  s  2s's
s  2s's
2s 's  s  3s 's
s  3s's
s'
y = 1/s
x = 1/s’
xy
x2
Activity 9: Thin Lenses
4
4
4
4
∑ �� =
∑ �� =
∑ ���� =
∑ �2� =
�=1
�=1
�=1
�=1
Activity 9: Thin Lenses
Use linear regression to solve for the � ���������. Then solve for the focal length f
using the relation � ��������� = 1⁄� .(3 points)
Compare the result in procedure A to procedure B. (1 point)