Focal Length and Magnification of a Thin Lens
EQUIPMENT NEEDED
- Optics Bench
- Convex Lens
PURPOSE
To determine the focal length, magnification and power of a thin lens
THEORY
For a thin lens: 1/f = 1/d0 + 1/dI
where f is the focal length, d 0 is the distance between the object and the lens, and dI is the
distance between the image and the lens.
PROCEDURE

Position the lens on the optics bench between the light box crosshairs (object) and the
screen. Make sure that the box and screen are far apart as possible.

Move the lens to a position where an image of the object (crosshairs) is formed on the
screen. Measure the image distance and the object distance. Record all measurements
in line 1 of the table.

Use a ruler to measure the object size and image size for this position of the lens. Record
your measurements in line 1 of the table.

Move the lens (not the screen or light source) to a second position where the image is
in focus. Measure the image distance, object distance, object size and image size.
Record all measurements in line 2 of the table.

Write down the local length of the lens printed on the lens holder above the data table.
DATA TABLE
Focal length of double convex lens (f) = _____________
Object
Distance
(cm)
Image
Distance
(cm)
Object Size
(cm)
Image Size
(cm)
1/ d0
(cm-1)
1/ dI
(cm-1)
f
(cm)
1
2
ANALYSIS

Using your measured object and image distances, determine 1/d0 , 1/di and f for each
position of the lens. Show your calculations on a separate sheet of paper and fill in the
values in the data table.

Determine the percentage difference between the average value of f (for the two positions)
and the value of f printed on the lens holder.
Work:

% Difference = _______________
Magnification of a lens is defined two ways.
First, it is defined as
Magnification = M = - dI / d0
1) Using dI and d0 values for line 1 in the table determine the magnification
Work:
M1 = ___________
2) Using dI and d0 values for line 2 in the table determine the magnification
Work:
M2 = ___________
Second, it is defined as
Magnification = Image size (hI)
Object size (h0)
1) Using hI and h0 values for line 1 in the table determine the magnification
Work:
2)
M 1 = ____________
Using hI and h0 values for line 2 in the table determine the magnification
Work:

M2 = ____________
Find the percentage difference for each value of magnification above for the two methods.
Work:
% Difference M1 = ____________
Work
% Difference M2 = _____________
QUESTIONS
1) Is the image formed by the lens erect or inverted?
2) Is the image real or virtual? How do your know?
3) Explain why, for a given screen-object distance, there are two positions where the image is in
focus.
4) Why is the magnification negative?