CONVEX LENS EXPERIMENT PURPOSE:
To find the focal length of a lens by two different methods (Part 1 and Part 2).
MATERIALS:
convex lens
white cardboard (screen)
meter stick
candle & base
2 supports for meter stick
support for lens
support for screen
PROCEDURE, PART 1:
First, remove all books and other things from your desk. To find the focal length of your lens, set up your equipment as shown in the diagram. Go to a dark corner of of the room, and aim the device at a window. Focus your image on the screen by sliding the screen back and forth. Try to focus on an object that is as far away as possible (a tree, etc.). In Table 1, record the distance between the lens and the screen. This is your first measurement of the lens’ Focal Length. In the same table, also calculate the values of 2.5f , 2f , 1.5f and and 0.5f . These distances will be used later.
PROCEDURE, PART 2:
We will now find the Focal Length of the lens by a second method. Instead of focusing on an object that is really far away, away, you will focus on a candle flame that is quite close. In a dark corner of the room, set up your equipment as shown in the diagram. Place the light source at a distance of 2.5f from from the lens. Now move the screen until a sharp picture of the flame appears. In Table 2, record the distance between the screen and the lens, the distance between the light source and the lens, and also describe what kind of image you see (magnified, inverted….). Repeat what you just did, but with the light source now placed at a distance of 2f from from the lens. Fill in Table 2 as before. Keep going, with the distances of 1.5f and and 0.5f . Put your equipment away before continuing with the calculations. The calculations will give you your your second estimate of the focal length of the lens.
DATA TABLE 1:
Focal Length, f (cm)
2.5 f (cm)
2 f (cm)
1.5 f (cm)
0.5 f (cm)
DATA TABLE 2:
Object at 2.5f from lens
Object at 2f from lens
Object at 1.5f from lens
Object at 0.5f from lens
do (cm) di (cm) Magnified / Reduced? Upright / Inverted?
CALCULATIONS:
1.
To find the Focal Length from Procedure 2, we need to use the equation to the right. We have 4 sets of data, so we can calculate the Focal Length 4 times and then calculate the average (for greater accuracy). Fill in the table below as part of your calculations…
Focal length calculated from the 2.5f setup (cm)
Focal length calculated from the 2f setup (cm)
Focal length calculated from the 1.5f setup (cm)
1 f
=
1 di
+
1 do
Focal length calculated from the 0.5f setup (cm)
Average:
2.
Calculate the % difference between the Focal Length from Procedure 1 and the average Focal Length from Procedure 2.
%error
=
measured value - actual value actual value
x 100%
LAB REPORT:
Don’t hand in this sheet. On your own paper, Make a Heading called DATA and copy the tables from this paper. Then make a Heading called CALCULATIONS, and do your work there. It is critical that you explain what you’re doing, showing all steps in your calculations and underlining your answers. Under the heading CONCLUSIONS, you will write a statement comparing your two values for Focal Length. Compare the answer you got from Procedure 1 with the answer you got from Procedure 2. In your opinion, were they close (considering the accuracy limits of the experiment). Also comment on what you should do different if you wanted better results. Hand in one report per group. It must be impeccably neat, or it won’t be marked.
