Kwun Tong Government Secondary School
S.6 Physics TAS Experiments 2005-2006
S.6 _____
Name:________________________
Class no.:_____
Date:____________
Experiment 3
Measurement of the focal lengths of a concave lens and a convex mirror
Objective:

To measure the focal length of a concave lens with the help of an auxiliary convex lens and the lens
formula

To measure the focal length of a convex mirror with the help of an auxiliary convex lens
Apparatus:
convex lens
1
concave lens
1
convex mirror
1
metre rule
1
white screen
1
ray box with a fine gauze grid
1
Experiment A: Measurement of the focal length of a concave lens using an auxiliary convex lens
Theory:
Step 1: Locating the real image formed by the auxiliary convex lens.
ray box
screen
image of the fine grid
Step 2: Adding the concave lens between the convex lens and the screen.
virtual object for the
concave lens
real image of the
concave lens
u
v
After repositioning the screen to catch the image of the fine grid again, u and v can be found and the
focal length of the concave lens f can be calculated by using the lens formula:
1 1 1
 
(Use real-is-positive convention, i.e. take u as negative.)
f u v
1
Procedures:
1.
Use a distant object to estimate the focal length of the convex lens, f.
Focal length of the convex lens, f
2.
= _______________ cm
Place the convex lens about 2f from the illuminated fine gauze grid and try to catch a sharp image
on the white screen.
ray box
screen
image of the fine grid
3.
Now place the concave lens about half way between the convex lens and the screen. Measure the
distance between the concave lens and the screen, u. Move the screen away from the lens until a
sharp image of the fine gauze grid is again obtained. Measure the new distance between the
concave lens and the screen, v.
u
v
Repeat step (3) by moving the concave lens either farther away or towards the convex lens and
readjust the position of the screen so as to obtain a focused image again. Obtain four more sets of
values of u and v. Tabulate the results below. Also calculate the values of the focal length of the
concave lens, f for each set of data.
u/cm
v/cm
f/cm
Average of f =
2
Experiment B: Measurement of the focal length of a convex mirror using an auxiliary convex lens
Theory:
Image is formed beside the
object
C
P
O, I
r
The theory is quite simple. First use the convex lens to form a real image at C. Then place the
convex mirror in between the real image and the convex lens. Move the convex mirror to and fro.
When the convergent beam is incident normally on the mirror, the reflected beam will be reflected back
along its original path. Thus an image I will be formed coincide with the object O. The distance
between C and the pole of the mirror is then the radius of curvature r.
Procedures:
1.
Use a convex lens to form an image on the white screen. The distance between the lens and the
screen should be about 0.5 m. Measure the distance between the lens and the screen, d.
ray box
screen
image of the fine grid
d
d = ____________ cm
2.
Now place the mirror between the lens and the screen. Adjust the position of the mirror until a
focused image of the source is obtained on the side of the light box and close to the grid.
Image is formed beside the
object
P
x
d
3
Measure the distance x between the centres of the lens and the mirror.
Radius of curvature of the convex mirror, r = d – x = _____________ cm
3.
Repeat steps (1) and (2) using a different initial position of the lens.
distance between the lens and the screen?
Tabulate your results below.
d/cm
x/cm
r/cm
What should be the least
f/cm
Average of f =
Report Guidelines:

Pay attention to the no. of significant figures of your data in your tables.

Briefly describe the procedures and state your precautions.

Sketch what you see on the screen.

Give a summary of your results found in the experiments
4