Page 210 - Physics_Form_2

Physics for Secondary Schools

Focal length of a convex lens The image of a distant object is formed at
The focal length of a converging lens the focal point of a convex lens. Therefore,
can be determined experimentally using the distance between the screen and the
various methods. Activity 5.15 describes lens is the focal length of the lens. Note
one of the methods. that, this is not a very accurate method

but a quick way of estimating the focal
FOR ONLINE READING ONLY
Activity 5.15 length of a convex lens.

Aim: To estimate the focal length
of a converging lens Example 5.4
An object is placed 12 cm from a
Materials: a converging lens, lens convex lens of a focal length of 18
holder, metre rule, plain cm. Using the lens formula, find the
paper (screen) position of the image.

Procedure Solution
1. Fix the lens on the holder and the For a convex lens, the value of
screen on a stand. focal length is positive. Therefore,
2. Place the screen and the lens on a f =+ 18 cm and u = +12 cm .
table in such a way that the light 1 1 1
from a distant object passes through f = u + v
the lens as shown in Figure 5.43.

3. Move the screen closer or away 1  1  1
from the lens until a sharp image of v f u
the distant object is focused on the 1 1
screen.  
18 12
Lens Screen
Rays  2 cm 3 cm
from 36 cm
distant
object  1

36 cm
Figure 5.43
v  36 cm
4. Measure the distance between the Because the value of v is negative, the
screen and the lens.
image is virtual (on the same side as

Question the object) and is 36 cm from the lens.
What is the distance between the Because the image is virtual, it is also
screen and the lens? erect.

204
Student’s Book Form Two

Physics Form 2 Final.indd 204 25/10/2025 10:28