AIM
To Determine the focal length of the concave mirror by obtaining the image of a distant object.
MATERIALS REQUIRED
A concave mirror, a metre scale, a mirror holder.
THEORY
1. A spherical mirror whose reflecting surface is curved inwards, i.e., faces towards the centre of the sphere is called a concave mirror.
2. It obeys the law of the reflection of light like a plane mirror.
3. Thus, the parallel beam of a ray of light coming from a distant object, such as the sun or a building can be considered as parallel.
4. When the parallel ray is incident on the reflecting surface of a mirror, then after reflection, the rays converge at a point, and this point is called principal focus of the concave mirror as shown in the figure.
5. since the image formed by the mirror is real so that it can be obtained on the screen.
6. The distance between the pole O and principal focus F of a concave mirror is called the focal length of the mirror. It is equal to half the radius of curvature of the mirror.
PROCEDURE
1. Choose a distant object like a tree or the sun to at as an object for our experiment.
2. Mount the concave mirror in a mirror holder.
3. Adjust the concave mirror in such a way that the rays of light coming from the tree fall directly on its reflecting surface.
4. Measure the horizontal distance between the wall and the centre of a concave mirror with the help of a meter scale.
5. Repeat the experiment by selecting the different distant objects at different distances to measure the focal length of the concave mirror.
OBSERVATION AND CALCULATION
S. No. |
The position of mirror ‘A’ (cm) |
The position of screen ‘B’ (cm) |
Difference between mirror and screen (B – A) |
Focal length ‘f’ (cm) |
1. |
20 cm |
37 cm |
17 cm |
F1 = 17 cm |
2. |
30 cm |
49 cm |
19 cm |
F2 = 19 cm |
3. |
40 cm |
58 cm |
18 cm |
F3 = 18 cm |
Mean Focal length 
= 18 cm
RESULT
The approximate focal length of the given concave mirror is 18 cm as determined by the above method.
According to the sign conventions, the focal length of a concave mirror is negative. Therefore,
f = - 18 cm
PRECAUTIONS
1. The mirror should be placed vertically in the lens holder.
2. There should not be any obstacle in the path of rays of light incident on the concave mirror
3. The meter scale must be correctly positioned between the wall and centre of the concave mirror.