EXPERIMENT 3
FOCAL LENGTH OF CONCAVE MIRROR AND CONVEX LENS
CONCAVE MIRROR
AIM
To determine the focal length of a concave mirror by obtaining the image of a distant
object.
MATERIALS REQUIRED
A concave mirror, a metre scale, a mirror holder.
THEORY
1. The nature of the image formed by a concave mirror, depends upon the position of
the object in front of it.
2. When a parallel beam of light coming from a distant object, such as tree or pole 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.
3. If a screen is placed at the point of focus, a sharp, real and inverted image of the
distant object is obtained.
4. The distance between the pole and principal focus of a spherical mirror is called
focal length of the mirror. It is equal to half the radius of curvature of the mirror.
PROCEDURE
1. Select a distant object such as a tree or pole or the sun.
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 on its reflecting surface.
4. Obtain a well-defined and sharp image on a white wall of the laboratory by
moving the mirror backward or forward.
5. Measure the distance between the wall and the concave mirror with the help of a
meter scale as shown in figure (a) or figure (b).
6. This will give you approximate focal length of the mirror.
7. Repeat the experiment by selecting the different distant objects at different
distances.
OBSERVATION AND CALCULATION
S.No Position of Position of Difference Focal length
Mirror(L) Screen(M) between
screen and f(cm)
mirror
1 10cm 22cm 12cm 12cm(f1)
2 15cm 27cm 12cm 12cm(f2)
3 20cm 32cm 12cm 12cm(f3)
FOCAL LENGTH OF CONCAVE MIRROR AND CONVEX LENS
CONCAVE MIRROR
AIM
To determine the focal length of a concave mirror by obtaining the image of a distant
object.
MATERIALS REQUIRED
A concave mirror, a metre scale, a mirror holder.
THEORY
1. The nature of the image formed by a concave mirror, depends upon the position of
the object in front of it.
2. When a parallel beam of light coming from a distant object, such as tree or pole 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.
3. If a screen is placed at the point of focus, a sharp, real and inverted image of the
distant object is obtained.
4. The distance between the pole and principal focus of a spherical mirror is called
focal length of the mirror. It is equal to half the radius of curvature of the mirror.
PROCEDURE
1. Select a distant object such as a tree or pole or the sun.
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 on its reflecting surface.
4. Obtain a well-defined and sharp image on a white wall of the laboratory by
moving the mirror backward or forward.
5. Measure the distance between the wall and the concave mirror with the help of a
meter scale as shown in figure (a) or figure (b).
6. This will give you approximate focal length of the mirror.
7. Repeat the experiment by selecting the different distant objects at different
distances.
OBSERVATION AND CALCULATION
S.No Position of Position of Difference Focal length
Mirror(L) Screen(M) between
screen and f(cm)
mirror
1 10cm 22cm 12cm 12cm(f1)
2 15cm 27cm 12cm 12cm(f2)
3 20cm 32cm 12cm 12cm(f3)
