Page 141 - NCERT Science Class 10 English Medium

The reflecting surface of a spherical mirror forms a part of a sphere.
This sphere has a centre. This point is called the centre of curvature of
the spherical mirror. It is represented by the letter C. Please note that the
centre of curvature is not a part of the mirror. It lies outside its reflecting
surface. The centre of curvature of a concave mirror lies in front of it.
However, it lies behind the mirror in case of a convex mirror. You may
note this in Fig.9.2 (a) and (b). The radius of the sphere of which the
reflecting surface of a spherical mirror forms a part, is called the radius
of curvature of the mirror. It is represented by the letter R. You may note
that the distance PC is equal to the radius of curvature. Imagine a straight
line passing through the pole and the centre of curvature of a spherical
mirror. This line is called the principal axis. Remember that principal
axis is normal to the mirror at its pole. Let us understand an important
term related to mirrors, through an Activity.

Activity 9.2
Activity
Activity 9.29.2
Activity 9.2
Activity 9.2
CAUTION: Do not look at the Sun directly or even into a mirror
reflecting sunlight. It may damage your eyes.
n Hold a concave mirror in your hand and direct its reflecting surface
towards the Sun.
n Direct the light reflected by the mirror on to a sheet of paper held
close to the mirror.
n Move the sheet of paper back and forth gradually until you find
on the paper sheet a bright, sharp spot of light.
n Hold the mirror and the paper in the same position for a few
minutes. What do you observe? Why?

The paper at first begins to burn producing smoke. Eventually it
may even catch fire. Why does it burn? The light from the Sun is converged
at a point, as a sharp, bright spot by the mirror. In fact, this spot of light
is the image of the Sun on the sheet of paper. This point is
the focus of the concave mirror. The heat produced due to
the concentration of sunlight ignites the paper. The distance
of this image from the position of the mirror gives the
approximate value of focal length of the mirror.
Let us try to understand this observation with the help
of a ray diagram.
(a)
Observe Fig.9.2 (a) closely. A number of rays parallel
to the principal axis are falling on a concave mirror. Observe
the reflected rays. They are all meeting/intersecting at a
point on the principal axis of the mirror. This point is called
the principal focus of the concave mirror. Similarly, observe
Fig. 9.2 (b). How are the rays parallel to the principal axis,
reflected by a convex mirror? The reflected rays appear to
come from a point on the principal axis. This point is called
(b) the principal focus of the convex mirror. The principal focus
is represented by the letter F. The distance between the
9.2
Figure 9.2
Figure 9.2
Figure
Figure 9.29.2
Figure
(a) Concave mirror pole and the principal focus of a spherical mirror is called
(b) Convex mirror the focal length. It is represented by the letter f.
136 Science

2024-25