Page 142 - NCERT Science Class 10 English Medium

The reflecting surface of a spherical mirror is by-and-large spherical.
The surface, then, has a circular outline. The diameter of the reflecting
surface of spherical mirror is called its aperture. In Fig.9.2, distance MN
represents the aperture. We shall consider in our discussion only such
spherical mirrors whose aperture is much smaller than its radius of
curvature.
Is there a relationship between the radius of curvature R, and focal
length f, of a spherical mirror? For spherical mirrors of small apertures,
the radius of curvature is found to be equal to twice the focal length. We
put this as R = 2f . This implies that the principal focus of a spherical
mirror lies midway between the pole and centre of curvature.

9.2.1 Image Formation by Spherical Mirrors

You have studied about the image formation by plane mirrors. You also
know the nature, position and relative size of the images formed by them.
How about the images formed by spherical mirrors? How can we locate
the image formed by a concave mirror for different positions of the object?
Are the images real or virtual? Are they enlarged, diminished or have
the same size? We shall explore this with an Activity.

Activity
Activity 9.3
Activity 9.39.3
Activity
Activity 9.3
9.3
You have already learnt a way of determining the focal length of a
concave mirror. In Activity 9.2, you have seen that the sharp bright
spot of light you got on the paper is, in fact, the image of the Sun. It
was a tiny, real, inverted image. You got the approximate focal length
of the concave mirror by measuring the distance of the image from
the mirror.
n Take a concave mirror. Find out its approximate focal length in
the way described above. Note down the value of focal length. (You
can also find it out by obtaining image of a distant object on a
sheet of paper.)
n Mark a line on a Table with a chalk. Place the concave mirror on
a stand. Place the stand over the line such that its pole lies over
the line.
n Draw with a chalk two more lines parallel to the previous line
such that the distance between any two successive lines is equal
to the focal length of the mirror. These lines will now correspond
to the positions of the points P, F and C, respectively. Remember –
For a spherical mirror of small aperture, the principal focus F lies
mid-way between the pole P and the centre of curvature C.
n Keep a bright object, say a burning candle, at a position far beyond
C. Place a paper screen and move it in front of the mirror till you
obtain a sharp bright image of the candle flame on it.
n Observe the image carefully. Note down its nature, position and
relative size with respect to the object size.
n Repeat the activity by placing the candle – (a) just beyond C,
(b) at C, (c) between F and C, (d) at F, and (e) between P and F.
n In one of the cases, you may not get the image on the screen.
Identify the position of the object in such a case. Then, look for its
virtual image in the mirror itself.
n Note down and tabulate your observations.

Light – Reflection and Refraction 137

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