Page 156 - NCERT Science Class 10 English Medium

usually represented by the letter O. A ray of light through the optical
centre of a lens passes without suffering any deviation. The effective
diameter of the circular outline of a spherical lens is called its aperture.
We shall confine our discussion in this Chapter to such lenses whose
aperture is much less than its radius of curvature and the two centres of
curvatures are equidistant from the optical centre O. Such lenses are
called thin lenses with small apertures. What happens when parallel rays
of light are incident on a lens? Let us do an Activity to understand this.

Activity 9.11
Activity 9.119.11
Activity 9.11
Activity
Activity 9.11
CAUTION: Do not look at the Sun directly or through a lens while
doing this Activity or otherwise. You may damage your eyes if you
do so.
n Hold a convex lens in your hand. Direct it towards the Sun.
n Focus the light from the Sun on a sheet of paper. Obtain a sharp
bright image of the Sun.
n Hold the paper and the lens in the same position for a while. Keep
observing the paper. What happened? Why? Recall your experience
in Activity 9.2.

The paper begins to burn producing smoke. It may even catch fire
after a while. Why does this happen? The light from the Sun constitutes
parallel rays of light. These rays were converged by the lens at the sharp
bright spot formed on the paper. In fact, the bright spot you got on the
paper is a real image of the Sun. The concentration of the sunlight at a
point generated heat. This caused the paper to burn.
Now, we shall consider rays of light parallel to the principal axis of a
lens. What happens when you pass such rays of light through a lens?
This is illustrated for a convex lens in Fig.9.12 (a) and for a concave lens
in Fig.9.12 (b).
Observe Fig.9.12 (a) carefully. Several rays of light parallel to the
principal axis are falling on a convex lens. These rays, after refraction
from the lens, are converging to a point on the principal axis. This point
on the principal axis is called the principal focus of the lens. Let us see
now the action of a concave lens.
Observe Fig.9.12 (b) carefully. Several rays of light parallel to the
principal axis are falling on a concave lens. These rays, after refraction
from the lens, are appearing to diverge from a point on the principal
axis. This point on the principal axis is called the principal focus of the
concave lens.
If you pass parallel rays from the opposite surface of the lens, you
get another principal focus on the opposite side. Letter F is usually used
to represent principal focus. However, a lens has two principal foci. They
are represented by F and F . The distance of the principal focus from
1 2
the optical centre of a lens is called its focal length. The letter f is used to
represent the focal length. How can you find the focal length of a convex
lens? Recall the Activity 9.11. In this Activity, the distance between the
position of the lens and the position of the image of the Sun gives the
approximate focal length of the lens.

Light – Reflection and Refraction 151

2024-25