Activity . 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. Hold a convex lens in your hand.
Direct it towards the Sun. Focus the light from the Sun on a sheet of paper. Obtain a sharp bright image of the Sun. 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 .
. 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.
. (a) and for a concave lens in Fig. . (b).
Observe Fig. . (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.
. (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 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 . . 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.