The rays of light falling on a convex lens in a direction parallel to principal axis of the lens, get refracted through the lens and meet actually at a single point `F` on the principal axis of the lens. This point is called principal focus of the lens.
Read the above passage and answer the following questions :
(i) Is principal focus of a convex lens, a real point ? Is the same true for a concave lens ?
(ii) A distinct image of a distant tree os obtained on a screen held at `40 cm` from a convex lens. What is its focal length ?
(iii) Our teachers and parants advise us to stay focussed. What does it imply ?

Explanation

(i) Yes, the principal focus of a convex lens is a real point. This is because rays refracted through convex lens meet actually at this point.
No, principal focus of a concave lens is a virtual point only.
(ii) f = distance of screen from the lens = 40 cm
(iii) Our parants and teachers advise us to stay focussed. It implies that we concentrate all our energies/efforts at a single point/problem so that we can resolve the same easily. Staying focussed means that we do not divert our energies and attention to several things at a time. This would lead us nowhere. Thus, the secret of success is to stay focussed.