Question

By drawing a neat diagram, describe the image formation by a concave lens when an object is
(a) at infinity.
(b) anywhere between optical centre and infinity.
[5 MARKS]

Answer 1

Each subpart: 2.5 Marks each

(a) Let a beam of parallel rays of light coming from an object placed at infinity fall on a thin concave lens. After refraction through the lens, the rays diverge and on being produced backward, they meet at the focus $F_2$ (i.e. to the observer, the rays appear to diverge from the focus $F_2$). Therefore, for an object placed at infinity, the image formed by a concave lens is situated at the focus on the same side of the lens as the object. It is virtual, upright and highly diminished.

(b) The formation of the image is shown in Fig. 5.38. It is formed on the same side of the lens as the object between the optical centre and the focus. The image formed is virtual, upright and diminished.