Question

(a) With the help of a labelled diagram, explain how a concave lens diverges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the concave lens on the diagram.
(b) State whether concave lens has a real focus or a virtual focus.
(c) List some things that concave lens and concave mirror have in common.

Answer 1

(a) When a parallel beam of light rays falls on a concave lens, the rays spread out (or diverge) after passing through the lens (figure). Since the refracted rays are diverging away from one another, they do not actually meet at a point. The diverging rays, when produced backwards appear to meet at a point F called the principal focus, on the left side of the lens.

(b) A concave lens has a virtual focus because the light rays do not actually pass through the focus.
(c) Few things which are common to both a concave lens and a convex mirror are as follows:

1. Both diverge a parallel beam of light.
2. Both form a virtual image at all times.
3. Both have a virtual focus.