Question

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.

Answer 1

 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.