Explain the optical center of a lens with the help of proper diagrams.
Optical center
Define the term principal foci of a concave lens and show them with the help of proper diagrams.
(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 convex mirror have in common.