An upright lens is divided horizontally into a number of parts and the diagram given below shows the magnified view of its lowest part .

(a) It is a concave (diverging) lens.

(b) The lowest part of the lens behaves like a small prism. From the diagram, this part is thicker at its lower edge and thinner at its upper edge, i.e., the base of this prism points away from the principal axis of the lens.

This means that as we move from the lower edge of the lens towards its middle, the lens becomes thinner. A lens which is thin at the middle and thick at the edges is a concave lens. Further, since each prism-like part bends the light towards its base, i.e., away from the principal axis, the lens diverges the incident light. Hence it is a concave lens.