A divergent lens will produce

If the central portion of a convex lens is wrapped in black paper as shown in the figure.

In the figure, an air lens of radii of curvature $10 \mathrm{cm}\left(R_1=R_2=10 \mathrm{cm}\right)$ is cut in a cylinder of glass $(\mu =1.5).$ The focal length and the nature of the lens is