Nuclear sizes are expressed in a unit called "Fermi" which is a unit of length equal to 10−15 metre (one femtometre), used in nuclear physics. It is similar to the diameter of a proton.
The lens which have atleast one inward curved surface is a concave lens. Concave ens is a diverging lens as it spreads out light rays that have been refracted through it.
A concave lens is thinner at its centre than at its edges.