Ray 1 is parallel to the principal axis. F. Similarly for ray 3. Ray 2 on the other hand is along the principal axis and is normal on the rectangular glass block. This ray passes un-deviated along the principal axis. This is the reason why a convex lens is called a converging lens.
Figure below shows how light rays are focused by a concave lens.
A concave lens may be considered to be made up of two prisms and a solid glass block in the centre. Prisms divert or refract light rays toward the direction of their bases. The prisms have their bases away from the optical centre. The central glass block is also placed edge on. Thus the arrangements of prisms and the glass block differs considerably in a concave lens.