A ray of light after refraction through a lens emerges parallel to the principal axis of the lens. The incident ray either passes through or appears to meet at:

In the diagrams below, $X{X}^{\text{'}}$ represents the principal axis, $O$ the optical centre and $F$ the focus of the lens. Complete the path of the rays $A$ and $B$ as they emerge out of the lens.

Study the diagram given below. Name the lens LL'.