(a) A convex lens is a converging lens because it converges a parallel beam of light rays passing through it at its focus.
Figure - A convex lens converges (brings closer) a parallel beam of light rays to a point F on its other side(right side)
(b) A concave lens is a diverging lens because it diverges the parallel beam of rays passing through it.
Figure - A concave lens diverges (Spreads out) a parallel beam of light rays.
