Ray Diagram for convex lens:

OR

Converging action of the convex lens as a combination of prisms:

A lens is made up of a set of prisms, the prisms in the upper part of a convex lens have their bases downwards, while the prisms in the lower part of the lens have their bases upwards. When a beam of parallel rays of light incident on a convex lens, the prisms in the upper part of the lens bend the incident rays downward and the prisms in the lower part of the lens bend the incident rays upwards. The central part which is a parallel sided glass block passes the incident ray undeviated. Thus the convex lens converges the parallel beam of light to a point called principal focus (F) of the lens and hence a convex lens is called a converging lens.