Question

Why concave lens diverge and convex lens converge light rays?

Answer 1

This has to do with geometry of lens.

Concave lenses are thinner at the middle. So Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens.

When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.

Convex lenses are thicker at the middle. So Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens.

When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.