A convex or converging lens is thicker at the centre than at the edges. It converges a beam of light on refraction through it. It has a real focus. Convex lens is of three types : Double convex lens, Plano-convex lens and Concavo-convex lens.
The concave lens is thinner at the centre than at the edges. It diverges a beam of Light on refraction through it. It has a virtual focus. Concave lenses are of three types: Double concave lens, Plano concave lens and Convexo concave lens.