Lens : A Lens is finite size medium having one or both surfaces as curved depending upon the curvature of surfaces, there are various types of lens.

Fig.8.1.2: Different Types of Lens

Convex Lens   : Thicker in the middle, thinner towards edges. Light rays falling on it bend towards center.
Concave Lens : Thinner in the middle, thicker towards edges. Light rays falling on it bends towards edges (away from the center)
Thin Lens         : The lens, whose thickness is much smaller compared to the distances associated with it (The focal lengths, the distance of object and image are measured from the center of the lens)