Lens is a position, of a transparent refracting medium which is bounding by the coaxial spherical surfaces. There are two types of lenses,
i) Convergent lenses : There are thick at centre and thin near edges. They bend the rays towards centre of the lens. They are divided into three categories.
ii) Divergent lens : These are thin at the centre and thick near edges. They bend the rays towards the edges of the lens. They are also further divided into 3 categories. The power of a lens is defined as the reciprocal of focal length of the lens. It is measured in diapters.