Define focal length of a lens

Find the focal length of a lens of power- 2.0 D.What type of lens this?

The focal length of a lens is expressed by which of the following:

The change in focal length of an eye lens is caused by the action of the:

A plano-convex lens is made of refractive index 1.6.If the radius of curvature of the curved surface is 60 cm,then focal length of the lens is:

Define the focal length of mirror

A thin converging lens has a focal length f in air. If it is completely immersed in a liquid , briefly explain how the focal length of the lens will vary?