For a lens in air, you have learned the conditions on its radii of curvature to make the lens converging or diverging. You have done some diving, and you realize that a lens has a different focal length underwater than it does in air. That starts you thinking about what would happen if a lens is placed in a medium with an index of refraction greater than that of the material of which the lens is made, but you are not even sure that it is possible to form images under such conditions. Lost in your thoughts, you drift off into dreamland.