There is a vessel completely filled with oil. In the middle of the vessel there is bubble on the surface of oil. explain whether the bubble will act as a convex lens or concave lens or both??

The bubble can act as of any type lens and it will depend on the relation of refractive index of the two mediums. As the forumla of focal length of a lens depends on the R.I. of the medium as well as the R.I. of the outer medium.