I was tackling a problem that came to my mind whether air bubble placed in water is converging or diverging lens. What I have concluded is that since bubble is sphere, we can assume it to be made of two similar plano convex lens. Now both of them will have same focal length but applying convention states that one would be positive and other would be negative. Thus the effective focal length would be zero and hence it wouldn't act like a lens but a glass slab. What do you say?