Question

when does a convex lens behave as a concave lens?

Answer 1

It's depends upon the refractive index of medium. So denser medium have greater value of (u). That's why the rarer medium convex lens placed in dancer medium behaves like as concave lens.

hope it's help u

Answer 2

when a lens filled with a substance say air and it is kept in an optically denser medium say water then at this time a convex lens behaves like a concave lens