Why does a lens(concave or convex) becomes invisible for sometine when dipped into water?

Question

what happens due to variation of refractive index?

Answer 1

It doesn't.  However, it becomes more difficult to see.

If the refractive index of water and glass were the same it would disappear as light would not see any boundary between the two.

The refractive index of water is 1.33 compared with common glass of 1.52.  This difference is enough to still be able to see the lens.

Answer 2

The shape of the lens (or the fact that it is a lens at all) is irrelevant.   When a transparent substance (glass, Lucite, etc.) is immersed in a liquid of the same refactive index, there is no distortion or difference between the light passing beside the object and the light passing through it.  Thus, the object appears invisible.