A convex lens is made of a material having refractive index 1.2. Both the surfaces of the lens are convex. If it is dipped into water $(\mu =1.33)$, it will behave like

The µ of the convex lens is greater than air so it will act as a convergent lens in the air. When it is dipped in water its µ is less than water. Now the rays entering and exiting the lens will bend toward or away from the normal in an opposite direction than when the lens was placed in air. So the convergent lens in the air will now behave as a divergent lens in water .