Figure below shows a lens of refractive index, $$\mu=1.4$$. $$C_1$$ and $$C_2$$ are the centres of curvature of the two faces of the lens of radii of curvature $$4 \mathrm{~cm}$$ and $$8 \mathrm{~cm}$$ respectively.

The lens behaves as a

When a biconvex lens of glass of refractive index 1.5 is dipped in a liquid, it acts like a plane sheet of paper. This means the refractive index of the liquid is

A hollow prism is filled with water and placed in air. It will deviate the incident rays

An object is placed at a distance of $$12 \mathrm{~cm}$$ from a convex lens on its principal axis and a virtual image of certain size is formed. If the object is moved $$4 \mathrm{~cm}$$ away from the lens, a real image of the same size as that of the virtual image is formed. The focal length of the lens in $$\mathrm{cm}$$ is