A thin concavo-convex lens has two surfaces of radius of curvature $R$ and $2R$. The material of the lens has refractive index $\mu .$ When kept in air, the focal length of the lens 

will be equal to $\frac{R}{\mu -1}$

 will be equal to $\frac{2R}{\mu -1}$

will depend on the direction from which light incident on it. 

will be the same, irrespective of the direction from which light is incident on it.