What are the factors affecting the refractive index of a lens?

Q. A lens is made of flint glass (refractive index $=1.5)$. When the lens is immersed in a liquid of refractive index $1.25$, the focal length :

Q. A concave lens of refractive index $1.5$ is immersed in a medium of refractive index $1.65$. What is the nature of the lens?

Q. What are the factors that influence refractive index.

Q. The refractive index of a converging lens is $1.4$. What will be the focal length of this lens if it is placed in a medium of same refractive index? Assume the radii of curvature of the faces of lens are $R_1$ and $R_2$ respectively.