A concave glass lens of refractive index $1.5$ is immersed in carbon di-sulfide (refractive index $=1.6$). Which of the following is true?
[ Assume both surfaces of the lens have same radius of curvature $=R$]

Diameter of a plano-convex lens is $6 \mathrm{cm}$ and thickness at the centre is $3 \mathrm{mm}$. If speed of light in material of lens is $2\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$ the focal length of the lens is (Speed of light in vacuum = $3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1})$

A converging lens having magnitude of focal length as $f_1$ is kept coaxially in contact with a diverging lens having magnitude of focal length as $f_2$. The focal length of the combination would be: