A convex lens of refractive index 1.5 and focal length f=18 cm is immersed in water. The difference in focal lengths of the given lens when it is in…

A convex lens of refractive index $\displaystyle 1.5$ and focal length $\displaystyle f=18 \mathrm{~cm}$ is immersed in water. The difference in focal lengths of the given lens when it is in water and in air is $\displaystyle \alpha \times \mathrm{f}$. The value of $\displaystyle \alpha$ is $\displaystyle \_\_\_\_$. (refractive index of water $\displaystyle =4 / 3$ )