Light enters from air into water which has a refractive index of $1.33$. Calculate the speed of light in water. The speed of light in air is $3.0\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$.

A convex lens of the focal length $25 \mathrm{cm}$ is placed in contact with a concave lens of focal length $20 \mathrm{cm}$. Find the focal length of the combination of lenses.  

A convex lens of the focal length $25 \mathrm{cm}$ is placed in contact with a concave lens of focal length $20 \mathrm{cm}$. Find the power of the combination. What is the nature of the combination?